Psychological Development

So far in this course, we have focused our attention on mental processes in mature, adult humans. We have taken mind and personality as givens, and asked two basic questions:

  • How does the mind work?
  • How does the mind mediate the individual's interactions with the social environment?

Now we take up a new question:Where do mind and personality come from?

Views of Development

In psychology, there are two broad approaches to the question of the development of mind:

Ontogenetic View of DevelopmentThe ontogenetic point of view is concerned with the development of mind in individual species members, particularly humans. This is developmental psychology as it is usually construed. Reflecting the idea that mental development, like physical development, ends with puberty, developmental psychologists have mostly focused on infancy and childhood. More recently, it has acquired additional focus on development across the entire "life span" from birth to death, resulting in the development of new specialties in adolescence, middle age, and especially old age.

Phylogenetic View of DevelopmentThe phylogenetic point of view is concerned with the development of mind across evolutionary time, and the question of mind in nonhuman animals. It includes comparative psychology, which (as its name implies) is concerning with studying learning and other cognitive abilities in different species (comparative psychology is sometimes known as cognitive ethology); and evolutionary psychology (an offshoot of sociobiology) which traces how human mental and behavioral functions evolved through natural selection and similar processes.Go to a discussion of the phylogenetic view.

              View of DevelopmentThe cultural point of view is concerned with the effects of cultural diversity within the human species. This approach has its origins in 19th-century European colonialism, which asked questions, deemed essentially racist today, about whether the "primitive" mind (meaning, of course, the minds of the colonized peoples of Africa, Asia, and elsewhere) was lacking the characteristics of the "advanced" mind (meaning, of course, the minds of the European colonizers).  Edward Burnet Tylor, generally considered to be the founding father of anthropology (he offered the first definition of "culture" in its modern sense of a body of knowledge, customs and values acquired by individuals from their native environments), distinguished between those cultures which more or less "civilized".  Stripped of the racism, the sub-field of anthropology known as anthropological psychology is the intellectual heir of this work. More recently, anthropological psychology has been replaced by cultural psychology, which studies the impact of cultural differences on the individual's mental life, without the implication that one culture is more "developed" than another.Go to a discussion of the cultural view.

The Ontogenesis of Personhood

Phylogenesis has to do with the development of the species as a whole. Ontogenesis has to do with the development of the individual species member. In contrast to comparative psychology, which makes its comparisons across species, developmental psychology makes its comparisons across different epochs of the life span.

Mostly, developmental psychology focuses its interests on infancy and childhood -- a natural choice, given the idea that mental development is correlated with physical maturation. At its core, developmental psychology is dominated by the opposition of nature vs. nurture, or nativism vs. empiricism:

  • Is the newborn child a "blank slate", all of whose knowledge and skills are acquired through learning experiences?
  • Or are some aspects of mental functioning innate, part of the child's genetic endowment, acquired through the course of evolution?

Nature and Nurture

The dichotomy between "nature" and "nurture" was first proposed in those terms by Sir Francis Galton (1822-1911), a cousin of Charles Darwin's who -- among other things -- expanded Darwin's theory of evolution by natural selection into a political program for eugenics -- the idea of strengthening the human species by artificial selection for, and against, certain traits.

Galton took the terms nature and nurture from Shakespeare's play The Tempest, in which Prospero described Caliban as:

A devil, a born devil, on whose nature Nurture can never stick.

(Here, from an early 20th-century English theatrical advertisement, is Sir Herbert Beerbohm Tree as Caliban, as rendered by Charles A. Bushel.)

Galton wrote that the formula of nature versus nurture was"a convenient jingle of words, for it separates under two distinct heads the innumerable elements of which personality is composed".

Galton's focus on nature, and biological determinism, was countered by Franz Boas (1858-1942), a pioneering anthropologist who sought to demonstrate, in his work and that of his students (who included Edward Sapir, Margaret Mead, Claude Levi-Strauss, Ruth Benedict, and Zora Neale Hurston),

the power of culture in shaping lives. It was nature versus nurture with the scales reset: against our sealed-off genes, there was our accumulation of collective knowledge; in place of inherited learning, there was the social transmission of that knowledge from generation to generation. "Culture" was experience raised to scientific status. And it combined with biology to create mankind ("The Measure of America" by Claudia Roth Pierpont,New Yorker, 03/08/04).

It is to Boas that we owe the maxim that variations within cultural groups are larger than variations between them.

Interestingly, Boas initially trained as a physicist and geographer, and did his dissertation in psychophysics, on the perception of color in water (don't ask).  But during a period of military service, he published a number of other papers on psychophysics, some of which anticipated the insights of signal detection theory, discussed in the lectures on Sensation.  From his dissertation, he concluded that even something as simple as the threshold for sensation depended on the expectations and prior experiences of the perceiver: the underlying sensory processes were not innate, and they were not universal.  They are in some sense acquired through learning. 

For a virtual library of Galton's works, see

For a sketch of Boas' life and work, see "The Measure of America" by Claudia Roth Pierpont,New Yorker, 03/08/04 (from which the quotes above are taken). 

For more detail on Boas and his circle, see Gods of the Upper Air (2919) by Charles King.  According to King, Boas and other early cultural anthropologists "moved the explanation for human differences from biology to culture, from nature to nurture".  In so doing, they were "on the front lines of the greatest moral battle of our time: the struggle to prove that -- despite differences of skin color, gender, ability, or custom -- humanity is one undivided thing".  As Ella Deloria, another of Boas's students (and sister of Vine Deloria, Jr., the Native American activist and author of Custer Died for Your Sins: An Indian Manifesto, published in 1969), recorded in her notes on his lectures, "Cultures are many; man is one".  King says that his book "is about women and men who found themselves on the front lines of the greatest moral battle of our time: the struggle to prove that -- despite differences of skin color, gender, ability or custom -- humanity is one undivided thing." 

  • Louis Menand, reviewing King's book in the New Yorker ("The Looking Glass, 08/26/201), notes that Boas and his group shifted the meaning of culture from "intellectual achievement" to "way of life".  This is something of a paradox, because cultural anthropologists are often portrayed by critics such as Allan Bloom (in The Closing of the American Mind, 1987) as cultural relativists.  It's OK, in this stereotype, for some aboriginal people to eat each other -- after all, it's their culture; and maybe, since they do it, we should try it too.  But as King and Menand point out, Boas and his students weren't cultural relativists.  They were interested in describing different cultures from within -- in recovering cultural diversity that was, even in the 1930s and 40s, being lost to the homogenizing effects of Westernization.  They were also interested in holding up other cultures a mirror through which they could understand their own.  As Menand points out,"The idea... is that we can't see our way of life from the inside, just as we can't see our own faces.  The culture of the "other" serves as a looking glass....  These books about pre-modern peoples are really books about life in the modern West....  Other species are programmed to "know" how to cope with the world, but our biological endowment evolved to allow us to choose how to respond to our environment.  We can't rely on our instincts; we need an instruction manual.  And culture is the manual."
  • Jennifer Wilson, reviewing Gods of the Upper Air in The Nation (05/18/2020), notes that "At a time when the country's foremost social scientists... were insisting that different cultures fell along a continuum of evolution, cultural anthropologists [like Boas and his circle] asserted that such a continuum did not exist.  Instead of evolving in a linear fashion from savagery to civilization, they argued, cultures were in a constant process of borrowing and interpolation.  Boas called this process "cultural diffusion", and it would come to be the bedrock of cultural anthropology...". 

The opposition between "nature" and "nurture" seems to imply that the "nurture" part refers to parental influence: how children are brought up.  As we'll see, parental influence appears to be a lot less important than we might think, compared to influences outside the home.  But more important,"nurture" is probably better construed as experience, as opposed to whatever is built into us by our genes.  Those "experiences" begin in the womb, and continue through old age to death; some experiences happen to us, some we arrange for ourselves, but whatever their origins they make us the people we are at each and every point in the life cycle.  This is the point of Unique: The New Science of Human Individuality (2020) by David J Linden a neuroscientist at Johns Hopkins University.  Being a neuroscientist, Linden focuses on how nature and nurture, meaning genes and internal, physiological changes instigated by events in the external environment, interact to produce individual differences.  But his essential point goes much beyond the genes and soup of neurogenetics and epigenetics.  Reviewing the book in the New York Times ("Beyond Nature and Nurture, What Makes Us Ourselves?", 11/01/2020), Robin Marantz Henig quotes Linden:

In ordinary English, "nurture" means how your parents raise you, he writes.  "But, of course, that's only one small part of the nonhereditary determination of traits."  He much prefers the word "experience", which encompasses a broad range of factors, beginning in the womb an carrying through every memory, every meal, every scent, every romantic encounter, evey illness from before birth to the moment of death.  He admits that the phrase he prefers to "nature versus nurture" doesn't roll as "trippingly off the tongue", but he offers it as a better summary of how our individuality really emerges: through "heredity interacting with experience, filtered through the inherent randomness of development". 

Still, a formula such as "genes and experience" doesn't come nearly as "trippingly off the tongue" as "nature and nurture", so stick with the latter.  Just understand what "nurture" really means.  And be sure to include the "and": it's not "nature versus nurture", or "nature or nurture".  As we'll see, nature and nurture work together in the development of the individual.

Of course, neither physical nor mental development stops at puberty. More recently, developmental psychology has acquired an additional focus on development across the life span, including adolescence and adulthood, with a special interest in the elderly.

The Human Genome

The Human
              GenomeThe human genetic endowment consists of 23 pairs of chromosomes. Each chromosome contains a large number of genes. Genes, in turn, are composed of deoxyribonucleic acid (DNA), itself composed of a sequence of four chemical bases: adenine, guanine, thymine, and cytosine (the letters A, G, T, and C which you see in graphical descriptions of various genes). Every gene is located at a particular place on a specific chromosome. And since chromosomes come in pairs, so do genes.

For a nice historical survey of genetics, see The Gene: An Intimate History by Siddhartha Mukherjee (2016).

Corresponding pairs of genes, contain information about some characteristic, such as eye color, skin pigmentation, etc. While some traits are determined by single pairs of genes, others, such as height, are determined by several genes acting together. In either case, genes come in two basic categories.Dominant genes (indicated by upper-case letters) exert an effect on some trait regardless of the other member. For example, in general the genes for brown eyes, dark hair, thick lips, and dimples are dominant genes.Recessive genes (indicated by lower-case letters) affect a trait only if the other member is identical. For example, in general the genes for blue eyes, baldness, red hair, and straight noses are recessive. The entire set of genes comprises the organism's genotype, or genetic blueprint.

One of the most important technical successes in biological research was the decoding of the human genome -- determining the precise sequence of As, Gs, Ts, and Cs that make us, as a species, different from all other species. And along with advances in gene mapping, it has been possible to determine specific genes -- or, more accurately, specific alleles, or mutations of specific genes, known as single-nucleotide polymorphisms, or SNPs -- that put us at risk for various diseases, and which dispose us to various personality traits. The most popular method for this purpose is a genome-wide association study (GWAS).  In what is essentially a multiple-regression analysis, employing very large samples, GWAS examines the relationship between every allele and some characteristic of interest -- say, heart disease or intelligence.  Of course, a lot of these correlations will occur just by chance, but there are statistical corrections for that.  

The Human Genome, Take 2Some sense of this situation is given by the "genetic" autobiography (A Life Decoded, 2007) published by Craig Venter, the "loser" (in 2001, to a consortium government-financed academic medical centers) in the race to decode the human genome -- though, it must be said, the description of the genome produced by Venter's group is arguably superior to that produced by the government-financed researchers. Anyway, Venter has organized his autobiography around his own genome (which is what was sequenced by his group in the race), which revealed a number of such genes, including:

  • on Chromosome 1, the gene TNFSF4, linked to heart attacks;
  • on Chromosome 4, the CLOCK gene, related to an evening preference (i.e., a "night person" as opposed to a "day person");
  • on Chromosome 8, the gene CHRNA8, linked to tobacco addiction;
  • on Chromosome 9, DRD4, linked to novelty-seeking (Venter is an inveterate surfer);
  • on Chromosome 18, the gene APOE, linked to Alzheimer's disease; and
  • on the X Chromosome, the MAOA gene (about which more later, in the lectures on Personality and Psychopathology), which is linked to antisocial behavior and conduct disorder (among genetics researchers, Venter has a reputation as something of a "bad boy").

None of these genes means that Venter is predestined for a heart attack or Alzheimer's disease, any more than he was genetically predestined to be a surfer. It's just that these genes are more common in people who have these problems than in those who do not. They're risk factors, but not an irrevocable sentence to heart disease or dementia.

Steven Pinker, a distinguished cognitive psychologists and vigorous proponent of evolutionary psychology (see below) went through much the same exercise in "My Genomic Self", an article in the New York Times Magazine (01/11/2009)While allowing that the DNA analyses of personal genomics will tell us a great deal about our health, and perhaps about our personalities, they won't tell us much about our personal identities.  Pinker refers to personal genomics, of the sort sold on the market by firms like 23andMe, as mostly "recreational genomics", which might be fun (he declined to learn his genetic risk for Alzheimer's Disease), but doesn't yet, and maybe can't, tell us much about ourselves that we don't already know.  He writes:

Even when the effect of some gene is indubitable, the sheer complexity of the self will mean that it will not serve as an oracle on what the person will do....   Even if personal genomics someday delivers a detailed printout of psychological traits, it will probably not change everything, or even most things. It will give us deeper insight about the biological causes of individuality, and it may narrow the guesswork in assessing individual cases. But the issues about self and society that it brings into focus have always been with us. We have always known that people are liable, to varying degrees, to antisocial temptations and weakness of the will. We have always known that people should be encouraged to develop the parts of themselves that they can (“a man’s reach should exceed his grasp”) but that it’s foolish to expect that anyone can accomplish anything (“a man has got to know his limitations”). And we know that holding people responsible for their behavior will make it more likely that they behave responsibly. “My genes made me do it” is no better an excuse than “We’re depraved on account of we’re deprived.”

A prominent GWAS by Daniel Benjamin, a behavioral economist, and his colleagues studied 1.1 million people and identified 1,271 single nucleotide proteins (SNPs, or genetic variants) which were, taken individually, significantly associated with educational attainment measured in years of schooling (Lee et al., Nature Genetics, 2018).  Taken together, these variants accounted for about 11-13% of the variance in educational attainment.  However, in order to maximize the likelihood of revealing genetic correlates (essentially, by reducing environmental "noise"), the study was confined to white people of European descent.  When the same analysis was repeated in a sample of African-Americans, the same genetic variants accounted for less than 2% of the variance in educational attainment.  None of these SNPs should be thought of as dedicated to educational achievement, especially in populations other than those in which the GWAS was conducted in the first place.  In the first place, formal schooling arose too recently to be subject to genetic selection.  In fact, the actual function of these SNPs is unknown, and it is likely that many of them control traits whose association with educational attainment is rather indirect.  So (to take an example from a story in The Economist, 03/31/2018), children who inherit weak bladders from their parents may do poorly on timed examinations.  In any event, the fact that there are SNPs associated with educational attainment, or any other psychosocial characteristic, doesn't mean that these traits are directly heritable -- or that focusing on genes is the best way to understand it, much less improve it.

Genetic Sequencing and Genetic Selection

Since the human genome was sequenced, and as the costs of gene sequencing have gone down, quite an industry has developed around identifying "genes for" particular traits, and then offering to sequence the genomes of ordinary people -- not just the Craig Venters of the world -- to determine whether they possess any of these genes. 

The applications in the case of genetic predispositions to disease are obvious -- though, it's not at all clear that people want to know whether they have a genetic predisposition to a disease that may not be curable or preventable.

  • Research has identified a genetic mutation for Huntington's disease, a presently incurable brain disorder, on Chromosome 4, but many people with a family history of Huntington's disease do not want to be told whether they have it (the folk singer Arlo Guthrie is a famous example).
  • Two genes, known as BRCA1 and BRCA2, substantially increase a woman's risk for breast cancer.  Some women with these genes, who also have a family history of breast cancer, have opted for radical preventive treatments such as double mastectomy.

On the other hand, there are some uses of genetic testing that are not necessarily so beneficial.

  • In certain cultures which value male children more highly than female children, genetic testing for biological sex may lead some pregnant women to abort female fetuses, leading to a gender imbalance.  For example, even without genetic testing, both China and India have a clear problem with "missing women" commonly attributed to sex-selective abortion. 

And there are other kinds of genetic testing which also may not be to society's -- or the individual's -- advantage.

  • A variant of the ACTN3 gene on chromosome 11 is present in a high proportion of athletes who compete at elite levels in "speed" and "power" spots (as opposed to "endurance" sports.  Almost as soon as this finding was announced, a commercial enterprise offered a genetic test which, for $149, would indicate whether a child had such a gene ("Born to Run? Little Ones Get Test for Sports Gene" by Juliet Macur, New York Times, 11/30/2010).  The danger is that children found to have the gene will be tracked into athletics, when they'd rather be librarians; and other children will be discouraged from sports, even though they'd be able to have careers as elite athletes.

The point is that genes aren't destiny, except maybe through the self-fulfilling prophecy.  So maybe we shouldn't behave as though they were.

From Genotype to Phenotype

Of course, genes don't act in isolation to determine various heritable traits. One's genetic endowment interacts with environmental factors to produce a phenotype, or what the organism actually looks like. The genotype represents the individual's biological potential, which is actualized within a particular environmental context. The environment can be further classified as prenatal (the environment in the womb during gestation), perinatal (the environment around the time of birth), and postnatal (the environment present after birth, and throughout the life course until death).

Because of the role of the environment, phenotypes are not necessarily equivalent to genotypes. For example, two individuals may have the same phenotype but different genotypes. Thus, of two brown-eyed individuals, one might have two dominant genes for brown eyes (BB), while another might have one dominant gene for brown eyes and one recessive gene for blue eyes (Bb). Similarly, two individuals may have the same genotype, but different phenotypes. For example, two individuals may have the same dominant genes for dimples, (DD), but one has his or her dimples removed by plastic surgery.

The chromosomes are found in the nucleus of each cell in the human body, except the sperm cells of the male and the egg cells of the female, which contain only one element of each pair. At fertilization, each element contributed by the male pairs up with the corresponding element contributed by the female to form a single cell, or zygote. At this point, cell division begins. The first few divisions form a larger structure, the blastocyst. After six days, the zygote is implanted in the uterus, at which point we speak of an embryo.

Personality, Behavior, and the Human Genome Project

Evidence of a genetic contribution to individual differences (see below), coupled with the announcement of the decoding of the human genome in 2001, has led some behavior geneticists to suggest that we will soon be able to identify the genetic sources of how we think, feel, and behave. However, there are reasons for thinking that behavior genetics will not solve the problem of the origins of mind and behavior.

  • In the first place, there's no inheritance of acquired characteristics.  So while there might be a genetic contribution to how we think, there can't be a genetic contribution to what we think.
  • In any event, there aren't enough genes. Before 2001, it was commonly estimated that there were approximately 100,000 genes in the human genome. The reasoning was that humans are so complex, at the very apex of animal evolution, that we ought to have correspondingly many genes. However, when genetic scientists announced the provisional decoding of the human genome in 2001, they were surprised to find that the human genome contains only about 30,000 genes -- perhaps as few as 15,000 genes, perhaps as many as 45,000, but still far too few to account for much important variance in human experience, thought, and action.(As of 2004, there were still a couple of gaps in our knowledge of the human genetic code, and by the time these are closed, the number of human genes may well have fallen again!)
  • The situation got worse in 2004, when a revised analysis (reported in Nature) lowered the number of human genes to 20-25,000.
    • By comparison, the spotted green pufferfish also has about 20-25,000 genes.
    • On the lower end of the spectrum, the worm c. elegans has about 20,000 genes, while the fruit fly has 14,000.
    • On the higher end of the scale, it was reported in 2002 that the genome for rice may have more than 50,000 genes (specifically, the japonica strain may have 50,000 genes, while the indica variety may have 55,600 genes).  It's been estimated that humans have about 24% of our genes in common with the sativa variety of rice. It's also been reported that 26 different varieties of maize (corn) average more than 103,000genes.
If it takes 50,000 genes to make a crummy grain of rice, and humans have only about 23,000 genes, then there's something else going on. For example, through alternative splicing, a single gene can produce several different kinds of proteins -- and it seems to be the case that alternative splicing is found more frequently in the human genome than in that of other animals. Alternatively, the human genome seems to contain more sophisticated regulatory genes, which control the workings of other genes.
  • Before anyone makes large claims about the genetic underpinnings of human thought and action, we're going to need a much better account of how genes actually work.
  • Of course, it's not just the sheer number of genes, but the genetic code -- the sequence of bases A, G, T, and C -- that's really important. Interestingly in 2002 the mouse genome was decoded, revealing about 300,000 genes. Moreover, it turned about that about 99% of these genes were common with the human genome (the corresponding figure is about 99.5% in common for humans and our closest primate relative, the chimpanzee). This this extremely high degree of genetic similarity makes sense from an evolutionary point of view: after all, mice, chimpanzees, and humans are descended from a common mammalian ancestor who lived about 75 million years ago (ya). And it also means that the mouse is an excellent model for biological research geared to understanding, treating, and preventing human disease, it is problematic for those who argue that genes are important determinants of human behavior. That leaves 300 genes to make the difference between mouse and human, and only about 150 genes to make the difference between chimpanzees and humans.Not enough genes.
  • Perhaps the action is not in the genes, but rather in their constituent proteins:indica rice has about 466 million base pairs, and japonica about 420 million, compared to 3 billion for humans (and about 2.5 billion for mice).

Why are there 3 billion base pairs, but only about 23,000 genes?  It turns out that most DNA is what is known as junk DNA -- that is, DNA that doesn't create proteins.  When the human genome was finally sequenced in 2001, researchers came to the conclusion that fully 98% of the human genome was "junk" in this sense.  But it's not actually "junk": "junk" DNA performs some functions.   

  • Some of it forms telomeres, structures that "tie off" the ends of genes, like the aglets or caps that prevent ropes (or shoelaces) from fraying at their ends.  Interestingly, telomeres get shorter as a person ages.  And even more interestingly, from a psychological point of view, telomeres are also shortened by exposure to stress (see the work of UC San Francisco's Elissa Epel).  So, stress does really seem to cause premature aging!
  • Junk DNA also codes for ribonucleic acid, or RNA.   
  • And junk DNA also appears to influence gene expression, which may explain some of the differences (for example, in susceptibility to genetically inherited disease) between genetically identical twins.

The paucity of human genes, and the vast amount of "junk DNA" in the genome, has led some researchers to concluded that the source of genetic action may not lie so much in the genes themselves, but in this other material instead. Maybe.  Or maybe DNA just isn't that important for behavior in complex, sophisticated organisms like humans.  Maybe culture is more important.  Something to think about.

For an extended discussion of junk DNA and its functions, see Junk DNA: A Journey Through the Dark Matter of the Genome (2015) by Nessa Carey.  Or excerpts printed in Natural History magazine, March, April, and May 2015).  Carey also wrote The Epigenetics Revolution: How Modern Biology is Rewriting Our Understanding of Genetics, Disease, and Inheritance (2012), also excerpted in Natural History, April and May 2012).

In any event, for all the talk about the genetic underpinnings of individual differences, as things stand now and behavior geneticists don't have the foggiest idea what they are.

"In the god-drenched eras of the past there was a tendency to attribute a variety of everyday phenomena to divine intervention, and each deity in a vast pantheon was charged with responsibility for a specific activity -- war, drunkenness, lust, and so on. 'How silly and primitive that all was,' the writer Louis Menand has observed. In our own period what Menand discerns as a secular 'new polytheism' is based on genes -- the alcoholism gene, the laziness gene, the schizophrenia gene.

Now we explain things by reference to an abbreviated SLC6A4 gene on chromosome 17q12, and feel much superior for it. But there is not, if you think about it, that much difference between saying 'The gods are angry' and saying 'He has the gene for anger.' Both are ways of attributing a matter of personal agency to some fateful and mysterious impersonal power."

---Cullen Murphy in "The Path of Brighteousness" (Atlantic Monthly, 11/03)

Embryological Development

What are the mechanisms of embryological development? At one point, it was thought that the individual possesses adult form from the very beginning -- that is, that the embryo is a kind of homunculus (little man), and that the embryo simply grew in size. This view of development is obviously incorrect. But that didn't stop people from seeing adult forms in embryos!

In the 19th century, with the adoption of the theory of evolution, the homunculus view was gradually replaced by the recapitulation view, based on Haeckel's biogenetic law that

"Ontogeny recapitulates phylogeny".

What Haeckel meant was that the development of the individual replicates the stages of evolution: that the juvenile stage of human development repeats the adult stages of our evolutionary ancestors. Thus, it was thought, the human embryo first looks like an adult fish; later, it looks like adult amphibians, reptiles, birds, mammals, and nonhuman primates. This view of development is also incorrect, but it took a long time for people to figure this out.

The current view of development is based on von Baer's principle of differentiation. According to this rule, development proceeds from the general to the specific. In the early stages of its development, every organism is homogeneous and coarsely structured. But it carries the potential for later structure. In later stages of development, the organism is heterogeneous and finely built -- it more closely represents actualized potential. Thus, the human embryo doesn't look like an adult fish. But at some point, human and fish embryos look very much alike. These common structures later differentiate into fish and humans. A good example of the differentiation principle is the development of the human reproductive anatomy, which we'll discuss later in the course.

Ontogeny and Phylogeny

For an engaging history of the debate between recapitulation and differentiation views of development, see Ontogeny and Phylogeny (1977) by S.J. Gould.

Neural Development in the Fetus, Infant, and Child

At the end of the second week of gestation, the embryo is characterized by a primitive streak which will develop into the spinal cord. By the end of the fourth week, somites develop, which will become the vertebrae surrounding the spinal cord -- the characteristic that differentiates vertebrates from invertebrates.

Bodily asymmetries seem to have their origins in events occurring at an early stage of embryological development. Studies of mouse embryos by Shigenori Nonaka and his colleagues, published in 2002, implicate a structure known as the node, which contains a number of cilia, or hairlike structures. The motion of the cilia induce fluids to move over the embryo from right to left. These fluids contain hormones and other chemicals that control development, and thus cause the heart to grow on the left, and the liver and appendix on the right -- at least for 99.99% of people. On rare occasions, a condition known as situs inversus, the position of the embryo is reversed, so the fluids flow left to right, resulting in a reversal of the relative positions of the internal organs -- heart on the right, liver and appendix on the left. It is possible that this process is responsible for including the hemispheric asymmetries associated with cerebral lateralization -- although something else must also be involved, given that the incidence of right-handedness is far greater than 0.01%.

  • In the second month, the eye buds move to the front of the head, and the limbs, fingers, and toes become defined. The internal organs also begin to develop, including the four-chambered heart -- the first characteristic that differentiates mammals from non-mammals among the vertebrates. It's at this point that the embryo changes status, and is called a fetus.
  • The development of the nervous system begins in the primitive streak of the embryo, which gradually forms an open neural tube. The neural tube closes after 22 days, and brain development begins.
  • In the 11th week of gestation the cerebral cortex becomes clearly visible. The cortex continues to grow, forming the folds and fissures that permit a very large brain mass to fit into a relatively small brain case.
  • In the 21st week of gestation synapses begin to form. Synaptic transmission is the mechanism by which the nervous system operates: there is no electrical activity without synapses. So, before this time the fetal brain has not really been functioning.
  • In the 24th week myelinization begins. The myelin sheath provides a kind of insulation on the axons of neurons, and regulates the speed at which the neural impulse travels down the axon from the cell body to the terminal fibers.

All three processes -- cortical development, synaptic development, and myelinization -- continue for the rest of fetal development, and even after birth. In fact, myelinization is not complete until late in childhood.

  • As far as the cerebral cortex is concerned, we're probably born with all the neurons we're going to get. The major change postnatally is in the number, or the density, of interconnections among neurons -- a process called synaptogenesis (as opposed to neurogenesis), by which the axons of presynaptic neurons increasingly link to the dendrites of postsynaptic neurons. Viewed at the neuronal level, the big effect of normal development is the proliferation of neural interconnections, including an increase in dendritic arborization (like a tree sprouting branches) and the extension of axons (so as to make contact with more dendrites).
  • At the same time, but at a different rate, there is also some pruning, or elimination, of synapses -- a process by which neural connectivity is fine-tuned. So, for example, early in development there is considerable overlap in the projections of neurons from the two eyes into the primary visual cortex; but after pruning, the two eyes project largely to two quite different segments of cortex, known as "ocular dominance columns". This pruning can continue well into childhood and adolescence. 
  • Neurons die. Fortunately, we are born with a lot of neurons, and unless neuronal death is accelerated by brain damage or something like Alzheimer's disease, neurons die relatively slowly. When neurons die, their connections obviously disappear with them.
  • The connections between neurons can also be strengthened (or weakened) by learning. Think of long-term potentiation. But LTP doesn't change the number of neural interconnections. What changes is the likelihood of synaptic transmission across a synapse that has already been established.
  • LTP is an example of a broader phenomenon called functional plasticity.
    • Violinists, who finger the strings with their left hands, show much larger cortical area in that portion of (right) parietal cortex that controls finger movements of the left hand, compared to non-musicians.
    • If one finger of a hand is amputated, that portion of somatosensory cortex which would ordinarily receive input from that finger obviously doesn't do so any longer. But what can happen is that the somatosensory cortex can reorganize itself, so that this portion of the brain can now receive stimulation from fingers that are adjacent to the amputated one.
    • And if two fingers are sewn together, so that when one moves the other one does also, the areas of somatosensory cortex that would be devoted to each finger will now overlap.
  • In each case, though, the physical connections between neurons -- the number of terminal fibers synapsing on dendrites -- don't appear to change. Much as with LTP, what changes is the likelihood of synaptic transmission across synapses that have already been established.
  • Finally, there's the problem of neurogenesis.  Traditional neuroscientific doctrine has held that new neurons can regenerate in the peripheral nervous system (as, for example, when a severed limb has been reattached), but not in the central nervous system (as, for example, paraplegia following spinal cord injury).  However, increasing evidence has been obtained for neurogenesis in the central nervous system as well.  This research is highly controversial (though I, personally, am prepared to believe it is true), but if confirmed would provide the basis for experimental "stem cell" therapies for spinal-cord injuries (think of Christopher Reeve).  If naturally occurring neurogenesis occurs at a rate greater than the rate of natural neuronal death, and if these new neurons could actually be integrated into pre-existing neural networks, that would supply yet an additional mechanism for a net increase in new physical interconnections between neurons -- but so far the evidence in both respects is ambiguous.

Studies of premature infants indicate that an EEG signal can be recorded at about 25 weeks of gestation. At this point, there is evidence of the first organized electrical activity in the brain. Thus, somewhere between the 6th and 8th month of gestation the fetal brain becomes recognizably human. There are lots of the folds and fissures that characterize the human cerebral cortex. And there is some evidence for hemispheric specialization: premature infants respond more to speech presented to the left hemisphere, and more to music presented to the right. At this point, in the 3rd semester of gestation, the brain clearly differentiates humans from non-humans.

Interestingly, survivability takes a big jump at this point as well. If born before about 24 weeks of gestation, the infant has little chance of survival, and then only with artificial life supports; if born after 26 weeks, the chances of survival are very good. If born at this point, the human neonate clearly has human physical characteristics, and human mental capacities. In other words, by some accounts, by this point the fetus arguably has personhood, because its has actualized its potential to become human. At this point, it makes sense to begin to talk about personality -- how the person actualizes his or her potential for individuality. 

What are the implications of fetal neural development for the mind and behavior of the fetus?  Here are some things we know. 

First, the fetus begins to move in the uterus as early as seven weeks of gestation.  While some of this is random, other movements seem to be coordinated.  For example, the fetus will stick its hands and feet in its mouth, but it will also open its mouth before bringing the limb toward it. 
  • Taste buds form on the fetal tongue by the 15th week, and olfactory cells by the 24th week.  Newborns prefer flavors and odors that whey were exposed to in the womb, which suggests that some sensory-perceptual learning is possible by then. 
  • By about the 24th-27th week, fetuses can pick up on auditory stimulation.  Again, newborns respond to sounds and rhythms, including individual syllables and words, that they were exposed to in utero -- another example of fetal learning. 
  • The fetus's eyes open about the 28th week of gestation, but there is no evidence that it "sees" anything -- it's pretty dark in there, although some light does filter through the mother's abdominal wall, influencing the development of neurons and blood vessels in the eye.  

The importance of early experience in neural development cannot be overemphasized.  Socioeconomic status, nurturance at age 4 (as opposed to neglect, even if benign), the number of words spoken to the infant, and other environmental factors all are correlated with various measures of brain development.

The Facts of Life

Much of this information has been drawn from The Facts of Life: Science and the Abortion Controversy (1992) by H.J. Morowitz and J.S. Trefil. Advances in medical technology may make it possible for fetuses to live outside the womb even at a very early stage of gestation, but no advance in medical technology will change the basic course of fetal development, as outlined here and presented in greater detail in the book. See also Ourselves Unborn: A History of the Fetus in Modern America by Sara Dubow (2010).

Nature and Nurture in Personality Development

So where does personality come from? We have already seen part of the answer: Personality is not a given, fixed once and for all time, whether by the genes or through early experience. Rather, personality emerges out of the interaction between the person and the environment, and is continuously constructed and reconstructed through social interaction. A major theme of this interactionist view of personality is that the person is a part of his or her own environment, shaping the environment through evocation, selection, behavioral manipulation, and cognitive transformation. In the same way, development is not just something that happens to the individual. Instead, the individual is an active force in his or her own development.

The Developmental Corollary to the Doctrine of Interactionism

Just as the person is a part of his or her own environment, the child is an agent of his or her own development.

              Human GenomeThe development of the individual begins with his or her genetic endowment, but genes do not act in isolation. The organism's genotype, or biological potential (sometimes referred to as the individual's "genetic blueprint", interacts with the environment to produce the organism's phenotype, or what the organism actually "looks like" -- psychologically as well as morphologically.

The individual's phenotype is his or her genotype actualized within a particular environmental context:

  • Two individuals can have different genotypes but the same phenotypes. For example, given two brown-eyed individuals, one person might have two dominant genes for brown eyes (blue eyes are recessive), while the other might have one dominant gene for brown eyes, and one recessive gene for blue eyes.
  • Two individuals can have the same genotype but different phenotypes. For example, of two individuals might both possess two dominant genes for dimples, one individual might have cosmetic surgery to remove them, but the other might not.

More broadly, it is now known that genes are turned "on" and "off" by environmental events.

  • Sometimes, the "environment" is body tissue immediately surrounding the gene. Except for sperm and egg cells, every cell in the body contains the same genes. But the gene that controls the production of insulin only does so when its surrounding cell is located in the pancreas. The same gene, in a cell located in the heart, doesn't produce insulin (though it may well do something else). This fact is the basis of gene therapy: a gene artificially inserted into one part of the body will produce a specific set of proteins that may well repair some deficiency, while the same gene inserted into another part of the body may not have any effect at all.
  • Sometimes, the "environment" is the world outside the organism. A gene known as BDNF (brain-derived neurotrophic factor), plays an important role in the development of the visual cortex, is "turned on" by neural signals resulting from exposure to light. Infant mice exposed to light develop normal visual function, but genetically identical mice raised in darkness are blind.

Nature via Nurture?

For some genetic biologists, facts like these resolve the nature-nurture debate: because genes respond to experience, "nature" exerts its effects via "nurture". This is the argument of Nature via Nurture: Genes, Experience, and What Makes Us Human (2003) by Matt Ridley. As Ridley writes:

Genes are not puppet masters or blueprints. Nor are they just the carriers of heredity. They are active during life; they switch on and off; they respond to the environment. They may direct the construction of the body and brain in the womb, but then they set about dismantling and rebuilding what they have made almost at once -- in response to experience (quoted by H. Allen Orr, "What's Not in Your Genes",New York Review of Books, 08/14/03).

The point is correct so far as it goes: genes don't act in isolation, but rather in interaction with the environment -- whether that is the environment of the pancreas or the environment of a lighted room.

But this doesn't solve the nature-nurture debate from a psychological point of view, because psychologists are not particularly interested in the physical environment. Or, put more precisely, we are interested in the effects of the physical environment, but we are even more interested in the organism's mental representation of the environment -- the meaning that we give to environmental events by virtue of such cognitive processes as perception, thought, and language.

  • When Ridley talks about experience, he generally means the physical features of an environmental event.
  • When psychologists talk about experience, they generally mean the semantic features of an environmental event -- how that event fits into a pattern of beliefs, and arouses feelings and goals.
As Orr notes, Ridley's resolution of the nature-nurture argument entails a thoroughgoing reductionism, in which the "environment" is reduced to physical events (such as the presence of light) interacting with physical entities (such as the BDNF gene). But psychology cannot remain psychology and participate in such a reductionist enterprise, because its preferred mode of explanation is at the level of the individual's mental state.

For a psychologist, "nurture" means the meaning of an organism's experiences. And for a psychologist, the nature-nurture argument has to be resolved in a manner that preserves meaning intact.

The point is that genes act jointly with environments to produce phenotypes. These environments fall into three broad categories:

  • prenatal, meaning the intrauterine environment of the fetus during gestation;
  • perinatal, referring to environmental conditions surrounding the time of birth, including events occurring during labor and immediately after parturition; and
  • postnatal, including everything that occurs after birth, throughout the course of the individual's life.

In light of the relation between genotype and phenotype, the question about "nature or nurture" is not whether some physical, mental, or behavioral trait is inherited or acquired. Better questions are:

  • What is the relative importance of nature and nurture?
  • Or even better, How do nature and nurture interact?

The Twin-Study Method

              Genetics: Twin StudiesMany basic questions of nature and nurture in personality can be addressed by using the techniques of behavior-genetics are used to analyze the origins of psychological characteristics. Perhaps the most interesting outcome of these experiments is that, while initially intended to shed light on the role of genetic factors in personality development. These behavior-genetic analyses show the clear role of genetic determinants in personality, but also reveal a clear role for the environment.

The most popular method in behavior genetics is the twin study -- which, as its name implies, compares two kinds of twins in terms of similarity in personality:

  • Monozygotic (MZ or identical) twins are the product of a egg that has been fertilized by a single sperm, but which subsequently split into two embryos -- thus yielding two individuals who are genetically identical.
    • Actually, MZ twins aren't precisely identical. Research by Dumanski and Bruder (Am. J. Hum Gen 2008) indicates that even MZ twins might differ in the number of genes, or in the number of copies of genes. Moreover, failures to repair breaks in genes can occur, resulting in the emergence of further genetic differences over the individuals' lifetimes. This discovery may have consequences for the determination of environmental contributions to variance, detailed below. But for most practical purposes, the formulas discussed below provide a reasonable first approximation.
  • Dizygotic (DZ or fraternal) twins occur when two different eggs are fertilized by two different sperm -- thus yielding individuals who have only about 50% of their genes in common.

A qualification is in order. A vast proportion of the human genome, about 90%, is the same for all human individuals -- it's what makes us human, as opposed to chimpanzees or some other kind of organism. Only about 10% of the human genome actually varies from one individual to another. So, when we speak of DZ twins having "50%" of their genes in common, we really mean that they share 50% of that 10%. And when we speak of unrelated individuals having "no" genes in common, we really mean that they don't share any more of that 10% than we'd expect by chance.

Of course, you could compare triplets, quadruplets, and the like as well, but twins are much more convenient because they occur much more frequently in the population. Regardless of twins or triplets or whatever, the logic of the twin study is simple: To the extent that a trait is inherited, we would expect MZ twins to be more alike on that trait than DZ twins.

Born in Ontario in 1934, the Dionne Quintuplets (all girls, all genetically identical) were the first quintuplets to live past infancy.  Their parents already had four children, and the birth of five more was completely unexpected.  The family's financial straits led their parents to sign over custody of the girls to the Canadian Red Cross, which built a special hospital and "observatory" for them across the street from their family home; their parents also agreed to display them at the Chicago World's Fair.  "Quintland" actually became a tourist attraction, complete with bumper stickers.  All of this was in an attempt to protect the children -- and of course it misfired badly, creating, among other things, a sharp division between the quints on the one hand, and their parents and four older siblings on the other.  Their story has been told in many books: the most recent, The Miracle and Tragedy of the Dionne Quintuplets by Sarah Miller (2019), traces their lives.  Spoiler alert: the tragedy is that they were exploited anyway; the miracle is that each girl grew up with her own individual personality and interests.


In 2009, the Crouch quadruplets, Ray, Kenny, Carol, and Martina, all received offers of early admission to Yale ("Boola Boola, Boola Boola: Yale Says Yes, 4 Times" by Jacques Steinberg, New York Times, 12/19/2009).  Now you don't have to be twins to share such outcomes (and the Crouch quadruplets obviously aren't identical quadruplets anyway!).  Consider The 5 Browns, sibling pianists from Utah, all of whom, successively, were admitted to study at Julliard.  How much of these outcomes is due to shared genetic potential?  How much to shared environment?  How much to luck and chance?  That's why we do twin and family studies.

The April 2018 issue of National Geographic magazine, devoted to race, featured Marcia and Millie Biggs, 11-year-old fraternal twin sisters living in England (shown here with their father, Michael; you'll see their photo from the magazine cover later in these lectures).  Their mother, Amanda Wanklin, calls them her "one in a million miracle".  The girls' differences in physical appearance are due entirely to the vicissitudes of genetic chance.  But while Millie is a "girlie" girl, Marcia is more of a "tomboy" (those are Marcia's words, not mine).  Where those psychological differences come from is a much more complex, and much more interesting, story -- as we'll see in what follows.

Twins in space!  Einstein's theory of special relativity predicts that a twin traveling through space at high speed will age less rapidly than his or her earth-bound counterpart.  That hypothesis hasn't been tested yet, because we don't have the ability to put a twin in a spacecraft that travels at close to light-speed.  But still, there are reasons to think that space travel, including exposure to microgravity and ionizing radiation, not to mention the stress and absence of a circadian clock, might have substantial physiological and psychological effects.  Taking advantage of the presence of two identical twins, Scott and Mark Kelly, on the roster of American astronauts, the National Aeronautics and Space Administration (NASA) subjected the pair to a battery of physiological and psychological tests before, during, and after one of them (Scott) completed a 1-year mission aboard the International Space Station (ISS).  The results, reported in 2019 (Garrett-Bakelman et al., Science 04/12/2019) did indeed uncover a number of physiological changes induced by a year in space.  Interestingly, Scott's telomeres -- portions of the chromosome that shorten with age -- actually did lengthen during spaceflight: score one, maybe, for Einstein.  Psychologically, frankly, the study was sort of impoverished, but revealed no significant decrements in cognitive speed, accuracy, or efficiency (speed-accuracy trade-off) inflight, compared to pre-flight; post-flight, however, there was some diminution in performance.

              Similarity in PersonalityAs far as personality goes, the Example Correlation: 60-Point Scale of Extraversion usual technique in twin studies is to administer some personality inventory, like the MMPI or CPI or NEO-PI to a large sample of MZ and DZ twins, thus obtaining scores representing each individual's standing on each personality trait measured by the inventory. Then we measure the similarity of the twins on each trait. The most common measure of similarity is the correlation coefficient, which summarizes the direction and strength of the relationship between two variables -- for example, between extraversion in one twin and extraversion in the other.

  • The correlation is positive if one individual has a high score and his twin does too.
  • The correlation is negative if one individual has a high score and his twin has a low score.
  • Correlations close to +1.0 or =1.0 indicate a strong relationship, positive or negative.
  • Correlations close to 0.0 indicate little or no similarity between the twins.

If we assume that a personality trait (or a physical trait like eye color, for that matter) is solely determined by the genes, and the environment has no effect, we would expect that following pattern of correlations:

  • For MZ twins,r = +1.0: the twins are genetically identical, and thus identical in personality.
  • For DZ twins,r = +0.50: because there is some degree of genetic similarity between the twins, we would expect some degree of similarity in personality as well.
  • For genetically unrelated individuals,r = 0.0: with no genetic overlap, there should be no similarity in personality.

An alternative measure of similarity is the concordance rate. Assuming that a person either has a trait or does not, on the hypothesis of exclusively genetic determination we would expect a concordance rate of 100% for MZ twins, and a concordance rate of about 50% for DZ twins (we will meet up with concordance rates again in the lectures on Personality and Psychopathology, when we discuss the origins of mental illness).

More generally, to the extent that a trait is inherited, we expect that MZ twins will be more similar to each other than DZ twins -- regardless of whether similarity is measured by the correlation coefficient or the concordance rate.

Once in a while, you'll read some extraordinary story about identical twins, separated at birth, who turn out remarkably similar as adults.  For examples, see Born Together -- Reared Apart: The Landmark Minnesota Twin Study by Nancy L. Segal (2012).  For example, one of the pairs of subjects in the Minnesota study were separated at birth, but both were named "Jim" by their adoptive parents, both married women named "Linda", both subsequently divorced, and both later married women named "Betty".  Both were chain-smokers, both drove Chevrolet cars, both were employed as deputy sheriffs, and both preferred to vacation at the same beach in Florida.  These coincidences are fun to read about, but let's be clear: there's no gene for driving Chevrolets, or for marrying women named "Linda".  Genes operate at an entirely different level.

Genes, Environments, and the "Big Five"

              Study: The "Big Five" Personality TraitsIn fact, a twin study of the "Big Five" personality traits by Loehlin and his colleagues (1992) showed that for each dimension of the Big Five, MZ twins were more alike than DZ twins. Studies using other personality inventories, such as the MMPI or the CPI, have yielded similar sorts of findings. Taken together, this body of research provides prima facie evidence for a genetic contribution to individual differences in personality.But genes aren't the only forces determining individual differences in personality. If they were, then:

  • the MZ correlations would be a perfect 1.0 and
  • the DZ correlations would be 0.50.

Physical characteristics like eye color and hair texture may come close to these values. Height and weight show high MZ correlations, but even these aren't perfectly correlated. The reason is that genotype alone is never sufficient to determine phenotype. Phenotypes always result from the interaction of genotypes with environmental factors. The typical MZ correlations for physical traits range upward from 0.50, suggesting high if not perfect heritabilities. By contrast, the typical MZ correlations for psychological traits range between 0.25 and .50 -- suggesting that genetic influences on personality are relatively weak, and environmental influences are correspondingly strong.

Note, however, that heritability can be misleading.  I owe the following example to Eric Turkheimer, a prominent behavior geneticist at the University of Virginia who has studied the heritability of both IQ and major mental illnesses like schizophrenia.  Consider the human trait of having two arms and two legs.  This is without any doubt completely determined by our genetic heritage (as Plato pointed out, we are featherless bipeds).  But if you look at the concordance between MZ and DZ twins for "armedness", you'll find almost perfect concordance: virtually 100% of the siblings of people with two arms also have two arms.  If then you just compare MZ and DZ correlations, you get a heritability of zero (0), or very close to it.  But we know that, barring rare incidents of accident or surgery, the number of arms is completely determined by the genes.

Of course, even perfect MZ correlations of 1.00 wouldn't be enough to clinch the case of exclusive heritability. Twins, especially MZ twins, share more than genes. They also share environments, and it is possible that MZ twins live in environments that are more alike than DZ twins (perhaps because MZ twins are of the same sex, or perhaps simply because they look more alike). This raises the question: How do we tease apart the genetic and environmental contributions to personality?

One way to address this question is to study identical twins who have been separate at birth and reared apart -- meaning that they share genes but not environments. Such cases do exist, and they are interesting, but the fact is that there are not enough of them to make a satisfactory sample. Moreover, many twins ostensibly "reared apart" really aren't. For example, twins might be "separated" for economic reasons, because their parents can't afford to raise both of them, and one of them reared by an aunt and uncle down the road. Even when twins are actually adopted out, adoption agencies often try to place adoptees with foster parents who resemble their biological parents in terms of age, educational levels, occupational status, and the like. Such twins probably share more of their environment than not.

Separated at Birth!

Identical twins separated at birth, and reared independently of each other, have often been taken as providing interesting evidence regarding the role of heredity and environment in personality and behavior. Naturally, a fair amount of interest lies in the similarities among the twins.

The study of identical twins reared apart has a history going back to Sir Cyril Burt's twin studies of intelligence (and the idea goes back to Sir Francis Galton), but got a serious boost from the "The Jim Twins", identical twin boys, born in 1939, and separated shortly after birth. When they were reunited at age 39, they were exactly the same height and weight. No surprise there, such physical properties should be under a high degree of genetic control. But it turned out that, as boys, they both had dogs named Toy, and taken vacations at the same Florida resort; they had both married and divorced women named Linda, and remarried women named Betty; and they had sons named James Alan and James Allan. Both smoked the same brand of cigarette and drank the same brand of beer. They both suffered from headaches, and they both bit their fingernails. Formal testing, by a group at the University of Minnesota, led by Thomas Bouchard (a prominent behavior geneticist) revealed that the two men were highly similar in terms of intelligence and other personality traits.

The Jim Twins started Bouchard and his colleagues on a search for other identical twins who had been raised apart, who were recruited for psychological testing, and confirmed a high degree of similarity. And, frankly, it's no surprise that IQ and other basic personality traits are also under some degree of genetic control. But let's get something straight: there's no gene for marrying women named Linda or Betty. It's important not to exaggerate what are, in fact, mere coincidences.

A more recent case is that of Tamara and Adriana. From the time Tamara Rabi began her undergraduate studies at Hofstra University, in New York, people started telling her that they knew someone else who looked just like her. It turned out that the other woman, Adriana Scott, was Tamara's identical twin sister. The two women had been born in Guadalajara, Mexico, and through a series of bureaucratic snafus, separated at birth and adopted by different American families. Adriana was raised as a Roman Catholic, Tamara as Jewish. Neither knew she had a twin.But both own a pair of large hoop earrings, both like to dance, and they had similar nightmares when they were children. Both their adoptive fathers died of cancer(see "Separated at Birth in Mexico, United at Campuses on Long Island" by Elissa Gootman,New York Times, 03/03/03).

Perhaps the most amazing case of this sort is "The Mixed-Up Brothers of Bogota", two pairs of identical twins, Jorge and William and Carlos and Wilber, born on the same day in the same Columbian hospital.  They were accidentally switched so that Jorge and Carlos were raised together as fraternal twins, as were William and Wilber.  Their story is told in "The Mixed-Up Brothers of Bogota" by Susan Dominus, New York Times Magazine, 07/12/2015).  In her article, Dominus briefly describes Bouchard's study, and its continuation by Nancy Segal, a psychologist at CS Fullerton.  Dominus's article has a strong biological cast to it: Similarities are explained by genetic similarities, differences are explained by "epigenetic" influences, about which there is more later in this supplement.  But there are thousands of genes, and thousands of epigenetic influences, and, frankly, the most parsimonious explanation of their differences is that they were raised in different environments!

In 2019, Three Identical Strangers, a fascinating documentary film (directed by Tim Wardle), presented on CNN, drew attention to the dramatic case of three identical triplet brothers (actually quadruplets, although the fourth brother died in childbirth) who were raised separately.  The men, Edward Galland, David Kellman, and Robert Shafran, were born in 1961 to a teen-aged unwed mother and separated at 6 months by the adoption agency under whose care they had been placed.  Although standard practice would attempt to keep adoptive siblings together, the brothers were deliberately placed into different homes as part of a study of the effects of different parenting styles.  They learned of their relationship only by accident, when one of the brothers happened to enroll at a college which had been attended by another; the third brother learned of the situation by reading a newspaper account of the reunion of the other two. The three brothers subsequently opened a restaurant together.  One of them eventually committed suicide. 

Despite having been designed by a prominent academic psychiatrist (actually a psychoanalyst, which may have been part of the problem), the study itself appears to have been a classic case of Bad Science (not to mention bad policy: after the triplets' situation came to light, one of their adoptive families said that they would have been happy to take in all three).  In the first place, it was too small to generate any meaningful conclusions.  Aside from the triplets, there were apparently only five sets identical twins also deliberately placed with different families.  And although the families themselves were blind to the fact that identical siblings were being reared separately, the researchers were not.  For example, at one point, at least, the same research assistant was assigned to do followup testing of all three of the triplets -- allowing plenty of opportunity for bias to creep into the collection of data.  The records of the study have been placed under seal at Yale University until 2065, so it will be a while until we find out just how bad a study it was.  

Identical twins raised apart also provide the plot of a Walt Disney-produced movie,The Parent Trap, starring Hayley Mills (1961; remade 1998), a television sitcom (Sister, Sister), and an episode of the X-Files ("Eve"). Other movies involving twins separated at birth include:

  • The Iron Mask (silent, 1929), remade as The Corsican Brothers (1971) and the Man in the Iron Mask (1997), all from the Dumas novel
  • Start the Revolution Without Me (1970)
  • Echo (1988)
  • Twin Dragons (1990)
  • Big Business (1988)
  • A Merry Mix-Up (1957), starring the Three Stooges
  • Twice Blessed
  • Double Impact (1991)
  • Equinox (1992)

See also:

  • Twin Stories, a documentary film by Fredric Golding (1997)
  • Twin Stories, a book by Susan Kohl (2001), based largely on interviews conducted at the annual Twin Days festival in Twinsburg, Ohio.
  • "A Thing or Two About Twins" by Peter Miller,National Geographic, January 2012.

For a review of scientific studies of identical twins reared apart, including Burt's and Bouchard's studies, see:

  • Identical Twins Reared Apart: A Reanalysis by Susan L. Faber (Basic Books, 1981)

Link to for a complete listing of films about twins -- and lots of other information for and about twins and other "multiples".

It's important to recognize that heritability estimates are accurate only for the environment from which a population was drawn. For example, as discussed in the lectures on Thinking, the heritability of IQ is higher in high-SES populations than it is in low-SES populations. Apparently, high socioeconomic status gives freer rein to genetic influences, allowing people "to be all they can be", while low SES constrains them, confronting individuals with artificial ceilings on attainment.

Separating Genes and Environment(s)

Components of Environmental Variance in PersonalityThe genetics-versus-environment question can also be approached in the context of standard twin studies. But the issue gets a little complicated, because it turns out that the "environment" comes in two basic forms:

  • The shared environment, also known as between-family variance, includes all the factors that children raised in the same family share, which differentiate them from children in other families. As a rule, children in the same family are raised by the same parents, share a single racial, ethnic, and cultural heritage, live in the same neighborhood, go to the same schools, and attend the same church, synagogue, or mosque. The shared environment includes all the things that siblings have in common.
  • The nonshared environment, also known as within-family variance, includes all the factors that differentiate among children raised in the same family. Even within a family, children differ in terms of such factors as gender (boy or girl) or birth order (first-born vs. latter-born). Children may have different interactions with their parents (parents treat children differently depending on age and sex), and develop different networks of friends and acquaintances outside the family. Different children within a family are also distinguished by non-systematic factors, which include all the things that happen randomly to one child but not to his or her brothers and sisters --chance encounters that can really make a difference in the individual's life. The nonshared environment is an umbrella term that refers to all the unique experiences that siblings have.

As it happens, the relative strength of both environmental components of personality, as well as the genetic component, can be estimated from the observed pattern of MZ and DZ correlations  Falconer & Mackay, 1996).

Distribution of "Big Five" Traits Consider, first the entire distribution of a trait within a population, from those individuals with the lowest scores on Extraversion or Neuroticism to those with the highest scores on these traits. This distribution is typically represented by a more-or-less "normal" distribution -- the famous "bell curve". Each person's score on a trait measure -- Neuroticism, Extraversion, whatever -- is a measure of the person's phenotype -- how he or she "turned out" with respect to that dimension of personality. The entire distribution of individual scores within a population is the total variance on the trait(s) in question.

Calculating Components of Population VarianceThis total variance in the trait (100%, or a proportion equal to 1.0) is the sum of genetic variance (i.e., variance in the trait that is accounted for by to genetic variability, or individual differences in genotypes) and environmental variance (i.e., variance in the trait that is accounted for by environmental variability, or individual differences in environments):

  • Total Variance on a Trait (T) = Genetic Variance (G) + Environmental Variance (E).

The environmental variance, in turn, is the sum of variance due to the shared environment and variance due to the nonshared environment:

  • E = Variance due to Shared Environment (ES) + Variance due to Nonshared Environment (ENS).

First, consider the comparison between MZ and DZ twins. By definition, MZ and DZ twins are identical with respect to the shared environment -- they are raised by the same parents in the same household. But MZ and DZ twins differ genetically: MZ twins are identical genetically, while DZ twins are no more alike, genetically speaking, than any two non-twin siblings. Thus, any difference in similarity between MZ and DZ twins must be due to genetic differences.

Genetic variance is a function of the difference between MZ and DZ correlations: the greater the MZ correlation compared to that of DZ, the more we can attribute similarity to shared genes than to shared environments (don't worry about where the "2" comes from: this is a technical detail):

G = 2 * (MZ - DZ).

Next, consider MZ twins raised together. By definition, MZ twins are identical with respect to both genes and the shared environment. They are the product of a single fertilized egg, and they are raised by the same parents in the same household. If the only contributions to variance were from the genes and the shared environment, they ought to be identical in personality. Therefore, any departure from a perfect correlation of 1.00 must reflect the contribution of the nonshared environment.

Variance due to the nonshared environment is a function of the MZ correlation: MZ twins share both genes and (shared) environment, so any MZ correlation less than a perfect +1.0 must reflect the contribution of the nonshared environment:

ENS = 1 - MZ.

Once we've estimated the contributions of the genes and the nonshared environment, variance due to the shared environment is all that's left, so it can be estimated simply by subtracting G and ENS from 1:

ES= 1 - G -E NS.

Here are some illustrative examples:

              and Environmental Contributions to VarianceIf the correlation for MZ twins is 1.00, and the correlation for DZ twins is .50, we have the situation described earlier: all the variance on the trait is attributable to genetic variance, and no variance is attributable to either sort of environmental factor, shared or nonshared.

              and Environmental Contributions to VarianceIf we reduce the MZ correlation substantially, but keep the DZ correlation pretty much the same, most of the variance is now attributed to the environment. There is some genetic effect, and some effect of each sort of environment.

Genetic and
              Environmental Contributions to VarianceIf we increase the MZ correlation, but also increase the DZ correlation, most of the variance is still attributable to the environment, but the strength of the genetic contribution diminishes markedly.

              and Environmental Contributions to VarianceIf we use MZ and DZ correlations that roughly approximate those found in Loehlin's study of the Big Five, we find evidence of a substantial genetic component of variance, but also a substantial environmental component. Most important, we find that the contribution of the nonshared environment is much greater than that of the shared environment. In fact, the effect of the shared environment is minimal.

Genetic and Environmental Components of ExtraversionIn fact, if we use Loehlin's data for Extraversion, that's exactly the pattern we find.The MZ correlation is about twice as high as the DZ correlation. The estimate for G is 48%, for ENS it's 52%, with nothing left over for ES.

              and Environmental Components of NeuroticismAnd also for Neuroticism.

A variant on the twin study is the adoption study, which compares the similarity between adopted children and their biological parents and siblings (with whom they share 50% of their genes, on average) and adopted children and their adoptive parents and siblings (with whom they share no particular genes). To the extent that genes contribute to individual differences on some variable, biologically related individuals should be more alike than biologically unrelated ones.

The two methods can be combined, in a way, by comparing identical twins reared together (who share genes and environment) and identical twins reared apart (who, ostensibly, share genes but not environment). I say "ostensibly", because some twins "raised apart" simply live in different households (like with grandparents or aunts and uncles) for economic reasons, but still have a great deal of family, school, and social life in common. So it's not necessarily the case that identical twins reared apart don't share a family environment.

The Big Five

              Study: The "Big Five" Personality TraitsConsidering Components of Variance in the "Big Five"
              Personality Traits all five traits examined in Loehlin's (1992) study of The Big Five, the results of actual twin studies of personality reveal that genetic factors account for approximately 40% of the variance on the Big Five traits; the nonshared environment accounts for approximately 50% of variance; and the shared environment accounts for less than 10% of variance. Apparently the family environment is not decisive for adult personality, and the nonshared environment is far more important.

To summarize, for each of The Big Five dimensions of personality, there is:

  • a substantial genetic component to variance;
  • the contribution of the nonshared environment is even greater than that of the genes;
  • the contribution of the shared environment is relatively trivial.


A somewhat different pattern occurs for individual differences in temperament, which some theorists consider to be the most "innate" of all personality characteristics. Allport (1961, p.34) defined temperament as follows:

Temperament refers to the characteristic phenomena of an individual's nature, including his susceptibility to emotional stimulation, his customary strength and speed of response, the quality of his prevailing mood, and all the peculiarities of fluctuation and intensity of mood, these being phenomena regarded as dependent on constitutional make-up, and therefore largely hereditary in origin.

Based on this definition, Buss, Plomin, and Willerman (1973) devised the EASI scale to measure individual differences in four aspects of temperament in MZ and DZ twins aged 4 months to 16 years:

  • Emotionality: level of emotional arousal, or intensity of emotional reaction (distress, anger, and fear) in objectively upsetting situations.
  • Activity Level: overall energy output, as indicated by the vigor and tempo of behavior.
  • Sociability: the tendency to approach other people, share activities, get others' attention.
  • Impulsivity: essentially, speed of response to stimulus.

The general finding was that all four dimensions were, as predicted by Allport, indeed, highly heritable, with heritability coefficients averaging .58 -- though there were some differences by age and gender.

  • For example, Activity yielded a heritability coefficient of .83 for boys under 55 months of age, but only .24 for girls in that group.
  • Similarly, Impulsiveness showed a heritability coefficient of .87 for young boys, but no heritability for young girls (the DZ correlation was actually higher than the MZ correlation!).


Temperament" is often considered to be an innate characteristic of personality, so it is not particularly surprising that it has a relatively large genetic component.  Other characteristics of the child, however, show evidence of a clear environmental contribution.  A case in point is attachment style, a term coined by John Bowlby, a British psychiatrist who was influenced by both Darwin's theory of evolution and Freud's psychoanalytic theory of personality.  From Darwin, Bowlby got the idea that the even the very young children have to learn how to survive in their environment -- an environment composed principally of their parents, and especially their mothers as their primary caretakers.  From Freud, he got the idea that the parent-child relationship determined the character of the child's later relationships with other adults, particularly their spouses (and their own children).  Bowlby's attachment theory argues that attachment security is an important feature of personality.  At first glance, it might seem reasonable to assess attachment security on a single dimension, from insecure to secure, in fact attachment theory describes four main types of attachment: "secure" and three different types of "insecure" attachment.  These are typically measured by a behavioral procedure known as the Strange Situation developed by Mary Ainsworth, an American-Canadian psychologist who worked closely with Bowlby. 

The Strange Situation assessment is conducted over a series of phases:

  1. The child (typically an infant, between 1 and 2 years old) and parent (usually the mother) are brought into a room. 
  2. The infant is allowed to explore the room while the parent sits by passively. 
  3. A a stranger enters the room and converses briefly with the parent.
  4. First Separation Episode: The parent leaves the room, and the stranger begins interacting with the infant.
  5. First Reunion Episode: Infants are typically distressed at the absence of the parent, and in any event the parent soon returns to the room. 
  6. Second Separation Episode: The parent and stranger both leave the infant alone in the room. 
  7. The stranger returns to the room and interacts with the infant.  
  8. Second Reunion Episode: Finally, the parent returns, picks up the infant, and the stranger leaves.

During the Separation and Reunion episodes, various aspects of the infant's behavior are observed and coded:

On the basis of these behavioral observations, the child is classified into one of four categories:

Obviously, attachment is a two-way street: between the child and the caregiver and between the caregiver and the child.  Therefore, we'd expect the environment to play an important role in individual differences in attachment style.  And this is just what is revealed by twin studies of attachment style.  The question is a little more complicated to answer than for extraversion, IQ, or temperament, because attachment style is a categorical variable, not a continuous variable, but the logic of the analysis is the same: are MZ twins more likely to share the same attachment style as DZ twins?  There have been a number of studies addressing this question (reviewed by Gervai, Child & Adolescent Psychiatry & Mental Health, 2009).

Roissman & Fraley concluded that the most important determinant of attachment style was the quality of parenting received by the child.  Interestingly, given the relatively large contribution of the nonshared environment, the quality of parenting differs even between identical twins!

IQ and Education

There's a different picture for individual differences in intelligence (measured by standard IQ tests) and education.

Family Studies of IQFamily studies show that the correlation between family members' IQ is, in turn, correlated with genetic resemblance.

  • MZ twins > DZ twins, even when the MZ twins are raised apart.
  • Biological siblings > adopted siblings.
  • Parents and their biological offspring > parents and their adopted offspring.

But there's also a clear family influence:

  • MZ twins raised in the same household are more alike than MZ twins raised apart.
  • Parents and their biological offspring raised in the same household > parents and their biological offspring raised apart.

              and Environmental Contributions to Variance in IQAggregating over the best available studies, a reasonable estimate for the MZ correlation for IQ is .86, and the corresponding estimate for DZ is .60. Plugging these figures into the equations, that means that about 52% of population variance in IQ is attributable to genetic variance. About 34% is attributable to the shared environment, and about 14% to the nonshared environment.

Genetic and Environmental Contributions to Variance
              in EducationWhen you look at educational attainment, you get somewhat similar correlations, and somewhat similar estimates.

Sex and Suicide

Here are two other examples of how behavior genetics can shed light on the role of nature and nurture in development.

Components of Variance in Adolescent and Young Adult
              BehaviorA study by Herden et al. (2007) looked at age of "sexual debut" -- that is, the age at which people had their first sexual intercourse. Here, genes account for about 31% of population variance, but the nonshared environment accounts for a whopping 59% of variance.

Genetic and Environmental Influences on SuicideAnd another study, by Fu et al. (2002), examined genetic and environmental contributions to suicidal behavior, defined broadly to include both suicidal ideation and actual attempts at suicide. Genes were more influential on suicidal ideation than suicide attempt (probably due to a genetic influences on depression), but in both cases the contribution of the nonshared environment was much stronger than that of the genes, or the shared environment.

Political Attitudes

There also appears to be a genetic contribution to political attitudes. Evidence for this comes from the Virginia 30K Twin Study, which followed 29,080 subjects residing in Virginia, and their first-degree relatives, including 2,648 MZ twins and 1,748 DZ twins. Among other questionnaires, these subjects were administered the Wilson-Patterson Attitude Inventory of opinions on various socio-political issues, such as school prayer, property taxes, busing, and abortion. Half the questions were posed so that an endorsement indicated liberal attitudes, and half were worded in the conservative direction. 28 of the items were expressly political in nature. The subjects were also asked to reveal their political party affiliation, if any.

Twin Study:
              Political AttitudesThe investigators devised two scales of liberalism-conservatism, plus a scale of "opinionation" indicating how strong their opinions were. On each scale, MZ twins were more alike than DZ twins.

              of Variance in Political AttitudesCalculating the components of variance, the investigators identified a substantial genetic contribution to both liberal-conservative attitudes and opinionation, though not so much to party affiliation as such (it's possible to be a relatively liberal Republican, and a relatively conservative Democrat). The shared environment was a relatively strong determinant of party affiliation: apparently, children tend to join their parents' political party. But in three out of four cases, the contribution of the nonshared environment was stronger than that of either the genes or the shared environment. Which just goes to show you how important individual experiences are to things like this.

Hatemi et al. (Behavior Genetics, 2014) obtained similar results in an even larger study.  Again employing the twin-study method, these investigators examined "left-right" or "liberal-conservative" political attitudes based on surveys of more than 12,000 twin pairs living in five democracies (Australia, Denmark, Hungary, Sweden, and the United States), surveyed over four decades (1980-2012).  They found that genetic factors accounted for about 40% of the variance in political attitudes; the shared environment, 12%; and the nonshared environment, 42%.  The only departure from this pattern is political-party affiliation, which is overwhelmingly determined by the shared environment.  Democrats tend to beget Democrats, and Republicans Republicans; but peoples attitudes toward specific issues, such as abortion or same-sex marriage, tend not to be passed from one generation to the next. 

Hatemi et al. went even further, conducting a genome-wide association study in three of the samples in an attempt to identify particular gene variants that might mediate the genetic contribution.  They found no plausible candidates, and concluded that individual differences in political attitudes, like individual differences in intelligence and other aspects of personality, were the product of large number of genes, each of which played a small role.  But from our point of view the most important finding is that environmental factors outweighed genetic factors, and the nonshared environment outweighed the shared environment.

As Irving Kristol once joked, "A neoconservative is a liberal who has been mugged by reality". If his identical twin hasn't been similarly mugged, he'll probably stay a liberal.

To sum up: Twin studies reveal genetic influences on personality and attitudes, and these are interesting, but by far the most surprising finding of behavior-genetics research is the evident power of the nonshared environment. We've been taught, at least since Freud (though that should have been our first clue that something might be wrong!) that the way children are treated in the family determines how they'll grow up. In fact, this widespread belief appears to be incorrect. There is an extensive literature on child-rearing practices, considering such things as age of weaning and toilet training (the sorts of things that interested Freud a great deal), and these aspects of childhood seem to have little influence on adult personality (Sears, Maccoby, & Levin, 1957).

Happiness (Subjective Well-Being)

One of the enduring puzzles in personality psychology is what accounts for individual differences in happiness or life satisfaction. Current theory favors the view that each of us has a sort of baseline level of happiness - -think of it as a sort of "happiness set point" around which we fluctuate, depending on what is going on in our lives at the moment (e.g., Kahneman et al., 1999). But where does this baseline level of happiness come from? The obvious answers -- like, "the richer you are the happier you are" aren't sufficient. Research shows a surprisingly low correlation between socioeconomic measures like income or wealth and self-rated happiness. A pioneering study by Lykken and Tellegen (1996) suggested that baseline happiness levels were substantially heritable -- meaning that we are born with a tendency to be happy or unhappy. But Lykken and Tellegen never claimed that happiness was all in the genes.

De Neve (2011; De Neve et al., 2011) examined the genetic and environmental contributions to happiness -- that is, one's sense of well-being and satisfaction with life. in the National Longitudinal Study of Adolescent Health (known as the Add Health Study), a survey of almost 27,000 American students enrolled in 80 high schools in that began in 1994-1995, and continues with regular follow-up interviews. At one point, the subjects were asked to indicate "How satisfied are you with your life as a whole?" -- the standard formulation used in such studies. Most respondents said they were at least fairly satisfied with their lives, but the important point has to do with the determinants of these ratings.

  • For a sub-sample of over 400 pairs of identical twins, the MZ correlation was .35.
  • For a sub-sample of over 400 pairs of same-sex fraternal twins, the DZ correlation was .13.

Using a somewhat more complicated formula than our "double the difference" heuristic, De Neve et al. calculated the following components of variance:

  • G = .33, meaning that approximately 33% of population variance was due to genetic variance.
  • ENS = .67, meaning that approximately 67& of population variance was due to the nonshared environment.
  • ES = 0, meaning that there was no contribution from the shared environment. Children from the same family are no more similar in happiness or life satisfaction than children drawn from different families.

So, these findings are consistent with the general pattern found with the Big Five: a significant genetic component, but also a significant contribution from the nonshared environment.

Rumination and Depression in Adolescence

Depression is a serious problem in adolescent (and adult) mental health, and rumination -- perseverative thinking about one's problems and feelings -- is a known risk factor for depression.  Moore and her colleagues studied genetic and environmental contributions to both rumination and depression.  Adolescents enrolled in the Wisconsin Twin Project completed a variety of questionnaires, yielding the following correlations and estimates of components of variance.

  • Depression: MZ, r = .53; DZ, r = .26.
  • Brooding: MZ, r = 22; DZ, r = .10.
  • Reflection: MZ, r = .40; DZ, r = .14.
  • Distraction: MZ, r = .40; DZ, r = .02.

Moore et al. used a slightly different procedure to estimate the components of variance.  In each case, it's obvious that there was a significant genetic contribution to variance.  The contribution of the nonshared environment was equally substantial, and the contribution of the shared environment was virtually nil.

Economic Behavior

The financial crisis of 2008 led some social scientists to try to understand how some bankers and investors, working in such a "rational" environment as the economy, could possibly have been so reckless. And, naturally, the question got frames in terms of nature and nurture, genes and environment. A study by Cronqvist and Siegel, two American business professors, analyzed data on savings habits in a sample of almost 15,000 Swedish identical and fraternal twins -- the largest twin registry in existence (again, you can get this kind of data in a country like Sweden, which has a comprehensive and efficient national healthcare system). Of course, Swedes are human, and so there was wide variation in savings behavior -- that is, how much of an individual's disposable income was saved for the future, instead of being spent in the present. The primary measure chosen was change in the individual's net worth between 2002 and 2006, adjusted for individual differences in gross income.

Comparing identical and fraternal twins, they got the following correlations:

  • MZ twins,r = .33
  • DZ twins,r = .16

Plugging these values into our rough-and-ready formula, we get the following estimates:

  • G = .32
  • ENS= .67
  • ES= .01

Actually, Cronqvist and Siegel applied a number of more sophisticated mathematical models to their data, but they all led to the same conclusion:

  • There was a significant genetic component to savings behavior -- perhaps related to the Big Five factor of conscientiousness.
  • There was also a substantial environmental component, but the nonshared environment was far more powerful than the shared environment.

Musical Ability

The old joke goes: "How do you get to Carnegie Hall?".  and the punchline is: "Practice, practice, practice!".  And it's true: Anders Ericsson and his colleagues (1993), surveying elite musicians, estimated that it took about 10,000 hours of disciplined practice in order to become an expert instrumentalist: this study gave rise to the "10,000 hour rule" popularized by Malcolm Gladwell in his book, Outliers (2008). 

But it's not all practice.  A group led by Miriam Mosing, a behavior geneticist at the Karolinska Institute in Sweden, finds that musical ability is a product of both genes and the environment -- and the critical environment is not the shared environment (e.g., whether the subject's parents were themselves musicians); it's the nonshared environment. 

  • In a 2013 study, Fredrik Ullen, Mosing, and their colleagues reported a study which tested subjects' ability to make auditory discriminations of pitch, melody, and rhythm.  
    • Taking advantage of the large databases available through the Swedish health system, Ullen et al. tested 6,881 twins.
    • They first established the validity of their test by showing that scores were positively correlated with such criteria as taking music lessons, playing an instrument, and years of formal musical training.
    • Collapsing across males and females, MZ twins had more similar scores on all three subscales than did DZ twins.
      • Rhythm: MZ, r = .51; DZ, r = .28
      • Melody: MZ = .57; DZ = .32
      • Pitch: MZ = .48; DZ = .29
    • Applying a slightly different mathematical model than our "double the difference" rule of thumb, Ullen et al. obtained the following estimates of the components of variance:
      • Rhythm: G = .50; ENS = .48; ES = .02.
      • Melody: G = .59; ENS = .40; ES = .00.
      • Pitch: G = .30; ENS = .52; ES = .19.
  • In a further analysis, Mosing and her colleagues (2014) also looked at the role of practice in a subsample of twins who reported that they played a musical instrument or actively sang  (i.e., not just in church, or the shower, or Friday nights at a karaoke bar).  It turns out that there's a genetic component to practice, too.
    • MZ twins self-reported practice times were more similar (r = .63) than those of DZ twins (r = .40).
    • Applying their formula yielded the following estimates: G, .41; ES, .21; ENS, .38.

It makes sense that the shared environment is a stronger determinant of practice than of musical ability per se.  It was probably the subjects' parents who encouraged them to take up music in the first place, and then insisted that they practice -- at least so long as they were living at home!.  But still, the nonshared environment trumps the shared environment.

Putting it All Together

The most comprehensive study of heritability was published by Tinca Polderman, Danielle Posthuma, and their associates in Nature Genetics for 2015.  Indeed, it is the most comprehensive genetic study of anything conceivable, because it's a meta-analysis of virtually all twin studies published from 1958 to 2012 -- covering 2,748 publications, 17,804 traits, and 14,558,903 twins (many of whom appeared in more than one study). These investigators used a slightly different method of estimated heritability, and the contribution of the shared and nonshared environment, than presented in this course, but the results they obtained are entirely compatible with those discussed here.  Still, they determined that, across all domains, the average heritability was 49%, and the contribution of the shared environment 17%, with the non-shared environment accounting for most of the rest of variation.  The largest heritabilities were associated with biological traits, with smaller heritabilities associated with psychosocial traits such as those we're concerned with in this course.  and the shared environment also had the largest influence in the biological domain.  However, all of the traits showed significant heritabilities -- not a single trait, biological or psychosocial, had an average heritability whose confidence interval included zero (0).  Most of the findings were consistent with an "additive" genetic model in which each trait is influenced by a number of different genes.  The table at the right shows the twin correlations, and estimates of heritability (h2) and shared environmental variance (c2) for the "Top 20" most-investigated traits in the literature.

There's a pattern here. For most psychologically interesting behaviors, genes may account for a significant proportion of individual differences, but by far the most important determinant is the nonshared environment.

Genes "for" Personality?

We'll turn to the nonshared environment in a moment, but first, let's explore the genetic component a little more. Is there really a "happiness gene"? Not exactly, but De Neves et al. did identify a particular genetic polymorphism that does seem to be involved in the genetic component.

And it's certainly possible that there are genes "for" certain basic individual differences in temperament, such as speed and strength of emotional response. There may even be genes for individual differences in some of the "Big Five" personality traits, such as extraversion and neuroticism. But there are unlikely to be genes for all the important individual differences in personality, for the simple reason that there aren't enough genes. The human genome contains about 22,500 genes -- and remember that about 90% of these don't vary from one individual to another, which leaves about 2,250 genes to work with. And when you consider that, in order to get the smooth bell-shaped normal distributions of personality traits, you need a model of polygenetic inheritance -- lots of genes making tiny contributions to each individual-difference variable -- you can see that the number of genes mounts up pretty quickly, and you pretty quickly run out of genes. So there's got to be something else going on, and that "something else" is going to be powerful, not trivial.

Moreover, consider the nature of some of the individual differences studied by behavior geneticists.

  • Based on an adoption study, Plomin and his colleagues (1990) reported a genetic contribution to the amount of time subjects spend watching television. But there can't be a gene for TV-watching, for the simple reason that genetic differences are a product of evolution, and there hasn't been enough time since the invention of television, by Philo Farnsworth and others in the late 1920s, for a TV-watching gene to evolve.  So if there is a genetic contribution to TV-watching, it's rather indirect.
  • There is a clear genetic contribution to individual differences in openness to experience, one of the Big Five dimensions of personality, but this "trait" was only identified in the 1960s, when "openness" became a salient feature of culture.Before that, this dimension was characterized as "intellectance" (i.e., looking intelligent) or culturedness. Personality characteristics that are so rapidly changeable don't allow evolution time to develop a relevant gene.
  • Similarly, other investigators have reported a genetic contribution to individual differences in political attitudes, but the liberal-conservative dimension on which these individual differences are measured had its origins in 18th- and 19th-century political philosophy. Again, there simply hasn't been enough time for a "political gene" to have evolved.


Identical twinsIdentical twins, whether raised together or apart, are strikingly similar on a host of psychological variables. But they're also strikingly different, even when they were raised together. Why should this be the case?

One answer lies in the complexities of genetic inheritance: it turns out that individuals with the same DNA might not be genetically identical after all. According to epigenetic theory, genes carry epigenetic tags, such as histones and methyl groups, that do not change an individual's DNA, but rather enhance or suppress the activity of particular genes -- turning them on or off, if you will. It's by virtue of epigenesis that embryological development happens -- how the single cell of the newly conceived zygote begins to differentiate into the specific cells that make up the various body parts of the developing fetus. And it's the basic mechanism for many forms of gene therapy -- inserting a gene into one (biochemical) environment will have one effect, while inserting the same gene into a different biochemical environment can have a quite different effect.

But the scope of epigenesis goes beyond embryological development. The result of epigenesis is that while two individuals can have the same genotype, they can will have different phenotypes even at the cellular level of analysis. The result can be -- here I'm taking an extreme example for purposes of illustration -- that two MZ twins who share the genotype for blue eyes, but one might end up with brown eyes. Further, according to theory, environmental factors like stress (war, child abuse, even social prejudice), poor nutrition, or other forms of deprivation (such as the economic deprivation of poverty), which can alter the chemical environment of the gene, might affect the activity of these epigenetic tags.

The interesting thing is that some of these environment-induced changes in gene expression -- essentially, turning some genes on or off -- can themselves be heritable.  So, for example, the effect of stress on one identical twin can alter that individual's genome, and perhaps the genes that he or she passes on to his or her offspring -- providing a genetic mechanism for the intergenerational transmission of the acquired characteristic.

Epigenetics is sometimes thought to prove Lamarck's idea of the inheritance of acquired characteristics -- that "acquired" modifications to the body (or, for that matter, behavior) can be passed on from parents to children.  It doesn't and they can't.  The prenatal environment can affect whether certain certain genes are turned on or off, and in this way what happens to a parent can affect a child's genetic endowment.  But that's not the same thing as the Lamarckian vision.  Besides, it's not at all clear that the altered genome can be transmitted genetically.  The altered genome will be preserved during mitosis, or ordinary cell division, but it is not at all clear that it will also be preserved during meiosis, in which cell division produces sperms and eggs.  Whatever intergenerational transmission occurs may be by virtue of pure environmental mechanisms.

In its broadest sense, epigenetics refers to everything that determines an individual's phenotype, other than his genotype. As such, it describes the effects of the environment on gene expression, and might be counted as an example of the person-by-situation interaction discussed at length in the lectures on Personality and Social Interaction.  That is, some environmental factor, such as stress levels, interacts with some aspect of the individual's genetic endowment, to alter some phenotypical aspect of behavior.  We'll see more examples of this gene-by-environment interaction in the lectures on Psychopathology and Psychotherapy.  But epigenetics is a biological construct, and it has only the vocabulary of biochemistry -- histones, methyl groups, stress hormones -- to describe that environment. But as psychologists, we are primarily interested in a different level of analysis -- the environment construed in psychological terms, as the person's mental representation of the environment. In psychological terms, what is important about stress are the levels of unpredictability and uncontrollability that cause it to occur; and whether the person subjectively experiences an environment as stressful -- regardless of whether it is "objectively" stressful. And what goes for stress also applies to other aspects of the environment: psychologists are always centrally interested in the individual's mental representation of the environment. How that mental representation is represented neurally, and what effects it will have biochemically, reflect a different level of analysis.

For a good account of epigenesis, see The Epigenetics Revolution: How Modern Biology is Rewriting Our Understanding of Genetics, Disease, and Inheritance by Nessa Carey (2012). See also Carey's articles on "Epigenetics in Action" in Natural History magazine: Part 1, "Deciphering the Link Between Nature and Nurture", appeared in the April 2012 issue; Part 2, appeared in May 2012.

See also "Epigenetics: The Evolution Revolution" by Israel Rosenfield and Edward Ziff, New York Review of Books, 06/07/2018 -- a sequel of sorts to their earlier essay, "Evolving Evolution", NYRB, 05/11/2006.

For a more technical overview, see "Effects of the Social Environment and Stress on Glucocorticoid Receptor Gene Methylation: A Systematic Review" by Gustavo Turecki and Michael Meaney, Biological Psychiatry, 2016.

In a somewhat similar manner, so-called jumping genes are segments of DNA that can replicate and insert themselves in new places in the genome, altering the activity of their neighbor genes. Jumping genes appear to be particularly active in the brain, leading some neurogeneticists to suggest that they are the key to human uniqueness: even identical twins are not, precisely, genetically identical, and these small differences in the genome are responsible for the differences in personality observed in MZ twins -- including differences in the concordance rates for various forms of mental illness, such as depression.

Well, maybe. Epigenetic theory is plausible, and it might even be true, in some cases. Epigenetic factors have been invoked to account for the relatively low concordance rates between MZ twins for certain mental illnesses, such as schizophrenia and autism (as discussed in the lectures on Psychopathology and Psychotherapy). But note that epigenetic theory defines "the environment" in purely biological, even biochemical terms -- not to put too fine a point on it, as the microenvironmental soup that the gene sits in. As psychologists, we're primarily interested in the social and cultural macroenvironment in which the individual person lives.

Frankly, by focusing on epigenetic factors, behavior genetics sometimes seem to want to have it all -- to be able to explain even differences between genetically identical individuals in genetic, biochemical terms. I noted a similar trend with respect to junk DNA.  Put another way, there is a certain class of biologically oriented individuals who want to discount the effects of the social and cultural environment at all costs. That's fine, if you're a biologist, because it's the very nature of the biological level of analysis to construe the environment in physical, chemical, and biological terms. But psychology is a social as well as a biological science, and so psychologists take a broader view of the environment -- one which considers social and cultural influences on their own terms, without reducing them to biochemistry.

Or, as E.B. White and Carl Rose (and, later, Irving Berlin) put it, "I say it's spinach (and the hell with it)". Let's look at the macroenvironment, or the social and cultural world in which the individual develops, from birth across the entire lifespan.

For an accessible discussion of epigenetic theory, see:

  • "Hidden Switches in the Mind" by Eric J. Nestler,Scientific American, 12/2011.
  • The Epigenetics Revolution: How Modern Biology is Rewriting Our Understanding of Genetics, Disease, and Inheritance (2012) by Nessa Carey.  Excerpted in Natural History magazine, April-June 2012.
For a discussion of jumping genes, see:
  • "What Makes Each Brain Unique" by Fred H. Gage (a distant relative of the famous Phineas Gage) and Alysson R. Muotri Scientific American, March 2012.

For excellent coverage of the modern science of heredity and genetics, see:

  • She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity by Carl Zimmer, a prominent science journalist.

Genetic Nurture -- or, "Unto the Next Generation"

One of the paradoxes of behavior genetics is that the same study that provides evidence of genetic contributions to behavior also can give evidence of environmental contributions.  So, for example, the twin-study method allows us to estimate the effects of heredity, the shared environment, and the nonshared environment.  Similar insights can come from the gene-wide association study (GWAS) method described earlier, which allows us to identify particular genes -- or, more precisely, particular alleles -- associated with particular characteristics.  For example, a recent study by Augustine Kong and his colleagues (Science, 2018), employing a huge sample of Icelandic probands (as subjects are commonly called in genetics research), to identify a number of different alleles which were significantly associated with educational achievement (EA, measured by years of education completed).  Recall that children receive half their genes from their fathers and half from their mothers.  Therefore, it follows that some of these alleles will be passed down from parents to their children, but others will not.  For each subject, Kong et al. identified which EA genes (as we'll call them) were transmitted by each parent to each of his or her children, and then examined the contribution of both transmitted and non-transmitted EA genes to EA in their children.  They found that direct genetic transmission from parent to child accounted for a significant proportion of the variance in the children's EA, as would be expected from the twin studies summarized earlier.  But there was also a significant effect of the parents' non-transmitted EA genes on their children's EA -- an effect about 1/3 as strong as the direct genetic effect.  Kong et al. call this effect genetic nurture -- that is, by virtue of the EA genes that they possess, the parents create an environment that affects EA in their children over and above the direct effect of the EA genes the parents have passed down to them.  This genetic nurture effect is one component in the shared environment of the children. 

If you think about it, genetic nurture effects involve more than parents and children.  Children get roughly 1/4 of their genes from each of their grandparents, too.  So some EA alleles are transmitted from grandparents to grandchildren directly, through the parents, but the non-transmitted genes also affect the environment in which the parents were raised, and also -- to the extent that the children are in contact with their grandparents -- the environment for the children.  And remember that children in the same family share about 50% of their genes in common.  So some parental EA genes, will get passed to proband John, but not to his siblings Jean and Don, and vice-versa.  But by virtue of the EA genes they have received, Jean and Don will create an environment that affects John's EA.  And vice-versa.  

Remember that children get half of their genes from each parent on average.  Now imagine that John didn't get his fair share of EA genes, but that Jean and Don got more than their fair share.  John might be genetically disadvantaged in this sense, but Jean and Don will use their genetic advantage to create an environment that promotes John's EA.  And if John got more than his fair share, and Jean and Don got less, they might create an environment that holds John's EA back. This is one of the ways in which the family environment ostensibly shared by siblings is not the same for each of them.  Now let's explore some other ways.

The Nonshared Environment

The fact is that, with respect to the major dimensions of personality, variability within a family -- i.e., the variance among children raised in the same family environment -- is almost as great as the variability between families -- i.e., the variance between children raised in different families. This result is sometimes misinterpreted as meaning that parents have no influence on their children. But it doesn't mean that at all.

What it means, first, is that there are other forces at work besides the parents, and these become increasingly important as the child begins to move beyond the family (e.g., by going to school or joining the soccer league)

In addition, it means that parents don't have the same effects on each of their children.

These differences in the nonshared environment are critical for our individual uniqueness. Put another way, the most important environmental determinants of personality are the unique experiences that we have in our lives.

The Nonshared Environment

One source of the nonshared environment, of course, is the extra-familial environment. Even if all children within a family are treated precisely alike by their parents, once they begin to interact with the world outside the family they will have experiences that tend to make them different.

Behavior-Genetic Analysis of Adolscent BehaviorAs examples of extra-familial influence, consider a series of studied by David Rowe and his colleagues (e.g., 1992), using behavior-genetic methods, of the sources of various aspects of adolescent behavior.

  • In one analysis, Rowe found that parents who smoke tend to have children who smoke, and the behavior-genetic analysis indicated that this influence was mediated by heredity (G), not the shared environment within the family (Es).
  • But it is also true that adolescents who smoke tend to have peers who smoke -- an effect of the nonshared environment outside the family (Ens).

Rowe and his colleagues found similar causal patterns for alcohol consumption, delinquency, sexual behavior, and pregnancy. In each case, the shared environment had relatively little impact on behavior, but peer groups had a powerful effect on whether adolescents experimented with tobacco, alcohol, sex, and misbehavior.

Similarly, a study by Kindermann (1993) of academic motivation among elementary-school children found that students tended to group themselves into cliques such as "brains" and "slackers" -- but also that membership in these cliques tended to be somewhat unstable, with individual children moving back and forth from one group to another. Interestingly, the children's attitudes toward school changed as they changed cliques. "Brains" lost interest in school if they moved to a "slacker" clique, and "slackers" gained interest if they moved to a "brains" clique -- despite the fact that their IQs remained constant, and parental influences presumably did so as well. It was the peer group that caused the changes to occur.

Based on results such as these, Harris (1995, 1998, 2006) has proposed a theory of group socialization which argues that peer groups and peer cultures, not parents, are the most powerful socialization forces impinging on a child. In fact, Harris argues that socialization is context-specific, and that children may behave quite differently depending on whether they are at home or away, and depending on which particular extra-familial context they're in.

As an example, she points to "code switching" among bilingual and bicultural children (as discussed in the lectures on "Language"):

  • Children born to Spanish-speaking parents, for example, may continue to speak Spanish inside the home, even though they will speak English with their peers.
  • Similarly, minority children from middle-class families may succumb to pressure from their peer groups to downplay academic achievement and other forms of "acting White" (Fryer, 2006).
  • As another example, consider food preferences. If parents are such powerful socialization agents, why do they find it so hard to get their children to eat what they want them to eat? Children want to eat what their friends like, not what their parents like.

Even within the family context, however, there are important differences among siblings. Harris (1995, 1999, 2006) has classified these within-family differences into four categories:

  • child-driven,
  • parent-driven,
  • relationship-driven, and
  • family context.

In addition to these within-family differences, there are also extra-familial influences that influence the development of personality and social behavior.

Harris on Parental Influence

In The Nurture Assumption: Why Children Turn Out the Way They Do (1998), Harris reproduced a famous bit of verse by Philip Larkin (1922-1985), an English poet.  Now, Larkin had issues: he once quipped that "Sex is too much fun to share it with others".  In "This Be the Verse" (1971/1974) Larkin reflected the dominant view of personality development in his time, influenced as it was by Freudian psychoanalysis -- that parental influence dominates the development of personality.

They fuck you up, your mum and dad.
They may not mean to, but they do.
They fill you with the faults they had
And add some extra, just for you.

But they were fucked up in their turn
By fools in old-style hats and coats,
Who half the time were soppy-stern
And half at one another’s throats.

Man hands on misery to man.
It deepens like a coastal shelf.
Get out as early as you can,
And don’t have any kids yourself.

To which, Harris replied with a bit of verse of her own:

Poor old Mum and Dad: publicly accused by their son, the poet, and never given a chance to reply to his charges. They shall have one now, if I may take the liberty of speaking for them:

How sharper than a serpent’s tooth
To hear your child make such a fuss.
It isn’t fair—it’s not the truth—
He’s fucked up, yes, but not by us.

Then by whom?  Peers and other elements of the nonshared environment.

Child-Driven Effects

Child-driven effects, also known as reactive effects, relate to the fact that each child brings certain physical and behavioral characteristics into the family, which in turn affect how he or she is treated by parents and others.

Some reactive effects reflect the environment's response to the physical appearance of the child. These reflect the evocation mode of the person-by-situation interaction.

  • The clearest example, discussed in the lectures on Personality and Social Interaction, concern how the child's biological sex -- male or female -- affects gender-role socialization. Here the physical appearance of the child structures the environment, by evoking differential treatment by others, according to cultural prescriptions for the proper socialization of boys and girls.
  • There may be other examples, having to do with the child's physical appearance:
    • whether the child is conventionally "pretty", or perhaps has some blemish or disfigurement;
    • whether the child physically resembles the parents or others in the immediate family.

Other reactive effects are instigated by the behavior of the child, not just his or her physical appearance. These reflect the manipulation mode of the person-situation interaction: there is something the child does, however unwittingly or involuntarily, to alter the environment in which he or she is raised.

  • The clearest reactive effects of the child's behavior have to do with individual differences in temperament -- by which we mean the person's speed and strength of emotional arousal. Temperament is usually thought of as a product of genetic endowment and physiology, which combine to give the child a generally "quiet" or "fussy" disposition. The child's temperament-related behaviors then interact with the parents to alter the environment in which the child is raised.
    • In a positive feedback loop, a child with a pleasant temperament might elicit positive treatment from the parents, while one with an unpleasant temperament might elicit negative treatment. Of course, the parents' response to the child will elicit subsequent behavior from the child himself. In this way, a vicious cycle can develop that strengthens the child's initial behavioral tendencies -- making a quiet child quieter, and a fussy child fussier.
    • In a negative feedback loop, a child with a quiet temperament might elicit "lower-limit control behaviors" from the parents, intended to increase activity levels, while one with an active temperament might elicit "upper-limit control behaviors" intended to decrease them.

Positive and Negative Feedback

Remember how positive and negative feedback are defined: They don't refer to pleasant or unpleasant consequences, like reward and punishment. Instead:

  • Positive feedback refers to any response that strengthens the stimulus that produced it.
  • Negative feedback refers to any response that weakens the stimulus that produced it.

Of course, such child-driven effects may extend beyond the child's interactions with his or her parents, as children begin to venture outside the home to school, playgroups, sports programs, and the like. Thus:

  • aggressive children may elicit aggressive behavior from other children;
  • introverted children may be ignored by their teachers.

In general, child-driven effects are unpredictable, because they greatly depend on the response to the child by the people who make up the child's environment -- their own personalities, beliefs, attitudes, goals, and the like.

Parent-Driven Effects

Parents don't just react to their children's appearance, behavior, or temperament. To some extent, parental behavior is independent of the physical, mental, and behavioral characteristics of the child. For example:

  • Sad as it may be, some parents will reject a child who is the product of an unplanned pregnancy (this doesn't always happen, of course: unplanned pregnancies can be joyful surprises for parents; but to think that they're all blessed events is asking too much. Sometimes the appearance of a child conflicts with the parent's other plans, or makes life difficult for the parent in some way (this is why it is so important that every child be actively wanted by its parents).
  • Some parents try to treat identical twins very similarly, with respect to dress, activities, and the like; other parents of identical twins will go out of their way to treat them differently. To the extent that identical twins are deliberately treated differently by their parents, of course, they are raised in different environments despite having identical genetic endowments; they have little by way of shared environment, and a great deal by way of nonshared environment.
  • Parents who have more than one child sometimes experience contrast effects on their perception of their offspring.
    • If the first child was "difficult", the second may be perceived as "easier" to raise.
    • If the first child was "easy", the second may be perceived as "difficult to raise -- even if there are no objective differences between the two children's behavior.
    • Fathers feel emotionally closer to children whom they believe resemble them, compared to children who resemble them less closely, or not at all (Heijkoop et al., 2009). Perhaps this is because they suspect that the children who don't look like them are not actually theirs (mothers don't have this problem, obviously): this would be the explanation given by evolutionary psychologists. But perhaps the effect is simply a consequence of the mere exposure effect: children who resemble their parents look more familiar, and thus tend to get higher preference ratings.
  • These different perceptions will naturally translate into different parenting behaviors, which will exaggerate the differences in home environments between the children.

Parental Styles

MacArthur & Wilson (1967; that's E.O. Wilson, the great evolutionary theorist) identify two major patterns of population dynamics, are associated with the abundance of resource sin the environment -- what evolutionary biologists call its "carrying capacity".

  • In r-selection, parents produce many offspring but "invest" relatively few resources in them, resulting in high levels of infant mortality (the "r" stands for rate of reproduction).
  • In K-selection, parents produce relatively few offspring but devote lots of resources to their nurturance -- theoretically increasing their  reproductive advantage (the "K", with initial capitals, stands for "capacity limit" in German).

The "r/K rule" seems to capture a lot of what happens in nonhuman species; and at first glance, seems to apply to humans as well.  People living in impoverished, underdeveloped societies tend to have large families, with relatively high levels of infant mortality, while middle- and upper-class families have relatively few children who tend to succeed in school and work (Mormons are a salient exception: in general, Mormons believe that parents should have as many children as they can afford).  However, a longitudinal study by Goodman et al. of the Uppsala Birth Cohort, a sample of about 14,000 people born in the Swedish capital between 1915 and 1929, and their descendants, showed that as SES improves, families grow smaller, and the children do indeed do better at work and school; but their children produced relatively few grandchildren and great-grandchildren (Goodman et al., 2012).  This is contrary to evolutionary theory, which (to make a long story short) defines "reproductive success" in terms of the number of grandchildren.   It is, however, completely compatible with a cultural explanation.  In developed societies, with reduced infant mortality, there's little reason to have lots of children, in the hope that a few of them will survive.

At the psychological level of analysis, UCB's Diana Baumrind (1971) has described four basic parenting styles:

  • Authoritative: Parents are warm and responsive to their children, but set high standards and set limits on their behavior.
  • Authoritarian: Parents are emotionally distant from their children, and set rules without explanation or negotiation.
  • Permissive: Parents are warm and loving, but undemanding and fail to set limits.
  • Indifferent: Parents interact very seldom with their children, doing little more than providing food and shelter.

According to Baumrind, these parenting styles are linked to behavioral differences in the children raised according to them.  But the direction of cause and effect is not entirely clear.  It might be, for example, that authoritative parents produce well-behaved children, but it is also possible that well-behaved children allow their parents to behave warmly toward them, and set higher standards for them.

A particular variant of the authoritarian parenting style has captured much popular attention lately: the "Tiger Mother" epitomized by Amy Chua, a Yale law professor (and graduate of El Cerrito High School), in her book, Battle Hymn of the Tiger Mother (2011; see also her essay, "Why Chinese Mothers Are Superior", Wall Street Journal, 01/08/2011).  Chua, who never allowed her two daughters to watch television or play computer games, and never accepted any school grade less than an "A", argued that "Chinese parents are better at raising kids than Western ones".  In her view, for example:
  • Western parents are primarily concerned about their children's self-esteem, while "Chinese" parents assume that their children are strong rather than fragile -- for example, demanding good grades because they believe that their children are capable of getting them.
  • Western parents believe they owe their children everything, while "Chinese" parents believe that their children owe them everything.
  • Western parents are too permissive, while "Chinese" parents believe that they know what is best for their children and freely veto their preferences and desires.

I Put "Chinese" in scare quotes because Chua acknowledges that the Chinese "Tiger Mother" is something of a stereotype, and that Tiger Mothers -- and, for that matter, Tiger Fathers -- are to be found in all cultures. But authoritarian parenting is more common in Chinese families than in American families.

Tiger Mothering may be good for children's academic and professional outcomes, but it also has drawbacks, as Kim Wong Keltner, sister-in-law of UCB's Prof. Dacher Keltner, has written in her rejoinder to Chua, Tiger Babies Strike Back (2014)Stephen Chen, working in the laboratory of UCB's Prof. Qing Zhou, has found that Chinese-American children raised by authoritarian parents show, on average, higher levels of anxiety and depression, and poorer social skills, than those raised by American-style authoritative parents (Chen, et al., 2014).  Of more importance, however, is the interaction between parents' and children's cultural orientation -- another example of the parent-child match and relationship-driven effect.  What is particularly deleterious is the mismatch that occurs when parents retain the child-rearing assumption of their "heritage" culture while their children adopt the assumptions of their "host" culture.  Zhou and her colleagues are now running workshops for Chinese-American parents in San Francisco, seeking to persuade parents of the benefits of a blend of "Chinese" authoritarianism (which is good for academic success) and "American" authoritativeness and, maybe, just a little permissiveness (which is good for mental health). 

Relationship-Driven Effects

In statistical terms, we can think of child-driven and parent-driven effects as main effects.  So, with two main effects, come the interaction between them.  Relationship-driven effects have to do with the "fit" between child and parent in terms of appearance and temperament. For example:

  • A quiet child may elicit quite different behaviors from a parent who is also quiet, as opposed to one who is active. In fact, home and school can be quite unhappy for many introverted children, who are pushed by their parents and teachers into social activities and group work, because they think they need to "get out and socialize more".
  • Similarly, an introverted parent may react quite differently to a quiet child, as opposed to one who is active.

Relationship-driven effects are related to the selection mode of the person-by-situation interaction, in that they involve the degree of "fit" between the child and the other people who make up his or her social environment.

For a journalistic account of children whose characteristics did not "match" with those of their parents, either by virtue of some disability (e.g., deafness, Down syndrome, autism, or schizophrenia) or some talent, see Far from the Tree by Andrew Solomon (2012; reviewed by Nathan Heller in "Little Strangers", New Yorker, 11/19/2012; also by Julie Myerson in "Coming Into Their Own", New York Times Book Review, 11/25/2012).  Parents and children adjust to each other, and that's Solomon's point, but it isn't always easy.  For one thing, children may have an identity that is different from their parents.  The parents might be able-bodied,. the child disabled in some way; the parents might be hearing, the child might be deaf; might be straight, but the child gay; the parents might be Catholic, but the child has converted to Islam.  These parent-child differences in horizontal identity can make it hard for parents and children to get along.

An interesting twist on relationship-driven effects is illustrated by a recent behavior-genetic study which shows how child personality can affect parenting behavior (Ayoub et al., Social Psychological and Personality Science, 2018).  In a study employing 1,411 children, twins and triplets, enrolled in the Texas Twin Project, the researchers (using an alternative genetic model to the one discussed earlier in these lectures), first confirmed that there is a significant genetic contribution to the Big Five personality traits -- but also that the nonshared environment accounted for 62-72% of the variance in childhood personality.  But in a variant on the usual twin-study method, the investigators also examined MZ and DZ correlations on parental warmth and stress.  The finding was that identical twins received more similar parenting than fraternal twins did, especially with respect to parental stress variable.  For example, children who scored high on agreeableness received more parental warmth and less parental stress than did those who scored low.  The children elicited different parental behaviors by virtue of personality characteristics that were partly heritable.  Childhood personality accounted for only a relatively small proportion of variance in parenting warmth and stress, but the effect was significant.  As the authors conclude, "parenting is a dyadic and dynamic process, whereby both parents and children influence each other".

Family-Context Effects

Family context effects relate to the children's "microenvironments" within a family. For example, in my family, there was a father, a mother, a girl and two boys. Therefore, my family microenvironment consisted of my parents, my sister, and my older brother. But my brother's family environment was different -- it consisted of my parents (who of course were also his parents), but it also included my sister and me, his younger brother. Similarly, my sister's family environment consisted of our parents and my brother and me. Different people in each environment. Put bluntly, I grew up in an environment that included a very popular cheerleader and a varsity basketball player; my sister and brother didn't. It's the same for every child in every family.


Aspects of the nonshared environment can be classified in many ways, but one that almost defies classification is serendipity -- chance encounters that shape our attitudes and personalities, and almost by definition constitute unique experiences. The word serendipity has its origins in a folktale first related in 1754 by Horace Walpole, and English writer, about three princes of Serendip, or Sri Lanka, who "were always making discoveries, by accident and sagacity, of things which they were not in quest of". In The Travels and Adventures of Serendipity (Princeton, 2004), the late Robert K. Merton and Elinor Barber trace both the origin of the word and the role that serendipity has played in the history of science -- for example, Alexander Fleming's accidental discovery of penicillin. In much the same way that simple chance will lead a scientist one way as opposed to another, simple chance can profoundly affect our lives and the way we lead them.

Birth-Order Effects?

Among the most controversial family-context effects involve birth order -- that is, systematic differences in personality between first-born and latter-born siblings in a family. Because there are no systematic genetic differences between first-borns and latter-borns (all brothers and sisters share a random 50% of their genes in common), any systematic differences between them must be due to their position in the family constellation.

But are there any such systematic differences owing to family constellation?

Until recently, most researchers held that birth-order effects were weak or inconsistent (Schooler, 1966; Ernst & Young, 1983). To be sure, there were occasional studies that demonstrated personality differences between first-borns and latter-borns, but there were lots of confounding variables that made the studies difficult to interpret:

  • By definition, first-borns are older than latter-borns, so any differences between them might be a product of age, not family constellation.
  • Also by definition, birth order is correlated with family size. You can't be a latter-born unless there are at least two children in the family, and you can't be the fifth-born unless there are at least five. Family size, in turn, is correlated with parents' education, occupation, and socioeconomic status. As a general rule, in Western countries at any rate, highly educated, wealthy, professional people have fewer children than poorly educated, poorer, working-class people. There are exceptions, of course: for example, members of the Mormon religion (Latter-Day Saints) are encouraged to have as many children as they can afford. But the fact that family size tends to be negatively correlated with socioeconomic status means that, in most populations, subjects who are first-borns will be from wealthier families, on average, than subjects who are latter-borns (it takes a little while to get your head around this, but you can do it). For that reason, differences between early-borns and latter-borns may be an artifact of differences in socioeconomic status.

Birth Order and Personality

Birth-Order and the "Big Five"None of these problems are intractable, however, provided that your sample sizes are large enough. And there are reasons for being interested in the possibility of birth-order effects on personality. For example, Frank Sulloway (1996), a historian of science who dabbles in evolutionary psychology, has argued that, in Darwinian terms, siblings complete with each other for their place in the family environment -- just like species and organisms compete for their environmental niches in nature.  This is known as the Family Niche Theory.

  • At least among males, Sulloway argues that first-borns have first choice, which makes them more traditional and acquiescent to authority.
  • By contrast, Sulloway argues, latter-borns have to find other ways of distinguishing themselves, making them more egalitarian and anti-authoritarian. From Sulloway's point of view, laterborns are "born to rebel" (which, not coincidentally, is the title of his book on birth-order effects).


As an example of the sort of "competition" process that Sulloway has in mind, consider the practice of primogeniture, quite common among the titled lords and landed gentry in England and elsewhere in Europe. In this practice, the first-born son inherited the father's estate, leaving the other sons to fend for themselves. As the saying went (more or less): the first-born got the title, the second-born son went into the military, and the third-born son went into the church. Parents wanted to marry their daughters off to first-born sons, so that they would not have to provide so much of a dowry (see, for example,Little Women or almost any 19th-century English novel). Note, too, that in royal successions, the crown passes from the king or queen to his or her eldest child (usually, in fact, the eldest son) -- regardless of his abilities or desire for the job (think about the House of Windsor in England: Charles get to be King when Elizabeth dies, while Andrew went into the Royal Navy and Edward became a filmmaker (not exactly the Church, but you get the picture). If the first-born son was disobedient, he could be disowned, and have to work for a living. No wonder, if Sulloway is right, that first-borns were more conservative and obedient to authority!

Sulloway, trained as a historian of science, found that scientists who made revolutionary contributions to their fields tended to be latter-borns. This led him to become interested in the wider psychological literature on birth-order and personality. In fact, a "meta-analysis" (i.e., a form of quantitative literature review that summarizes and aggregates the outcomes of many studies) performed by Sulloway, revealed systematic birth-order effects on personality:

  • Neuroticism: firstborns > laterborns: Firstborns tend to be more jealous, more anxious, more neurotic, more fearful, and more likely to affiliate with others under conditions of stress.
  • Extraversion: firstborns > laterborns: Firstborns tend to be more extraverted, more assertive, and more likely to exert leadership in groups.
  • Agreeableness: laterborns > firstborns: laterborns tend to be more easygoing, more cooperative, and more popular with others.
  • Conscientiousness: firstborns > laterborns: Firstborns tend to be more responsible, more achievement oriented, more organized, and more planful.
  • Openness: laterborns > firstborns: Firstborns are more conforming, more traditional, and more closely identified with their parents' attitudes, beliefs, values, and goals.

              and the "Big Five"Technically speaking, Sulloway counted the number of comparisons on each dimension that gave positive, negative, or null findings with respect to his Darwinian hypotheses. For each Big Five dimension, he found that far more studies supported his hypothesis than contradicted it. Lots of studies yielded unambiguous findings, though, leading to some controversy over his interpretations.

  • Of the total of 196 comparisons, only a minority of the studies (72, or 36.7%) confirmed his hypotheses. However, if you adopt the standard criterion for statistical significance --p < .05, meaning that a result has a 5% probability of occurring randomly, this is more than we'd expect by chance (196 x .05, or 9.8). Conversely, even fewer studies (14, or 7.1%) yielded clearly negative results.
  • Of the 86 comparisons that yielded definitive findings one way or another, the vast majority (74. or 86.1%) were positive, confirming his hypothesis.

The power of Sulloway's meta-analysis comes from the fact that he made his predictions in advance, based on his reading of Darwinian theory. However, his analysis involved a lot of interpretation, and it would be nice to see his findings confirmed in a study expressly designed to test his hypotheses.

Achivers and Rebels in CaliforniaSuch Achivers and Rebels in Canada a study was performed by Paulhus et al. (1999), based on samples drawn from student populations in both California and Canada. Paulhus asked subjects to think about themselves and their siblings, and to nominate who in their family was the "achiever" and who was the "rebel". In both samples, Paulhus found that subjects were more likely to nominate the first-born child in their family as the "achiever", and a laterborn child as the "rebel", than we would expect by chance.

  • For example, if there were two children in a family, we would expect 50% (1/2) of firstborns to be nominated as "achievers", and 50% of laterborns (the remaining 1/2) to be nominated as "rebels", just by chance. On the contrary, in the California sample Paulhus 65% of "achiever" nominees were firstborns, and 61% of "rebel" nominees were laterborns.
  • If there were three children in a family, we would expect 33% (1/3) of firstborns to be nominated as "achievers", and 67% of laterborns (the remaining 2/3) to be nominated as "rebels", just by chance. On the contrary, in the California sample 37% of the "achiever" nominees were firstborns, and 71% of the "rebel" nominees were laterborns.
  • The findings originally obtained in the California sample were subsequently replicated in a Canadian sample.

Birth Order and PersonalityIn two other studies, Paulhus and his colleagues also 044BirthOrderPaulhus4.jpg (59301 bytes)found other respects in which firstborns differed from laterborns. For example, there were more firstborns nominated as the "scholastic achiever" in the family, and more laterborns nominated as the "liberal" in the family, than we would expect by chance. These departures from statistical expectations are sometimes small, but these small effects accumulate to provide significant support for Sulloway's hypotheses.

Note that Sulloway and Paulhus found significant personality differences between first- and laterborn children, but these differences do not necessarily validate Sulloway's "Darwinian" theory. The personalities of first- and laterborn children may differ in significant ways for reasons that have nothing to do with competition for an environmental niche. There may be other expectations. For example, parents may impose their own expectations more strongly on the firstborn, and give laterborns more freedom. Royal families (like the House of Windsor in England) want to produce "an heir and a spare", but once the heir proves up to his assigned job, his younger sibling may be given a great deal of freedom to pursue his own interests. In England, Edward looks like a rebel only because Charles is doing his duty.

Birth Order and Intelligence

Perhaps the most controversial claim about birth-order is that firstborns are more "intelligent", as measured by standard "IQ" tests, than laterborns. It was this hypothesis that was specifically rejected by the Schooler (1966) and Ernst & Young (1983) studies cited earlier. However, a provocative study by Zajonc and his colleagues has revealed an interesting (if small) effect of birth order on general intelligence. These effects, however small, has led Zajonc to develop a confluence model of development that recapitulates the major themes of these lectures:

  • The person is a part of his or her own environment.
  • The child is an agent of his or her own development.

The first study involved an analysis of data collected in the Netherlands as part of a study of the effects of the Dutch famine of 1944 (Zajonc & Markus, 1974). As part of routine testing for the military draft, the Dutch government administered a nonverbal IQ test (Ravens Progressive Matrices) to every Dutch male who reached 19 in the years 1963-1966. Zajonc and Markus then plotted mean IQ scores as a function of both family size and birth order.

The results revealed a significant interaction of birth order with family size on IQ. Specifically:

  • Average IQ declines with family size.
  • Within each family size, average IQ declines with birth order.
  • The last-born child shows a greater decline in IQ than any other birth rank.
  • The rate of decline in IQ diminishes with later birth ranks.
  • An only child has a lower average IQ than the firstborn of a two-child family.

Before we go on, please note that the effects just described are very small. Note the Y-axis on the accompanying figure: the difference between the top and bottom means amounts to only about 10 IQ points. The differences noted above achieve statistical significance only by virtue of the huge sample size involved.

Given the fact that individual differences in IQ are only weakly related to social outcome in the first place, the differences revealed in Zajonc & Markus's analysis are of no practical significance. However, as we will see, they are of considerable theoretical import. Big theories can be built on small effects, and that is as true in psychology as it is in physics.

The Confluence Model

In order to explain the joint effects of family size and birth order on IQ, Zajonc and Markus proposed a confluence model of intellectual development. This model traces the mutual intellectual influences among children, and their parents, as they develop. The major features of the model are as follows:

  • The Dilution Effect: A newborn child effectively diminishes the intellectual resources available within a family. Newborns literally don't know very much, and their lack of declarative and procedural knowledge drags the family down.
  • The Growth Effect: Each child contributes more intellectual resources to the family as he or she grows up. Over time, this growth brings the family average back up.
    • But if more siblings come into the family, each new child is born into a progressively diminished environment. This is an extension of the dilution effect.
    • At the same time, each child in the family is growing up. The intellectual grown of early-born siblings progressively enhances the intellectual environment for the whole family, and counteracts the dilution effect created by the laterborns.

The actual effects of dilution and growth depend on the spacing of the siblings.

    • If siblings are spaced closely together, the dilution effect is increased.
    • If siblings are spaced farther apart, the dilution effect is weakened.
    • In large families, some earlyborns are much older than some laterborns. Therefore, the dilution effect is weakened for these laterborn children.

  • The Teaching Effect: Earlyborns also profit from the presence of laterborns, because they get intellectual stimulation from teaching younger siblings.
  • The Last-Child Handicap: The last-born child doesn't get the benefit of the teaching effect, simply because there are no younger siblings for him or her to teach. Therefore, he or she is at a special disadvantage.
  • The Only-Child Handicap: For the same reason, an only child also doesn't get the benefit of the teaching effect. This puts only children at a disadvantage compared to the firstborns of small families. In a sense, the only child is both a firstborn and a last-born.

The theory makes the interesting prediction that twins and triplets should be even more disadvantaged than only children, because their birth produces a big dilution effect. This is, in fact the case, but of course the outcome depends on details of birth order, spacing, and the like. Some other implications of the theory:

  • Children from single-parent households may be at a special disadvantage, because there is a stronger dilution effect with only one parent in a household. Of course, there may be other adults present, such as grandparents or paramours, who can substitute for the missing parent.
  • Children from extended families may be at a special advantage, because there are lots of adults around to counteract the dilution effect.

A Continuing Controversy

Although I use birth-order effects on personality and intelligence to illustrate the general idea of within-family differences, it has to be emphasized that the effects of birth order are very small in absolute terms.  Judith Rich Harris, for example, thinks that birth order is a relatively unimportant influence on personality.

One thing for sure is that birth-order research is complicated. 

In the first place, we have to distinguish between "between-family" and "within-family" designs like Paulus's.  In between-family designs, the individuals are unrelated to each other.  For example, in the Zajonc and Markus study, all the subjects were 19 years old, so one subject would be a first-born from one family while another subject would be a latter-born from a different family.  Between-family designs are, typically, unable to control for between-family differences such as socioeconomic status. family structure, and number of siblings (though Zajonc and Markus did).  In within-family studies, all these factors are controlled for, except that birth order is necessarily confounded with age: first-borns must, necessarily, be older than latter-borns.  There's no perfect study.

In the second place, birth-order studies are necessarily large, involving a huge number of subjects, and therefore have to make some compromises.  For example, most rely on self-reports of personality, rather than actual behavior (though Zajonc and Markus had objective IQ test data). 

Zajonc's essential findings were confirmed in a sample of 252 799 male Norwegian military draftees (Bjerkdahl et al. Intelligence, 2014).  The results were the same regardless of whether the study analyzed data between families (as Zajonc's study did), or within families (which removed any potential confounds).  Mean IQ declined with birth order -- although, again, the differences were too small to be of any practical significance.

The largest study of birth order to date, by Damian and Roberts (J. Res. Personality, 2015), was a between-family study involving some 377,000 American high-school students who participated in Project Talent, a long-standing, nationwide, longitudinal study, and found that the average correlation between birth order and intelligence was .04, and the average correlation between birth order and personality was .02.  Although these correlations are small in absolute terms, at the level of the population they are not insignificant.  For example, the correlation between aspirin and reduction in heart attacks is actually relatively low, corresponding to a correlation of .034 (Rosenthal, 1990) -- yet, on a population level, the correlation is high enough that physicians typically put patients at risk for heart attacks on an aspirin regimen, and the first thing you should reach for if you're having a heart attack is the aspirin bottle. 

Another large-scale study, involving three multinational samples totaling more than 20,000 subjects, confirmed a small correlation between birth-order and IQ (Rohrer et al., 2015).  There were no correlations between birth order and personality, as measured by the Big Five, except for openness to experience, where there was a small effect of both order on the "intellect" facet only, paralleling the effect on objectively measured intelligence.  This study was notable for its inclusion of both between-family and within-family analyses.  

Getting back to birth order, nobody says that first-borns are necessarily smarter than latter-borns, or that they're more conscientious and less rebellious.  You have to look at the family dynamics in question.  The trends that we see at the population level, small as they are (and they are very small), nevertheless illustrate a larger point about within-family differences.  There are lots of other potential within-family differences besides birth order.

Like the question of birth order effects itself, Sulloway's work is highly provocative, but it is also highly controversial -- not just for the Darwinian interpretation he puts on his findings, but also with respect to the findings themselves. For critiques of Sulloway's work, see:

For another view of within-family differences, see The Pecking Order: Which Siblings Succeed and Why by Dalton Conley (Pantheon, 2004).

The confluence theory tells only part of the story of intellectual development . There are lots of other things going on, both genetic and environmental. But the major assumptions of the theory illustrate the basic points made throughout these lectures on personality, social interaction, and psychological development:

  • The individual is a part of his or her own environment.
  • The environment itself is dynamically changing as different individuals enter and leave it.
  • The individual is constantly influenced by these environmental changes.
  • The individual reciprocally influences the environment by virtue of his or her own changes, regardless of where these changes come from.

The person is a part of his or her own environment.

The person is an agent of his or her own development.

The nonshared environment is such a powerful force in personality development because everyone creates a unique environment for him- or herself, in interaction with other people, through the various modes of person-by-situation interaction:

  • Evocation;
  • Selection;
  • Manipulation;
  • Transformation.

The uniqueness of the individual's environment is the sum total of many different effects, acting alone and in combination. And the uniqueness of the environment, as it is shaped by the individual him- or herself, in turn contributes to the uniqueness of the individual.

There is a sort of paradox in development, which is that the universal processes of development are universal, but these widely shared processes come together in such a way as to produce a unique individual.

in Psychological TermsThe implications of these interactions for development are profound, because they mean that, in psychological terms:

Every child is born to different parents, raised in a different family,

lives in a different neighborhood, attends a different school, and worships in a different church.

In medieval times, philosophers debated the idea of contingentia mundi -- the idea that the world world we actually live in is not the only one that is possible.  And we now know, scientifically, that this is the case: if that comet had not struck the Earth, killing the dinosaurs, things would have been very different for us humans (and other mammals!).  What goes for the Earth goes for the individual as well.  The people we are are not the only ones that are possible.  Each of us is contingent: we have been shaped by a whole host of forces, each of which could have been different from what it was; and we have shaped ourselves, and each of us could have made different choices than the ones we made.  Personality, like the world itself, is contingent.

Consider the Snowflake...

The individual's unique personality is shaped by its environment, which in turn is shaped by the person, in a complex dynamic system. If you're looking for a metaphor for this process, you might consider the snowflake, as Adam Gopnik did in the following comment, which appeared in The New Yorker ("All Alike", 01/03/2011).

In the cold, thoughts turn to snowflakes, heralds of winter.For the past three decades, at this time of year, a twinkling snowflake has been hoisted above the intersection of Fifth Avenue and Fifty-seventh Street. It's a giant, galumphing thing, which makes the crossroads of the world resemble the main intersection of a Manitoba town. Closer[to the New Yorker offices], Starbucks on Forty-seventh and Sixth, even has a sign that reads, "Friends are like snowflakes: beautiful and different". This thought seems so comforting, so improving and plural-minded, that one begins to wonder whether it is truly so. are snowflakes really different--or, rather, how different are they, really?

A quick trip to the New York Public Library and a few request slips (and, let it be said, a little Googling) later, one arrives at the compelling figure of Wilson (Snowflake) Bentley, the great snowflake-ologist, hero of the best movie Frank Capra never made. Bentley was a Vermont semi-recluse who had a lovely and inexplicable devotion to snow. In 1885, at the age of nineteen, he photographed his first snowflake, against a background made as dark as black velvet.... Bentley, over his lifetime, took portraits of five thousand three hundred and eighty-one snow crystals (to give them their proper scientific name; flakes are crystals clumped together) and inserted into the world's imagination the image of the stellar flower as the typical, "iconic" snowflake, along with the idea of a snowflake's quiddity, its uniqueness.

It turns out, however (a few more slips, a bit more Googling), that Bentley censored as much as he unveiled. Most snow crystals -- as he knew, and kept quiet about -- are nothing like our stellar flower: they're irregular, bluntly geometric. They are as plain and as misshapen as, well, people. The Fifth Avenue snowflakes are the rare ones, long and lovely, the movie stars and supermodels, the Alessandra Ambrosios of snow crystals. The discarded snowflakes look more like Serras and Duchamps; they're as asymmetrical as Adolph Gottliebs, and as jagged as Clyfford Stills.

But are they all, as Starbucks insists, at least different? Another flurry of catalogue searching reveals a more cheering, if complex, truth. In 1988, a cloud scientist named Nancy Knight (at the National Center for Atmospheric Research -- let's not defund it) took a plane up into the clouds over Wisconsin and found two simple but identical snow crystals, hexagonal prisms, each as like the other as one twin to another, as Cole Sprouse is like Dylan Sprouse. Snowflakes, it seems, are not only alike; they usually start out more or less the same.

Yet if this notion threatens to be depressing -- with the suggestion that only the happy eye of nineteenth-century optimism saw special individuality here -- one last burst of searching and learning puts a brighter seasonal spin on things. "As a snowflake falls, it tumbles through many different environments," an Australian science writer named Karl Kruszelnicki explains. "So the snowflake that you see on the ground is deeply affected by the different temperatures, humidities, velocities, turbulences, etc, that it has experienced on the way." Snowflakes start off all alike; their different shapes are owed to their different lives.

In a way, the passage out from Snowflake Bentley to the new snowflake stories is typical of the way our vision of nature has changed over the past century: Bentley, like Audubon, believed in the one fixed image; we believe in truths revealed over time -- not what animals or snowflakes are, but how they have altered to become what they are. The sign in Starbucks should read, "Friends are like snowflakes: more different and more beautiful each time you cross their paths in our common descent." For the final truth about snowflakes is that they become more individual as they fall -- that, buffeted by wind and time, they are translated, as if by magic, into ever more strange and complex patterns, until, at last, like us, they touch earth. Then, like us, they melt.

The metaphor isn't perfect, because the environment acts on the snowflake, but the snowflake doesn't really act on the environment. But it's  not a bad start. the uniqueness of the individual's personality is a product of the individual's interaction with the environment, chiefly the nonshared environment -- an environment that is very much the individual's own making.  

Gender Dimorphism

Aspects of Gender DimorphismThe interaction of nature and nurture can be seen when we look at the development of gender dimorphism:

  • gender identity (one's sense of oneself as male or female);
  • gender role (the person's adoption of characteristically masculine or feminine aspects of thought and action); and
  • erotic orientation, also known as sexual orientation or sexual preference (as heterosexual, homosexual, bisexual -- or, for that matter, not sexual at all).

              Imperatives of Gender Some role differences, known as the procreative imperatives, are built into us by our biology:

  • impregnation in males,
  • menstruation, gestation, and lactation in females.

But gender role goes beyond the demands of reproduction to include, at least in this culture:

  • typically masculine characteristics of agency and instrumentality; and
  • typically feminine characteristics of communality and expressiveness.

It is now very clear that this developmental process is not a simple matter of genetic determination. Rather, it reflects a complex interaction between genetic/biochemical processes (the phyletic imprimatur) and social/environmental processes (the social imprimatur) -- an interaction that is made more complex by the fact that

the developing child is both a target and an instigator of his or her own development.

Gender Differentiation in Fetal Development

              Human GenomeAs noted earlier, the normal human cell possesses 46 chromosomes, arranged in 23 pairs. Two of these, making up a single pair, are the sex chromosomes, known as X and Y. Normally, males carry one X and one Y chromosome (XY), while females carry two X chromosomes (XX). Genes for sex-linked traits are located on these sex chromosomes. Note that because each parent contributes one chromosome of each pair to his or her offspring, and the mother can contribute only X chromosomes to this process, in the final analysis the father determines the sex of his child: if he contributes an X chromosome, the child will be genetically female (XX); if he contributes a Y chromosome, the child will be genetically male (XY).

              Program for Gender DimorphismAlthough the fetus is genetically male or female from the beginning (because its cells carry the XX or XY chromosome pairs from the beginning), early in gestation the fetus is otherwise undifferentiated with respect to gender. That is, although it carries the XX or XY chromosomal endowment, it has no outward appearance of being male or female. This is because at this stage the fetus's gonadal tissue is undifferentiated. . Remember the debate between recapitulation and differentiation as basic themes in development? In these terms, the undifferentiated gonadal tissue of the early fetus is a primordial structure which will eventually differentiate into the more complex structures representing the male and female reproductive anatomy).

Undifferentiated Gonadal Tissue Primordial Structure
              (Anlagen)In technical terms, the structures in this undifferentiated gonadal tissue contain the anlagen (or foundation) of the male and female reproductive systems:

  • the outer cortex will become the ovaries of the female;
  • the inner medulla will become the testes of the male;
  • the Mullerian ducts will become the internal reproductive organs of the female -- the uterus, fallopian tubes, and inner portion of the vagina;
  • the Wolffian ducts will become the internal reproductive organs of the male -- the vas deferens, seminal vesicles, and ejaculatory ducts; and
  • the genital tubercle, situated above a single urogenital slit (itself surrounded by urethral folds and labio-scrotal swellings will become the external genitalia: the vagina and clitoris of the female, the penis and scrotum of the male.

After about six weeks of gestation, sexual differentiation begins. In response to genetic messages (carried on the X and Y chromosomes), one set of structures begins to develop while the other one becomes vestigial. If the fetus carries the XY genotype, the inner medulla will grow into the testes of the male, and the outer cortex regresses; if the fetus carries the XX genotype, the outer cortex will grow into the ovaries of the female, while the inner medulla vestigiates.

"Nature's Rule": Add Something to
              MasculinizeThe genes themselves appear to play no further role in what happens. Rather, further sexual differentiation occurs by virtue of hormones secreted by the gonads -- and in particular, those male hormones secreted by the testes. There may be a role for the female hormones in genetically XX fetuses, but this is not clear at present. As a rough approximation, further sexual differentiation appears to reflect what the biologists call "nature's rule":

add something to masculinize.

Without masculinization instigated by the male gonadal hormones, the remaining gonadal tissue will naturally differentiate into the female reproductive system. So, in a sense, at this point the program for sexual dimorphism passes from the genes to the hormones.

Simone de Beauvoir and The Second Sex

In 1949, Simone de Beauvoir (1908-1986), the French writer and existentialist philosopher (and longtime companion of Jean-Paul Sartre), published a book,The Second Sex (1949; English edition 1953), which is rightly regarded as instigating the feminist revolution of the 1960s (these things take time: Mary Wollstonecraft published The Vindication of the Rights of Woman in 1792, and the feminist revolution among middle-class women in the United States didn't really begin until Betty Friedan, who had once been a graduate student in psychology at UC Berkeley, published The Feminine Mystique in 1963).

In her book, de Beauvoir begins with, and details the various ways in which, throughout history and across cultures, women have been relegated to subordinate status. For example, in the Genesis myth, Eve was created from one of Adam's ribs, as a kind of afterthought by God. Closer to our own time, Freud held that women were diminished men (that's why he thought they were obsessed with penis envy). As de Beauvoir put it, so far as history and culture is concerned, man was the essential "Subject", an "Absolute"; woman the inessential "Other". In expressing the fundamental doctrine of existentialism, Sartre had written that "Existence precedes essence". Similarly (in perhaps her most famous passage), de Beauvoir wrote "One is not born, but rather becomes a woman". For de Beauvoir, there is no "essence" to womanhood or femininity; the details of gender role are imposed on the individual by the culture, and can be accepted or declined by the individual as a matter of free choice. Of all the many good books of feminist theory and doctrine, de Beauvoir's remains perhaps the most thorough and convincing, but in a sense she got the title wrong. Biologically speaking, anyway, the female is the "first" sex.

A Note of Caution: It turns out that the only English edition of de Beauvoir's book is seriously deficient, with many technical words and phrases simply mistranslated, and large sections of the French original simply cut out. The problem is that the editor who bought the English rights thought that she was buying a sort of French sex manual, and the person who took responsibility for the translation was a zoologist whose knowledge of philosophy was practically nonexistent and whose knowledge of French dated from high school and college. You get the gist, especially if you already know something about the argument (or de Beauvoir, or existentialism), but if you read the book very closely important parts of it don't really make sense, which does a disservice to the quality of de Beauvoir's thought and writing. See "Lost in Translation" by Sarah Glazer,New York Times Book Review, 08/22/04).

The hormones secreted by the testes have effects on other structures in the initially undifferentiated gonadal tissue (again, there may be independent effects of female hormones in genetically XX individuals, but this is a controversial point in endocrinology).

  • In the third month of gestation, a Mullerian inhibiting substance appears to stop the development of the Mullerian duct system (I say "appears to" because the MIS is at present known only by inference -- we know this happens, but we don't exactly know what does it). At the same time, fetal androgen promotes the development of the Wolffian duct system into the male internal reproductive system. In the absence of MIS and androgen, the Mullerian ducts develop into the female internal reproductive system.
  • In the third and fourth months of gestation, we observe more effects of fetal androgen. The genital tubercle forms around the urethra into a penis rather than a clitoris; and the labio-scrotal swelling fuses into a scrotum rather than a vagina. Again, in the absence of this dose of androgen, these structures will develop into the clitoris and vagina of the female.
  • In successive months, the vaginal canal will connect the external and internal reproductive anatomy of the female.
  • In the seventh month of gestation, the testes descend from the abdomen into the scrotum of the male.

Outcome of
              Fetal Gender DifferentiationWhen everything goes as programmed, after nine months of gestation a human baby is born with a set of external genitalia that are recognizably male or female, and a corresponding set of male or female internal reproductive organs.

Anomalies of Gender Differentiation

PseudohermaphroditismBut sometimes things don't run quite the way they're programmed, and the child is born sexually ambiguous -- individuals who are known technically as pseudohermaphrodites.

Chromosomal XX Individuals. If a genetic female somehow experiences an environment to which androgen has been added, she will be born with female internal genitalia, but most likely an enlarged clitoris and fused vaginal labia; rarely, such a girl will be born with a normal penis and scrotum (of course, the scrotum will be empty, because there are no testes to descend into it). This occurs in two principal ways.

  • In the female adrenogenital syndrome, there is a natural failure of the adrenal glands to function properly, resulting in the circulation of androgen to a fetus that is genetically female. There are no effects on the internal reproductive anatomy, but the external genitalia are masculinized. These children receive surgical correction of the external genitalia. At puberty (because they have malfunctioning adrenal glands) they also receive cortisone therapy to counter the adrenal failure. As a result of this therapy, the girl develops a characteristically feminine physique, menstruates, and can conceive and bear children.
    • The female adrenogenital syndrome is also known as congenital adrenal hyperplasia.  It is very rare, but sometimes pregnant women whose fetuses are at risk for FAS/CAH are prescribed dexamethasone, a steroid drug, to prevent the condition.  Although this drug is often effective, such prescriptions are currently "off-label", and not approved for this purpose by the Food and Drug Administration.  
  • In progestin-induced pseudohermaphroditism, a pregnant woman (with a personal or family history of difficult pregnancy) receives synthetic hormones to prevent miscarriage. In some cases, the hormone treatment results in a masculinization of the external genitalia, which is corrected surgically. Because there is no problem with the endogenous hormones, there is no need for cortisone therapy to feminize the physique or induce menarche.

In both cases, the children are raised as girls.

Chromosomal XY Individuals. In a genetic male, the failure of the Mullerian-inhibiting substance can leave the fetus with a set of male external genitalia, but both the female and male internal reproductive systems. Note, however, that such an individual has only testes (the gonadal tissue becomes either testes or ovaries); thus, he cannot menstruate or gestate. The children are raised as boys.

  • In the androgen-insensitivity syndrome, a genetic defect causes the androgen which circulates naturally to the male fetus to have no effect. The result is that the child is born without male external genitalia (except, perhaps an enlarged clitoris). Following surgical correction, including removal of the testes, the children are raised as girls. At the time of puberty, natural estrogen (which circulates to males as well as females, but which is suppressed by androgen in hormonally normal males) feminizes the physique. However, because these girls do not possess the internal reproductive anatomy of females (under genetic control, the inner medulla differentiated into testes while the outer cortex regressed, and the Mullerian inhibiting substance works even if the androgen does not!), they have no ovaries. Therefore, they will not menstruate, and will be infertile.

Guevodoces. An interesting syndrome, originally discovered in an isolated area of the Dominican Republic (but also documented in an isolated village in Puerto Rico), involves genetically male individuals (chromosomal XY) who are born with a particular defect in their androgen system known as 5-alpha-reductase deficiency syndrome. Because they do not undergo masculinzation in utero, these children are born with apparently female external genitalia; if the condition is undiagnosed, they are raised as girls. At puberty, however, the flow of natural testosterone induces masculinization: the voice deepens, the child develops a typically masculine muscular structure, breasts do not develop as expected and -- surprise! -- the child's scrotal tissue balloons, testes descend, and what originally appeared to be a clitoris enlarges into a functioning penis. Hence the popular name for this condition --guevodoces.

What's especially interesting about this syndrome is that the children readily shift their gender identities, and corresponding gender roles, from feminine to masculine. This is because their culture is prepared for the possibility of a spontaneous "sex change" from previous cases known in the village -- it's as if they say "Oh, that happened to Uncle Jose, too!". The boys' adolescent and adult behavior, including sexual behavior, is not are appreciably different from that of "normal" males.

Before the arrival of modern medicine, the condition went undiagnosed until adolescence. Now, the condition is diagnosed at birth, either through chromosomal testing or through palpation of the groin (which reveals the undescended testes), and the children are identified and raised as boys, from birth -- appearances to the contrary notwithstanding.

Jeffrey Eugnedies' novel Middlesex (Farrar, Straus & Giroux, 2002; he also wrote The Virgin Suicides) uses the fictional life of Cal (nee Calliope) Stephanides, a Greek-American with "guevodoces syndrome", as a metaphor for the identity crises of immigrant, "hyphenated" ethnic Americans, as well as places "like Berlin, like Korea, like all the other places in the world that were no longer one thing or the other".

Klinefelter's Syndrome, 47 XXY. In another condition, a chromosomal male has an extra X chromosome, thus 47XXY rather than 46XY (this occurs in fewer than 1/500 male births). Common consequences include "feminized" physique, infertility, delayed motor and speech development, difficulties with reading and writing. As adults, these individuals often gradually lose both sexual potency and interest. Many of these symptoms can be reversed with hormone replacement therapy, replacing the testosterone that is missing naturally.

These anomalies of gender differentiation sometimes raise the question of gender identity: is a person male or female? And not just a question about how the individual identifies him- or herself -- but also issues of how s/he is identified by other people. Because of previous gender-related controversies -- including the fact that, in the 1936 Berlin Olympic Games, the Germans cajoled a male athlete, Hermann Ratjen, into living as a woman for three years before entering "her", renamed Dora, into the high jump competition ("she" lost); and the practice, in certain countries of Communist Eastern Europe, of doping female athletes with testosterone and other steroids in order to enhance their performance -- in the 1970s the International Olympic Committee began testing female athletes to confirm their "femaleness" by inspecting their chromosomal material for the presence of a telltale X chromosome. But Stella Walsh, a Polish sprinter who also competed in the 1936 Olympic games, apparently suffered from androgen-insensitivity syndrome -- although chromosomally male, she identified herself as a woman and had lived as a woman all her life. Under current rules, she would have been disqualified from competition. At the same time, since 2004 the IOC's rules have allowed transsexual women -- that is, chromosomally male individuals who identify themselves as women and have undergone sex-reassignment surgery and post-operative hormone-replacement treatment -- to complete as women (though as of 2008, no openly transsexual individuals have qualified for the competition). Like Stella Walsh, these individuals would also have failed a chromosomal test of gender [see "The XY Games" by Jennifer Finney Boylan,New York Times 08/03/08.].

              Five Sexes... And More?All of which raises the questions: What is the proper criterion for being male or female? Chromosomal sex? Body morphology? Gender identity?  And how many categories of gender are there, anyway?  The usual two? Or are there at least two more, to cover conditions like androgen-insensitivity?

The Case of Caster Semenya...

The issue of gender ambiguity and gender determination crops up in athletics from time to time. Consider, for example, the case of Caster Semenya, a South African athlete who won the Gold Medal in the women's 800-meter race at the World Track and Field Championships, held in Berlin, August 2009. She beat the previous world record by a full two seconds, after which some other competitors, and their coaches, revived the question, which had long been on many observers' minds, of whether she was, in fact, female. The IAAF, the sport's governing body, conducted an extensive series of biological and psychological tests, and decided that she could continue to compete as a woman. However, the Federation did not release the actual results of the testing -- an admirable (and ethical) protection of Semenya's privacy rights, but a move that deprived her competitors of the right to understand the ruling, professionals an opportunity to scrutinize the tests themselves and their findings, and the public at large of a "teachable moment" to learn about the complexities of gender dimorphism.

Semenya took some time off from elite competition, but returned to the track in August 2010, again winning her event, though by less of a margin than in 2009. Still, the questions persisted, and it probably would have been better, in the long run, if the Federation had explained its ruling in more detail. Diane Cummins, one of Semenya's competitors in the 2010 meet, suggested that the issue will not go away for a while:

"We have levels that we are not allowed to test over, so even if she's a female, she's on the very fringe of the normal female athlete biological composition from what I understand in terms of hormone testing. So from that perspective I think most of us sort of just feel like literally we are running against a man because what we know to be female is a certain testosterone level. And if that isn't the case, they need to change everything" [quoted in "Semenya Returns, and so Do Questions", by Christopher Clarey,New York Times, 08/23/2010).

Semenya competed for South Africa in the 2012 Summer Olympic Games, in London, carrying her country's flag in the opening ceremony, and taking home a silver medal.

For a good article on the Semenya case, see "Either/Or: Sports, Sex, and the Case of Caster Semenya" by Ariel Levy, New Yorker, 11/30/2009.

See also "On the Basis of Testosterone" by Grace Huckins, Scientific American, 02/2021.

Current Olympic standards employ a cutoff of 10 nanomoles of testosterone per liter of blood, but there is still some overlap, with some elite female athletes falling within normal limits (remember "The Rule of 66, 95, and 99") for males, and vice-versa.  In 2018, facing increasing challenges from female athletes with excessively high levels of natural testosterone (a condition called hyperandrogenism), the International Association of Athletics Federations issued a new standard for women competing in certain track and field events where high levels of testosterone (between 5 and 10 nm/l) give women a clear advantage compared to women with lower levels.  Such athletes must keep their testosterone levels within the "normal" range of 0.12 to 1.79 nm/l, compared to the normal range for males of 7.7 to 29.4 nm/l (remember 95% confidence intervals?) -- employing hormone therapy, essentially doping with estrogen (which itself may not be healthy), if necessary.  If they cannot keep within these standards, they will be disqualified from international competition, or be required to compete against men, or to change to events that are not covered by the new rule.

Perhaps a better standard is to have no standard at all, but simply to distinguish between natural testosterone and the artificial variety which might be used in doping (they're chemically different).  Males, after all, aren't disqualified for having high testosterone -- why should females?  Maybe you should only be disqualified for doping.  But isn't taking hormone therapy to suppress one's natural levels of testosterone a form of doping? 

On the other hand... there's the other hand.  Which is to say that this is one of those controversies that distinguishes between sex as a biological category, and gender identify and role as psychosocial categories.  Although the ovaries do release some small amount of natural testosterone, pretty much the only way to get really high levels of natural testosterone is to be be endowed with testes, and the only way to be endowed with testes is to be genetically XY.  And really high levels of natural testosterone (i.e., in the typically male range) have consequences for other "secondary" sex characteristics, such as increased strength, muscle type and mass, heart size, oxygen carrying capacity of the blood, and muscle-to-fat ratio -- and a 10-12% performance gap favoring males in track and field.   For this reason, Doriane Lambelet Coleman, a legal scholar who herself was a world-class competitor in track and field, has suggested that the IAAF is on the right track.  She writes ("Sex, Sport, and Why Track and Field's New Rules on Intersex Athletes are Essential", New York Times, 05/01/2018):

Advocates for intersex athletes like to say that sex doesn't divide neatly.  That may be true for gender studies departments, but at least for competitive sports purposes, they are simply wrong.  Sex in this context is easy to define and the lines are cleanly drawn: You either have testes and testosterone in the male range or you don't....  Pick your body part, your geography, and your socioeconomic status and do your comparative homework.  Starting in puberty there will always be boys who can beat the best girls and men who can beat the best women.  Because of this, without a women's category based on sex, or at least these sex-linked traits, girls and women would not have the chance they have now to develop their athletic talents and reap the many benefits or participating and winning in sports and competition.  Eric Vilain, a geneticist who specializes in differences of sex development, has been blunt about it: removing sex from the eligibility rules would "be a disaster for women's sport... a sad end to what feminists have wanted for so long".

In any event, in 2019 the Court of Arbitration for Sport, a kind of Supreme Court for the athletic world, ruled that, henceforth, women like Semenya, who have very high testosterone levels, cannot compete in middle-distance races, such as the ones she excels in, unless they take medication to reduce their testosterone levels to the "normal" range for women.  Semenya herself sees this as, essentially, a bill of attainder -- a ruling specifically enacted to apply to her ("Caster Semenya, Hero in South Africa, Fights Hormone Testing on a Global Stage" by Karen Zraick, New York Times, 05/02/2019). 

In 2020, in the run-up to the Tokyo Olympics (rescheduled to 2021 because of the Covid-19 pandemic), Human Rights Watch, a non-governmental organization, issued a 120-page report on athletes like Semenya with differences of sexual development (DSDs), demanding that sex testing of female track and field athletes be stopped see "Rights Group Demands End to Sex Testing of Fmale Track Athletes" by Jere Longman, New York Times, 12/05/2020).

...and Johnny Saelua...

An athlete who has gotten much less attention, not least because he plays for a team that perennially loses, is Johnny Saelua, a soccer player from American Samoa. He's what's known in Polynesian culture as a fa'afafine -- a "third sex" of biological males who identify themselves as female. Unlike transgender individuals in the West, however, Polynesian culture readily accepts this gender-related category. Still, there are some difficulties: Saelua plays for the men's team (which won a World Cup match for the first time in 2011). Still, as Saelua told the Sydney Morning Herald,

''When I go out into the game, I put aside the fact that I'm a girl, or a boy, or whatever, and just concentrate on representing my country,'

...and Bradley/Chelsea Manning...

In August 2013, Army Pfc. Bradley Manning was convicted on charges of leaking vast quantities of US government secrets and sentenced to 35 years in prison.  Manning immediately announced that he ha always felt that he was "female", and that his transgender status had caused him to experience considerable amounts of emotional stress during two deployments in the war in Iraq.  Henceforth, he asked to be addressed as "Chelsea" Manning.  He also asked to receive hormone therapy to help him achieve a more feminine physique.  The Army has no provisions for transgender soldiers, and neither military nor civilian prisons typically provide hormone therapy for transgender prisoners ("'I am a Female,' Manning announces, Asking Army for Hormone therapy" by Emmarie Huetteman, New York Times, 08/23/2013).

The case also raised issues of journalistic practice: given that Private Manning considers himself to be a woman in a man's body, what pronoun should they use.  The public editor for the New York Times argued that, henceforth, the Times should refer to Pvt. Manning with "she" ("'He'? 'She'? News Media Are Encouraged to Change" by Christine Haughney, New York Times, 08/23/2013).

...and Andrej/Andreja Pejic...

For several years, beginning in 2010, the Australian fashion model Anrej Pejic cultivated a distinctly androgynous look modeling menswear for several European designers, including Marc Jacobs and Jean Paul Gaultier -- and, for that matter, modeling bridal wear for Gaultier as well.  What Pejic did not tell people was that he was transgender, taking synthetic hormones to suppress his masculine development.  In 2014, Andrej completed sex-reassignment surgery, and now seeks work as a women's fashion model under the name Andreja.  For more, see "Will the Fashion World Accept Andreja Pejic as a Woman?" by Matthew Schneier, New York Times Style Section, 09/07/2014).

...and Bruce/Caitlin Jenner...

In 1976, Bruce Jenner went on the Wheaties box after winning the gold medal in the 1976 Olympic Decathlon.  He subsequently married into the Kardashian family, of reality-TV fame ("Keeping Up With the Kardashians"), having two children with his wife, Kris.  In 2015, after years of rumors about his changing, more femininized appearance, Jenner announced that he was transgender and renamed himself Caitlyn.  Apparently, he had experimented with cross-dressing and hormone replacement therapy before Marrying Kardashian; they separated in 2013 and divorced in 2015, citing "irreconcilable differences", before Jenner's announcement; still, he remains part of the extended Kardashian clan). The widespread publicity given to Jenner's announcement, including an interview on ABC's 20/20 program with Diane Sawyer, a two-part special of Keeping Up With the Kardashians, a fashion spread in Vanity Fair magazine, and an eight-part TV documentary planned for July 2015), and the overwhelmingly positive reaction to Caitlyn's announcement, marks the widespread public acceptance of transgender individuals.

...and Jonas and Wyatt/Nicole....

In many ways, interest in sex-reassignment and transgender individuals began with the famous John/Joan case presented by John Money.  You remember, John lost his penis during a botched circumcision, and the decision was made to raise him as a girl, along with his identical twin brother Brian -- a decision that did not work out as planned.  The John/Joan case has an interesting parallel, with -- apparently -- a much better outcome: the case of Nicole Maines, recounted in Becoming Nicole: The Transformation of an American Family by Amy Ellis Nutt (2015).  Image: New York Times, 10/22/2015).  Whereas John lost his penis accidentally, and never wanted to become Joan, Nicole was clear from a very early age: at the tender age of 3, Wyatt told his father, "I hate my penis".  Later, though still a child, Wyatt hit his identical twin brother Jonas: when asked why, he said "Because he gets to be who he is and I don't".  Later, with support from her parents (and her brother), Nicole sued the state of Maine for sex discrimination, because she was not allowed to use a women's rest room at school, and used the award to pay for gender-reassignment surgery.

...and John...

One of the most interesting cases of gender fluidity appeared in the "Ask Amy" personal advice column, written by Amy Dickinson and syndicated by the Los Angeles Times, published in the East Bay Times (03/09/2013).  The writer's gay son had married a transsexual man who had not had surgical correction of his (female) genitalia, and had become pregnant.  This case makes it clear that we have moved way beyond the Manichean dichotomies of male and female, masculine and feminine, heterosexual and homosexual.

...and Pauli...

Anna Pauline ("Pauli") Murray (1910-1985) was a pioneer in both the civil rights and women's movements.  Born in North Carolina, she graduated from Hunter College (Jim Crow laws kept her out of the UNC), and as a law student at Howard (Harvard Law rejected her, because at the time it did not admit women, though she did obtain a master's degree in law from UC Berkeley) devised the legal strategy that Thurgood Marshall and other top civil-rights attorneys employed in Brown v. Board of Education (1954) to overturn its notorious decision in Plessy v. Ferguson (1896) that permitted racial segregation in public facilities.  A friend of Langston Hughes and Eleanor Roosevelt, Murray was one of the founders of the National Organization of Women.  She was the first African-American to receive the doctoral degree in jurisprudence from Yale Law School -- and, later, the first African-American woman to be ordained a priest in the Episcopal Church.  A residential college at Yale is named for her, and her childhood home in Durham, North Carolina, is listed on the National Register of Historic Places.  And it may well be that Murray was also what we would now call transgender.  She was sexually attracted to women -- which, at the time, was enough for her to be labelled as "lesbian".  But given the state of medical and psychological knowledge at the time, that was pretty much the only option.  But Murray saw herself as a blend of genders.  "Maybe two got fused into one with parts of each sex, male head and brain (?), female-ish body, mixed emotional characteristics", "one of nature's experiments; a girl who should have been a boy", "very natural falling in love .e., as a man] [with the female sex".  As if to confirm the diagnosis, Murray herself sought hormone treatments; and when she underwent an appendectomy, she instructed the surgeon to look for signs of male internal genitalia.  For more information, see The Firebrand and the First Lady by Patricia Bell-Scott (2016) and Jane Crow: The Lie of Pauli Murray by Rosalind Rosenberg (2017; reviewed in "Saint Pauli" by Kathryn Schulz, New Yorker, 04/17/2017, from which the quotations and image are drawn).

...and Lisa Davis's Daughter (a Cautionary Tale)...

It's important not to confuse gender role with gender identity.  These days (I'm writing , 04/19/2017this in 2017), with all the media attention on transgender individuals (e.g., Caitlin Jenner), and the implications of the increasing visibility of transgender individuals for public policy (e.g., bathroom laws), it's sometimes tempting to assume that someone who does not conform to traditional gender roles must be either homosexual or transgender.  But this isn't true, not by a long short.  Most boys and men have a male gender identity and adopt a masculine gender role, just as most girls and women have a female gender identity and adopt a feminine gender role.  Lisa Selin Davis has the assumption of many people, including teachers and physicians, as well as longtime family acquaintances, that her unnamed, 7-year-old, admittedly "tomboyish" daughter is transgender ("My Daughter is Not Transgender.  She's a Tomboy", New York Times, 04/19/2017).  One of her teachers -- this is a 7-year-old --put it this way: "I just wanted to check.  Your child wants to be called a boy, right?  Or is she a boy who wants to be called a girl?  Which is it again?".  Davis points out that her daughter is not gender nonconforming, but rather is gender role nonconforming.  She's what used to be called, as still is called, a "tomboy".  She's very clear about being a girl.  She's just not a girl the way many other girls are girls.  As Davis puts it (and the same goes for boys):

"The kids get it.  But the grown-ups do not.  While celebrating the diversity of sexual and gender identities, we also need to celebrate tomboys and other girls who fall outside the narrow confines of gender roles.  Don't tell them they're not girls."

...and Alicia Roth Weigel's Surgery (a Cautionary Tale).

Earlier in these lectures, I discussed cases of children born with ambiguous genitalia, who underwent gender-reassignment surgery at a very young age, and in some cases were not informed of this fact until much later in childhood.  A first-person account of this experience was provided by Alicia Roth Weigel, a genetic male (XY) with complete androgen insensitivity syndrome ("Intersex, and Erased Again", New York Times, 10/24/2018). 

Imagine knowing that every aspect of your physiology, from your height to your cup size, was chosen off a menu — not by nature but by doctors and family members.

From the second I was born, decisions were made by medical professionals about which of two gender categories my body should fit into. For me, surgery to remove my gonads as an infant was the first stop on the track to female — but the train didn’t stop there. My family was consulted about how 5 feet 8 inches seemed like an optimal height, and informed on how hormone levels and sequences could be measured to achieve just that. The ideal breast size for my frame was also discussed; I can still remember the male doctor nodding approvingly. I was also given a dilator before even hitting my teens, so my vagina would be ready for penetrative sex.

“Disconcerting” would be one — euphemistic — way to put it.

I was born intersex, with XY chromosomes but Complete Androgen Insensitivity. If you’re not sure what that means, I don’t blame you. By some estimates, almost 2 percent of the world’s population is intersex like me but is still living in the shadows because of societal stigma and shame. Stigma knows no borders, and neither did my body, apparently: I didn’t respond to androgen hormones in the womb, and thus stopped developing at a certain point — a point between what we consider to be the binary sexes, hence “intersex.” I was ultimately born with female anatomy on the outside but with internal testes instead of ovaries. As a result, doctors, alongside my parents, decided when I was still a baby that I would be raised as a girl. This decision has shaped the course of my entire life but was made without my consent.

The gonadectomy surgery performed on my body was internal but opened the floodgates for a sequence of physical alterations that would affect my appearance and identity. Any subsequent decisions made about my body that involved me, at an age of informed consent, were constrained by this first choice: to render me traditionally female. Regardless of my liminal genetic code — or rather, regarding it as a threat to societal norms — the train to my idealized gender presentation had already left the station. Why were all these decisions fast-tracked onto my body? Not because they were medically necessary — I would have been perfectly healthy just living and growing as little old me — but because they were vital to “normalize” me.

The desire to force-fit people into societally conditioned boxes has led to sterilizing children and enacting medically unnecessary surgeries on them. These surgeries are irreversible, lead to physical and emotional scarring, and their subjects are un-consenting. They are, to put it bluntly, the coercive application of Western cultural ideals to everyday human bodies....

I’ve experienced firsthand the consequences of the gender binary in what’s often a non-binary world. It isn’t good for anyone. Certainly not trans people, but also not for a population that’s larger than many think — and that has spent years trying to convince people that our bodies are good enough as they are.

Personal Change and Social Change

Although transgender individuals are relatively rare, their increasing public visibility has led to increasing social acceptance.  What was once a movement for "gay rights" became a movement for "gay and lesbian rights", which then added bisexuality, and then transgender -- yielding the acronym LGBT.  And these changes have also been reflected in the law and social policy -- especially here in California.

  • Beginning in 2014, in what is thought to be the first such law in the United States (the "School Success and Opportunity Act", so named because it is intended to make school environments more comfortable for gender-nonconforming students), the State of California required all public schools, from kindergarten to grade 12, to allow transgender students to choose freely whichever restrooms or locker rooms they choose, according to their gender identity -- whether formally designated for boys or girls, men or women.  The law also allowed transgender students to join any athletic team -- whether boys' football or girls' field hockey
  • In September 2014, the University of California announced a new policy to create single-stall gender-neutral bathrooms on each of its 10 campuses.  The University also began to allow transgender students to use a name (like Andreja) other than the one (like Andrej) listed in their official campus records. 
  • Mills College, a private, women's college in Oakland (the oldest such college in the West), which declined to go coeducational when many other women's schools (such as Vassar) did, announced new policies on transgender students.
    • Anyone who identifies as a woman, regardless of gender assignment at birth, is eligible for admission.
      • Applicants who received a female gender assignment at birth, but who now consider themselves neither male or female, may also apply.
      • A person who was born female but is now legally considered male may not apply.
    • Enrolled students who change their gender identity from female to male may continue to attend the school.

In 2015, the status of transgender individuals reached into the level of public policy, when the US Department of Education (DoE) ordered that schools "generally must treat transgender students consistent with their gender identity".  In 2016, is strengthened the policy, indicating that schools which discriminated against transgender students, including in matters such as bathrooms and locker rooms, could lose their federal subsidies.  Subsequently a federal judge in Texas issued an injunction preventing the Obama administration from enforcing these guidelines.  Some other states followed suit, setting up the conditions for bringing transgender rights before the US Supreme Court.

In 2016, the Court agreed to hear just such a case: Gloucester County v. G.G. (Docket #16-273).  The case involved high-school student, Gavin Grimm (image from the New York Times, 10/29/2016), who was designated female at birth but identifies as male.  Originally, Grimm's high school allowed him to use boys' bathrooms, but the local school board overruled this decision.  Interestingly, the girls in the high school objected to this decision, on the grounds that they identified Grimm as male, and didn't want any boys in their bathrooms and locker rooms.  Grimm complained that the requirement that he use a private bathroom was humiliating, and brought suit against the school board.  At issue is a 1975 regulation adopted under Title IX of the Civil Rights Act of 1964 -- which, while prohibiting discrimination on the basis of sex (itself a "poison pill" originally slipped into the legislation by opponents of racial desegregation, in an attempt to prevent passage of the bill), prohibited In a split decision, the US Court of Appeals ordered the school to allow Grimm to use the boys' bathroom.  The regulation clarified the prohibition on sex discrimination by allowing institutions to provide sex-specific separate bath and dressing rooms.  The appeals court, agreeing that the regulation was ambiguous with respect to gender identity (not a big issue in 1975!), sided with Grimm.  The school board appealed, and the Supreme Court agreed to hear the case.  In its decision, the Supreme Court vacated the Appeals Court ruling, sending the case back to the lower courts for reconsideration in light of the DoE's new 2016 policy guidance.

So, rather than gender-nonconforming people having to accommodate to cultural norms, cultural norms are assimilating new gender identities.

For a journalistic account of how one all-women's college, Wellesley, is adjusting to transgender students (and vice-versa), see "Sisterhood is Complicated" by Ruth Padawer, New York Times Magazine, 10/19/2014.

For a cross-cultural perspective on these issues, see Gender Revolution, a special issue of National Geographic magazine, January 2017.

Or maybe not.  In 2018, it became known that the Trump Administration was considering a new DoE policy that would adopt a new, narrow definition of gender in strictly biological terms ("'Transgender' Could Be Defined Out of Existence Under Trump Administration" by Erica L. Green, Katie Benner, & Robert Pear, New York Times, 10/22/2018).  According to the draft policy, "Sex means a person's status as male or female based on immutable biological traits identifiable by or before birth.  The sex listed on a person's birth certificate, as originally issued, shall constitute definitive proof of a person's sex unless rebutted by reliable genetic evidence." 
The Times article notes that "The new definition would essentially eradicate federal recognition of the estimated 1.4 million Americans who have opted to recognize themselves -- surgically or otherwise -- as a gender other than the one they were born into....  The move would be the most significant of  series of maneuvers, large and small, to exclude the population from civil rights protections....  For the last year, the Department of Health and Human Services has privately argued that the term 'sex' was never meant to include gender identity or even homosexuality, and that the lack of clarity allowed the Obama administration to wrongfully extend civil rights protections to people who should not have them."

Writing in response to the Trump Administration's decision, Alicia Roth Weigel (see above) wrote:

I woke up Sunday morning to the news that the Trump administration is planning changes to federal civil rights laws that would define sex “as either male or female, unchangeable, and determined by the genitals a person is born with,” and that any confusion would be clarified through genetic testing. Most people have interpreted this effort as a blow to transgender rights — and it is. But amid all this, the fate of intersex people seems to have been forgotten.

Where would such a change leave me? My body would throw this Trumpian test for a loop — my naturally occurring genitalia don’t match the “correct” genetic code in this forced-binary paradigm that seeks to override biology.

Here’s another curveball: What Mr. Trump’s memo defines as “unchangeable” is anything but. I know this because the process of realizing a gender via hormones and surgeries, analogous to the process the administration is seeking to marginalize and discourage among trans people, is one imposed on intersex children all the time — but in our case, it’s done before we can understand or agree. It’s not just the government that is forcing an unnatural gender binary; medicine has been doing so for ages.

Meanwhile, in 2019 the Vatican's Congregation for Catholic Education, the office which regulates Catholic education, issued a guidance document entitled "Male and Female He Created them: Towards a Path of Dialogue on the Question of Gender Theory in Education" which was less dialogical than doctrinaire in nature.  It argued that gender flexibility, intersexuality, and transgenderism ignored essential biological differences between the sexes, led to ambiguous notions of masculinity and femininity, confused young people, and threatened traditional families. 

Hormonal Effects on Mind and Behavior

Psychological Consequences of PseudohermaphroditismPseudohermaphroditic children are of interest because they are genetic females who experience the effects of male hormones, and genetic males who do not experience these effects. Therefore, at least in principle, these the children provide an opportunity to observe the effects of prenatal hormones on behavior -- observations that might provide evidence of a hypothetical "masculinization of the brain" underlying the differences between masculine and feminine gender roles. And, in fact, there is some evidence for hormonal effects on behavior. Thus, for example, girls with the female adrenogenital syndrome and progestin-induced pseudohermaphroditism generally appear more vigorous and aggressive -- in terms of gender role stereotypes, more "tomboyish" -- than control girls; by comparison, the genetic males with the androgen-insensitivity syndrome, who are raised as girls, are behaviorally indistinguishable from control girls. This evidence is controversial, however, and shouting matches regularly erupt when it is presented and discussed at scientific meetings.

Sometimes, pregnant women who suffer from severe diabetes will be treated with exogenous estrogen and progesterone to prevent miscarriage. These female hormones do suppress the action of the androgen that would normally circulate to genetically and hormonally male fetuses, but appears to have no effects on the external or internal reproductive anatomy. These children are, of course, raised as boys. They are relevant to this discussion only because some early literature indicated that they were somewhat less aggressive and competitive than other boys. However, it should also be noted that the mothers of these children are also sicker than the mothers of control boys, and this by itself may inhibit vigorous play -- and effect which has sometimes been attributed to the lack of "masculinization of the brain". When we add controls for race, age, social class, and especially maternal illness during pregnancy, however, the difference disappears. Therefore, the behavioral differences appear to be caused by environmental, rather than biological factors.

Program for
              Gender DimorphismIn any event, this process, sometimes called the phyletic imprimatur, leaves the developing fetus and newborn child with a set of external genitalia, and an internal reproductive system, that are more or less recognizably male or female. At this point, the influence of biological factors stops, temporarily, and the individual's biographical history takes over as the parents and others structure an environment corresponding to their conceptions of how boys and girls should be raised -- an important part of gender-role socialization referred to as the social imprimatur.

In other words, the program for sexual differentiation (or gender dimorphism) passes from the hormones to the (social) environment). However, the program will be passed back to the hormones later, at the time of adolescence (for both sexes), and again at the time of menopause (for females). Actually, from birth on, it is probably best to think of the program being passed back and forth between the hormones and the environment. This is what is meant by the interaction of nature and nurture.

I've Seen Both Sides Now....

In classical mythology, Hermaphroditus, the son of Hermes and Aphrodite (get it?), shared a body with Salmacis, a nymph.

Another mythological character, Tiresias, was a blind soothsayer, the most famous prophet in ancient Greece. His prophecies play a central role in the story of Oedipus, who unknowingly killed his father and married his mother (the story of Tiresias is told in Ovid's Metamorphoses, and he appears in many other Greek tragedies, Homer's Odyssey, and T.S. Eliot's poem,The Waste Land. As the legend goes, Tiresias was out for a walk when he came upon two huge snakes who were copulating. When he struck the female with his staff, he was instantly turned into a woman. Seven years later, he saw the same two serpents copulating again; this time he struck the male, and was changed back into a man. Because he had experienced both sides of love, Tiresias was called upon to settle an argument between Hera and Zeus, as to who enjoyed lovemaking more. Tiresias agreed with Zeus that women enjoyed sex more than men, whereupon he was blinded by Hera. To compensate Tiresias for his loss, Zeus gave him the gift of prophecy and a long life. The gift was apparently heritable: one of Tiresias' daughters became an oracle at Delphi (or maybe this was an effect of the environment).

In another version of the legend, Tiresias was blinded by Athena when he accidentally saw her bathing; as compensation, she gave him the gift of prophecy. In another, Tiresias was blinded not by Athena but according to the laws of Cronos, as punishment for beholding an immortal without his or her consent.

  • An excellent fictional treatment of intersexuality is Middlesex by Jeffrey Eugenides, which traces the life of Cal (also known as Calliope), who is hermaphroditic due to 5-alpha-reductase deficiency. The book won the Pulitzer Prize for Fiction in 2002.
  • Another novelistic treatment of bisexuality and intersexuality is John Irving's In One Person (2012).
  • Yet another, this one aimed expressly at young adults, is None of the Above (2015) by Ilene Wong, a physician who was inspired by a patient with androgen-insensitivity syndrome.  Kristin Lattimer, the protagonist, is a genetic male with AIS, raised as a girl with the aid of hormone treatments, who begins to discover the truth about herself after a particularly unfortunate incident in the back seat of her boyfriend's car.

Tiresias is a myth, and Eugenides' Calliope/Cal is a fictional character, and there are no true hermaphrodites, but some cases of male pseudohermaphroditism come close, at least in some respects:

  • In one case described by John Money and Anke Ehrhardt (Man and Woman, Boy and Girl, 1972), a child was actually given the choice as to whether s/he would be a boy or a girl.
  • Some "transgendered" individuals undergo sex-change operations after they have lived for some time as adults. Famous cases include Christine (nee George) Jorgensen, James/Jan Morris, the travel writer, and Dierdre (nee Donald) McCloskey, the economist.
    • Jorgensen (1926-1989) was the first case of gender-reassignment surgery to come to wide public attention in the United States.   After serving in the US Army in World War II, Jorgensen underwent sex-reassignment surgery, and hormone treatment, in Denmark in 1952.  She became engaged in 1959, but was denied a marriage license because she was listed as male on her birth certificate.  See her book, Christine Jorgensen: A Personal Autobiography (1967).
    • Morris (1926-2020), a wonderful travel writer (check out books on Venice, Trieste, and Hong Kong, among other cities -- plus my personal favorite, Last Letters from Hav, a "travelogue" about a fictional city) was the first to report (from 22,000 feet!) the 1953 conquest of Mt. Everest by Edmund Hillary and Tenzing Norgay -- one of the great journalistic scoops of the 20th century.  See her series of memoirs, Conundrum (1974), Pleasures of a Tangled Life (1989), In My Mind's Eye (2019), and Thinking Again (2021).  Reviewing this last book, Hermione Lee writes ("The Wanderer", New York Review of Books, 02/25/2021):
Jan Morris's remarkable life was made up of many journeys....[At the time James Morris joined the Hillary expedition, having never climbed a mountain before,] [t]his young man had been aware from the age of about three of being in "the wrong body".  From 1964 Morris began the transition that culminated in the publication of Conundrum in 1974 under the name Jan Morris.  Apart from the Everest scoop [just in time for the coronation of Queen Elizabeth II], the other thing Morris was famous for was the journey to becoming Jan Morris, and Conundrum gives a brave, tender, candid, and pragmatic account of that difficult process, long before gender dysphoria was a well-known condition, and long before transitioning was the much-discussed public and political issue that it is now.  The painful struggle she had in the first thirty years of her life to understand and deal with her condition, when she "was dark with indecision and anxiety," often involved -- as for many others wrestling with this "conundrum" -- profound depression and thoughts of suicide: "For if there had been no hope of ending my life as a woman, I would certainly have ended it for myself as a man". 
    • McCloskey (1942-), an economic historian and an expert on cliometrics, the application of statistics to the study of history (Clio was the Greek muse of history); after her transition, she also made important contributions to feminist economics.  See Crossing: A Memoir (1999).    

In the famous "John/Joan" case, also treated by Money, "John", an infant boy (real given name: Bruce), lost his penis through an accident during circumcision. He was subsequently "re-assigned" to be raised as a girl, renamed "Joan" (actually "Brenda"), castrated, and his external genitalia surgically corrected. In their book, Money and Ehrhardt describe this case as a successful instance of gender-reassignment, but they lacked long-term followup. The case was all the more interesting because the child was one of a pair of identical twins; "her" identical twin was uninjured during the circumcision, and was raised as a boy. The case was also highly controversial, because its early apparent success implied that masculinity and femininity were learned, rather than based on biology -- thus contravening the doctrine (favored by both psychoanalysis and evolutionary psychology) that "biology is destiny", and that gender identity and role are encoded in the genes (Milton Diamond was an especially vigorous critic). But was it successful? Apparently, Brenda was never comfortable as a girl, and as an adolescent chose to live as a boy, changed his name to "David", and later underwent what might be called "re-correction" surgery; he subsequently lived as a man, married, and went public about his own case (he appeared on "Oprah" in 2000). When he committed suicide, at age 38, his family implied that he had been a victim of a "botched medical experiment"; they also noted that he had been depressed by the suicide, two years previously, of his twin brother Brian, who suffered from schizophrenia, as well as the recent loss of his job and separation from his wife ("David Reimer, 38, Subject of the John/Joan Case",New York Times, 05/12/04; see also "Being Brenda" by Oliver Burkeman & Gary Younge,the Guardian, 05/12/04).

The sad fate of David Reimer is commonly held up as a demonstration that gender identity and role are encoded in the genes, and can't be changed by environmental manipulation. However, in considering the outcome of the Reimer case, a few points should be borne in mind:

  • Reimer's identical twin brother Brian suffered from schizophrenia; because schizophrenia is to a large extent heritable, David inherited some disposition to schizophrenia as well. Although neither his parents nor his doctors could have known this at the time (both children were less than 2 years old), Bruce was probably not the best candidate for involuntary sexual reassignment surgery.
  • Reimer was about 1 year old when he underwent sex-reassignment surgery, but close to 2 years old when her parents began to treat her as if she was a girl -- by, for example, making her wear dresses. However, children start noticing their own and others' gender by about 2 years of age. Brenda's initial resistance to dresses might have been a product of her initial gender identity as male. Or, perhaps more simply, perhaps she wanted to be dressed in the same way as her brother Brian. Little girls often express an interest in the functional clothing, and more freedom in play, given their brothers.
  • Despite Brenda's parents' valiant efforts to treat her as a girl, they may have been unsure about the success of the "experiment", and thus given her mixed messages. Along these lines, Money's own post-surgical treatment of Brenda, which included psychotherapy sessions intended to reinforce her new gender assignment, may have backfired by drawing attention to the fact that she was not, in fact, a "normal" girl.

The point of all of this is not to say that Money was entirely right after all, but only that the critics might not also be entirely right that gender identity and role are encoded in the genes. The case of David Reimer is more complex, on both sides, than it would initially appear to be.

The John/Joan case is related in depth by John Colapino in an article, "The True Story of John/Joan" (Rolling Stone, 11/11/97), and a book,As Nature Made Him: The Boy Who Was Raised as a Girl (HarperCollins, 2000). In general, Colapinto views Reimer as a victim of radical environmentalism, if not (early) radical feminism.

For insight into the life and mind of an adolescent female-to-male (FTM) transgender person, see , "About a Boy: Transgender Surgery at Sixteen" by Margaret Talbot ( New Yorker , 03/18/2013). Written at a time when the US Supreme Court was hearing two cases regarding same-sex marriage, Talbot writes that "Transgenderism has replaced homosexuality as the newest civil-rights frontier...".

Joel Meyrowitz has provided an authoritative overview of transsexualism in How Sex Changed: A History of Transsexuality in the United States (Harvard, 2002).

Michael Bailey has argued that transsexualism is not, as commonly portrayed, a case of "a woman trapped in a man's body" ( or the reverse). Bailey argues that transsexualism comes in two forms:homosexual transsexuals, gay men who are so effeminate, in terms of gender-role behavior, that they want to take on a female gender identity as well -- and the body that goes with it (Bailey also argues, from his extensive survey data, that even gay men who are not transsexuals tend to be extraordinarily effeminate); and autogynephilic transsexuals, heterosexual men who are sexually stimulated by the thought, or the act, of a male-to-female sex change. Bailey presents his arguments and data in the provocatively titled book,The Man Who Would Be Queen: The Science of Gender-Bending and Transsexualism (Joseph Henry Press, 2003).

Why do I spend so much time on questions of gender identity and role, when other intro courses spend most of their time on cognitive development?  Because, as a personality psychologist, I believe that gender identity and role are central issues for self and social interaction.

Postnatal Hormonal Influences

              Hormonal InfluencesThe effects of the sex hormones do not stop with the differentiation of the internal and external genitalia. They come back on the scene at least two more times, at puberty and in old age, each time interacting with the social environment.

Puberty. At puberty, the program for gender dimorphism passes back to the hormones, as indicated by such milestone events as menarche (onset of menstruation) in girls and nocturnal emissions ("wet dreams") in boys. The most obvious post-natal effects of the sex hormones are the development of such secondary sex characteristics as the deepening of the voice in males and the development of breasts in females, and the masculinization or feminization of overall body shape. These physical changes are instigated by sex hormones, testosterone and estrogen, secreted by the testes and ovaries, respectively.

Interestingly, there is now evidence that puberty may begin long before adolescence. In girls, for example, the pituitary hormones associated with puberty, along with secretions from the ovaries, are known to begin at about age 9, and breast development often begins between 10 and 11 years of age. Martha McClintock and Gilbert Herdt, researchers at the University of Chicago, have noted that children experience a spurt in physical growth around age 6, accompanied by the appearance in the skin of oil-producing sebaceous glands similar to those associated with pimples in adolescence (Current Directions in Psychological Science, 12/96). They also reported that signs of sexual attraction, heterosexual or homosexual, can be observed in children as young as 9 or 10 years of age; interestingly, this is also about the time that girl-boy teasing begins. This attraction should not be confused with sexual desire, much less sexual activity; these kick in later, as the individual approaches and enters adolescence. Rather, at this early age there appears to be only a more or less clear "leaning" toward one sex or the other. McClintock and Herdt suggest that these changes are related to the secretion by the adrenal glands of a form of androgen known as dihydroepioandrosterone (DHEA), which begins at about age 6 and increases to a critical level at about age 10 -- a point which they call arenarche, by analogy to menarche. DHEA reaches adult levels at about age 18 before diminishing over the rest of the life course.

In any event, the hormonal effects of puberty interact with the social imprimatur as parents and others impose culturally bound standards for adolescent behavior. In some cultures, for example, sexual experimentation is permitted, even encouraged; in others, sex is strictly prohibited until marriage. In some cultures, boys and girls are permitted to date, and even engage in light sexual activity ("petting"); in other cultures, boys and girls can meet only under conditions of strict supervision; in still other cultures, marriages are arranged by the parents, and the engaged couple may have only minimal contact with each other before their wedding day. In all cultures, parents and others scrutinize adolescents for signs of "normality", and the adolescents' gender identities, gender roles, and erotic orientations are strengthened and challenged.

Middle and Old Age. Later in life, there are further dramatic changes in hormone levels. These are most obvious in women, particularly the sudden drop in estrogen levels, and cessation of menstruation, known as menopause. Recent evidence suggests that there may be a male version of menopause as well, known as partial androgen deficiency in aging men (PADAM), or andropause. Although men produce sperm throughout their adult lives, they experience a gradual decline in testosterone levels as they age (about 0.5% per year after age 30), which in turn may be associated with fatigue, loss of muscle tone and bone density, and "decreased libido" (a term derived from Freud's term for the sexual drive). Note, however, that the diagnostic criteria for andropause overlap greatly with those for depression. In the absence of laboratory tests showing abnormally low levels of testosterone, it is not at all clear that andropause is a legitimate diagnosis, or that HRT is a legitimate treatment.

Here again, the hormones interact with the social environment. For example, the cessation of menstruation, and the loss of child-bearing capacity, may challenge women's gender identities and roles. Similarly, age-related erectile difficulties may challenge those of men.

Natural Condition or Manufactured Illness?

As with menopause, it has been suggested that andropause be treated with hormone replacement therapy, and several drugs, such as Androgel (a product of Unimed Pharmaceuticals), have been marketed for that purpose. In fact, between 1997 and 2001 prescriptions for testosterone almost doubled ("Male Hormone Therapy Popular But Untested" by Gina Kolata,New York Times, 08/19/02).As with menopause, this suggestion has most frequently been made by the pharmaceutical industry -- giving rise to the suggestion that, like menopause and its pharmaceutical treatment, andropause and male HRT are diseases and treatments that have been "manufactured" by Big Pharma for economic gain (see "Hormones for Men" by Jerome Groopman,New Yorker, 07/29/02).In 2002, the results from a major study of HRT in healthy women suggested that the treatment did more harm than good, increasing the risk of heart problems, and leading many physicians to discontinue the treatment and many of their patients seeking alternatives. Similarly, because testosterone can promote increase prostate cancer and increase the risk of heart attacks and strokes, a long-term study of HRT in men was also discontinued in 2002.

In women, menopause is a real condition, but it is something that occurs naturally, in the course of aging, and it not at all clear that it should be treated as if it were a disease that could be cured with the right drug -- especially, as in the case of HRT, the risks are so great. This would constitute a "medicalization" of normal aging. The situation is even less clear, and the danger of inappropriate medicalization even greater, in the case of andropause, in that levels of testosterone do not drop as quickly, or with such consequences (such as hot flashes) as estrogen levels drop in women. HRT is valid as a treatment for hypogonadism, a rare genetic condition (also known as Klinefelter's syndrome, or chromosomal XXY) that can also be induced by chemotherapy or radiation therapy for cancer, and for certain disorders of the pituitary gland. But it is not at all clear that HRT should be considered a pharmaceutical "fountain of youth" -- for either women or men.

              for Gender DimorphismIn summary, the "program" for gender dimorphism of identity and role begins with the sex chromosomes (XY or XX), which differentiate the gonadal tissue into testes or ovaries; and continues with hormones secreted by the testes, which differentiate the internal and external reproductive anatomy. This phyletic imprimatur, a process of gender dimorphism that is common to all humans, and is under direct biological control, endows the fetus with characteristically male or female reproductive anatomy.

At birth the program for gender dimorphism passes from the genes and the hormones to the social environment, as the parents classify the newborn child as male or female, and begin the process of raising him or her according to cultural concepts of masculinity and femininity. Early in childhood, the child recognizes his or her own gender, identifies him- or herself as male or female, and begins to model his or her attitudes, beliefs, and behavior on others of his or her "own kind". These social learning processes, which are under environmental control and vary from one culture to another, is known as the social imprimatur. Everyone undergoes gender dimorphism of identity and role, but the outcome differs from one individual to the next, and from one culture to the next, depending on the details of the social imprimatur.

The social imprimatur encompasses many different elements:

  • The individual's biographical history.
  • Gender-role socialization, in which parents and others impose on the child culturally specific concepts of masculinity and femininity.
  • The development of the child's self-concept as male or female, and consequent identification of others of his or her "kind".
  • Social learning processes, including:
    • the direct experience of rewards and punishments for gender-appropriate and -inappropriate attitudes and behaviors;
    • vicarious learning from the example of others;
    • learning by precept, or deliberate instruction in how to think and behave.

Gender Dimorphism Beyond Reproductive AnatomyThe point of all this is that gender dimorphism extends far beyond the obvious matters pertaining to reproductive anatomy. There are at least three other aspects of gender dimorphism that we have to consider.

  • Gender identity, or the individual's private experience of being male or female (or, in some cases, transgender).
  • Gender role, or the public expression of masculine or feminine characteristics, as deemed appropriate in one's culture.
  • Erotic orientation, or one's sexual attraction towards other people, whether heterosexual, homosexual, bisexual, or asexual.

Gender Identity

Gender role socialization is not simply imposed on the child from outside: the child is also an active agent of his or her own gender socialization, as he or she acquires an identity as a little boy or little girl.

Beginning at about age 2, children notice (as it were) their own genitalia, and identify themselves as boys or girls. (The cartoon on the left captures the situation beautifully: In the original, the caption reads "There is a difference!".  I first encountered this cartoon in the 1960s in a "QSL" card mailed by an amateur radio operator, whose identity I have since forgotten.  Apparently, it's now available as a tattoo.)

This self-identification has a number of consequences

  • The child divides people into two categories, according to sex, and identifies him- or herself as the same as some, and different than others.
  • The child attaches a positive affective valence to his or her own gender.
  • The child begins to actively model him- or herself on others who are similarly endowed.

Children's recognition of gender is perfected by about age 3. By this time, they strongly prefer objects labeled as "for" their own gender.

Differences in gender-role behavior are not reliably observed before age 2, but they are well established by the time the child goes to school:

  • Children prefer playmates of the same gender.
  • Girls prefer to play in smaller groups than boys.
  • Boys engage in more roughhousing than girls.
  • Boys playing in groups are more likely to fight than girls.
  • Girls are more likely than boys to turn to adults as resources.

Actually, children learn both gender roles, and adopt the role that is appropriate to their gender identity as male or female. Even so, there is some asymmetry in their preferences:

  • Boys actively avoid activities that have been stereotyped as "for girls".
  • Girls tend to show less stringent gender-role differentiation.
  • Similarly, fathers enforce stricter gender boundaries than mothers do.

The active participation of the child in his or her own gender-role socialization illustrates the principle that the child is an active agent of his or her own development. Once the child has categorized him- or herself as male or female, he or she begins the active process of learning and performing the roles deemed appropriate for his or her gender. In this way, gender role socialization illustrates the complex interactions that play out between nature and nurture, and between the person and the environment.

Gender Role

Gender-Role DimorphismIn western society, gender roles have traditionally been divided into two major dimensions.

  • The traditional masculine gender role can be summarized in terms of agency and instrumentality. Men, and even boys, are expected to be active and independent, to exercise leadership, to be competitive, and to be oriented toward achievement outside the family.
  • The traditional feminine gender role emphasizes communality and expressiveness. Women, and even girls, are expected to be sensitive and empathic, to be concerned for others' welfare, to seek cooperation and interpersonal harmony , and to be oriented toward achievement inside the family.

Link to an interview with Janet Taylor Spence.

Bacha Posh in Afghanistan...

                PoshThe distinction between gender identity and gender role is dramatically illustrated by the common practice, in Afghanistan, in which pre-adolescent girls masquerade as boys. In Afghanistan, there is considerable social pressure on families to produce sons, and pity and even contempt directed toward families that have none. Sons are valued more highly than daughters, on sheer economic grounds; and in Afghanistan's tribal culture, only sons can inherit the family's wealth or continue the family name. So when a son doesn't happen, parents will sometimes take a daughter (sometimes the youngest, who may be seen as a kind of "last try"), dress her as a boy, and present her a such to outsiders. (There is even a superstition that doing so will increase the family's chance of actually producing a male child.) The children are generally referred to as bacha posh, a Dari phrase meaning "dressed as a boy".

Inside the home, unless there are visitors, the bacha posh are dressed and treated as girls. Outside the home, of when there are visitors, they are dressed and treated as boys. They will play with other boys, go to school with boys, and even work as boys outside the home. Nobody is fooled, exactly, but it does relieve some of the social pressure. At puberty, most bacha posh return to their culturally sanctioned roles as girls -- partly because the fact that they're not boys becomes pretty obvious, and partly because their parents fear the consequences of pubescent girls being around pubescent boys (and men). But, apparently, most they never lose their female gender identities. How the boys they play with (and the men who teach them in school) deal with this is an interesting question.

While most bacha posh revert to a feminine gender role, some -- having gotten a sense of the privileges accorded to boys and men -- resist the traditional feminine gender role to which they have been re-assigned. Shukria Siddiqui, a former bacha posh who is now employed as an anesthesiology nurse, had a difficult time making the adjustment. "She had no idea how to act in the world of women.... For years, she was unable to socialize with other women and uncomfortable even greeting them.

"I had to learn how to sit with women, how to talk, how to behave", she said.... When you change back, it's like you are born again, and you have to learn everything from the beginning. You get a whole new life. Again."

The bacha posh in the illustration is Mehran Rafaat, age 6, pictured with her older twin sisters Benefsha and Beheshta (when Mehran's mother produced three girls in a row, the negative attitude from her mother-in-law led her to raise Mehran as a bacha posh). At the time the photo was taken, their mother, Azita Rafaat, was an For her part, Mehran seems to have taken to the masculine gender role quite well. Azita said of her daughter, Mehrat, My daughter adopted all the boys' traits very soon You've seen her -- the attitude, the talking -- she has nothing of a girl in her".

Like the guevodoces, the bacha posh constitute a natural, living laboratory for the study of gender issues. It will be interesting to follow them, and see if they constitute a force for social change within Afghanistan. Once girls have had a taste of what it is like to be boys, and once boys have had a taste of girls who can behave like boys, what will happen to traditional gender roles in Afghan culture. Interestingly, Mehran's mother, Azita, herself functioned as a kind of bacha posh when she was a girl, helping out in her father's shop, and dressing like a boy to run errands. With the overthrow of the Taliban, she became an elected member of Afghanistan's parliament, with a salary of $2,000 per month in salary (Mehran's father, by contrast, was unemployed, and functioned as a "house-husband"). Coincidence? Maybe not.

(Photo by Adam Ferguson, from "Where Boys Are Prized, Girls Live the Part" by Jenny Nordberg,New York Times, 09/21/2010, which was the source for this sidebar and is a terrific article, highly recommended).

...and Wakashu in (Edo) Japan

Another example of gender fluidity was a sort of "third gender" in Japan during the "Edo period", beginning in 1603 and lasting until the "opening" of Japan to Western influences in 1868.  These wakashu were the "beautiful youths", adolescent males, appreciated as representing the ideal of beauty, for whom it was permissible to engage in sexual relations with both men and women. The wakashu were not necessarily homosexual males (lesbianism  This cultural practice was discontinued in the late 19th century, as Japan adopted Western notions of gender and sexuality.  Homosexuality (though not same-sex marriage) is legal in Japan, which has a thriving gay subculture, but even homosexual men are expected to marry women and produce offspring.  Some androgynous Japanese men identify as "genderless danshi".

For more, see "A Third Gender: Beautiful Youths in Japanese Prints", an exhibition at The Japan Society in New York, 2017, reviewed in "The 'Indescribable Fragrance' of Youths" by Ian Buruma, New York Review of Books, 05/11/2017).

In many ways, it would seem that masculinity and femininity are polar opposites -- that they anchor opposite ends of a single dimension of gender role. And, indeed, that's how these personality characteristics are measured in traditional questionnaire measures of personality, such as the Mf (Masculinity-Femininity) scale of the Minnesota Multiphasic Personality Inventory (MMPI) or the Fe (Femininity) scale of the California Psychological Inventory (CPI).

Psychological AndrogynyBut, more recently, some feminist psychologists have argued convincingly for an alternative conception in which masculinity and femininity are construed as independent dimensions of personality (recall that a similar debate took place with respect to the relation between positive and negative emotionality). Such a scheme yields four basic categories of gender role:

  • Masculine sex-typed individuals score high on traditional measures of phenotypically "masculine" agency and instrumentality, and low on traditional measures of phenotypically "feminine" communality and expressiveness.
  • Feminine sex-typed individuals score high on phenotypically feminine traits, and low on phenotypically masculine traits.
  • Androgynous individuals score high on both masculinity and femininity.
  • Undifferentiated individuals score low on both scales.
    • Perhaps this is because they simply have not developed a particular stance with respect to gender role.
    • Alternatively, it is possible that these individuals, or at least some of them, have transcended traditional gender roles entirely.

Androgyny was a new scientific concept in the 1970s, but it was foreshadowed by one of the earliest documents of feminist theory: Woman in the Nineteenth Century (1845) by Margaret Fuller, the lone woman in the "Transcendentalist Club" that included Ralph Waldo Emerson and Henry David Thoreau. As Judith Thurman writes (reviewing a number of biographies of Fuller):

Margaret was a strapping girl who preferred boys' strenuous activities to girls' decorous ones.... [Her platonic but] amourous friendships informed Fuller's prescient notion of gender as a bell curve -- the idea that there are manly women, womanly men, and same-sex attractions, all of which would be considered perfectly natural in an enlightened society.... It was an "accursed lot", Fuller concluded, to be burdened with "a man's ambition" and "a woman's heart", though the ambition, she wrote elsewhere, was "absolutely needed to keep the heart from breaking". ("An Unfinished Woman", by Judith Thurman, New Yorker , 04/01/2013.)

The terms "tomboy" and its rough male equivalent, "sissy", have long been used to label gender-nonconforming girls and boys (though, in practice, "sissy" is generally more pejorative.  For a historical survey of "tomboyism" (with occasional forays into sissyhood), see Tomboy: The Surprising History and Future of Girls Who Dare to be Different (2020) by Lisa Selin Davis (reviewed by Lisa Damour in "'Tomboy' Looks at Gender Roles, and Role-Playing, Through the Ages", New York Times Book Review, 11/08/2020).

How Big Are Gender Differences, Really?

Men and women, and boys and girls, differ from each other psychologically in obvious and subtle ways.  In addition to obvious differences in masculinity (agency) and femininity (communality), it's also been claimed that there are gender differences in cognitive ability: males are generally held to be superior in mathematical and spatial ability, for example, and females to be superior in verbal ability.  All of these differences fit the cultural stereotypes (at least so far as Western culture is concerned), but how big are these differences, really.  Are they big enough to justify obvious differences in social outcomes, such as the fact that there are many more men than women teaching math and science at the college level? 

Oddly enough, for all the studies -- and there are umpteen thousands of them -- documenting gender differences in performance on this or that task, it was only relatively recently that anyone viewed this literature from the standpoint of effect size -- which, as you'll remember from the lectures on Methods and Statistics, measures the strength of an effect -- in this cases, differences between two groups classified by gender.  The first comprehensive review of gender differences was published by Eleanor Macoby and Nancy Jacklin in 1974, but these researchers did not have the statistical tools of meta-analysis available to them at the time, and they had to present only a verbal, impressionistic summary of the literature.  Since then, however, the technique of meta-analysis has been developed, allowing researchers to combine the results of a large number of studies, and summarize their findings in a single quantitative score representing effect size -- that is, taking all of the studies together, the magnitude of the difference between males and females on various measures.  These meta-analyses of the literature confirm that many of these gender differences exist -- but they also show how remarkably small most of them really are.

Among the first researchers to notice this, and to make a big deal out of it, was Janet Shibley Hyde, a professor at the University of Wisconsin (whose PhD was from UC Berkeley). 

  • Linn & Peterson (1984) conducted a meta-analysis of 172 studies of gender differences in spatial ability, and found that there were, overall, moderate differences favoring men, with an overall effect size of about d = .43.
  • But Hyde & Linn (1988) conducted a meta-analysis of 165 studies of gender differences in verbal ability, and found that the effect size (d) was a "small" 0.11 favoring females.
  • And Hyde et al. (1990) conducted another meta-analysis of mathematics ability, and found an even smaller effect -- a truly "trivial" effect -- favoring males, d = .05.
  • Hyde and Plant (1995), responding to Eagly (1995), found that 60% of meta-analyses of gender differences yielded only "very small" and "small" effect sizes, compared to 35% of meta-analyses of various psychological, educational, and behavioral interventions; and only 13% of gender studies yielded "large" or "very large" effects, compared to 26% in the other topic areas.  

OK, so there are differences, on average, in spatial, verbal, and mathematical abilities between males and females.  The economist Lawrence Summers, who was Secretary of the Treasury during the Clinton Administration and later President of Harvard University (and author of a paper famously entitled "There Are Idiots"), pointed to this last difference, especially, in explaining why there were so few women among the math and science faculty and graduate students at Harvard -- implying that women just didn't have the quantitative chops to succeed at the highest level in these fields (in this he was supported by Steven Pinker, a distinguished Harvard psychologist; in the resulting brouhaha, Summers was forced to resign from the presidency, though he remains a professor in the Economics Department).  But Summers left out two important points.

  1. Although there were more men than women on the faculty of math and science departments at Harvard, there were also more men than women on the faculty in the English Department!  So whatever accounts for the gender imbalance, it has to be more than raw ability.  Maybe there's some gender discrimination as well (you think?).
  2. Although there are gender differences on average, they're pretty small.  Even allowing for the gender difference, there are plenty of women available with strong math abilities -- enough to occupy a fair share of faculty slots.  

Hyde's papers set of an avalanche of meta-analyses of gender differences in various abilities and traits.  In 2005, Hyde summarized the results of these analyses with her gender similarities hypothesis, "that males and females are similar on most, but not all, psychological variables. That is, men and women, as well as boys and girls, are more alike than they are different" (p. 581).  Surveying the 46 published meta-analyses available to her at the time, she found a mean (unsigned) difference of d = .21 between males and females.  The figure at left depicts what a d of 0.21 looks like, in terms of the normal distribution.  Fully 78% of the psychological gender differences uncovered in the literature were, in terms of effect size, "very small" or "small" in magnitude; only 9% counted as "large" or "very large".  As Hyde pointed out, these differences hardly justify the title (and argument) of John Gray's 1992 best-selling book, Men Are From Mars, Women Are From Venus! (see also Hyde, 2007).  

And to put the icing on the cake, Zell and his colleagues (2015) reviewed a total of 106 meta-analyses, and confirmed Hyde's essential findings: an unweighted average d score of 0.21, right in the middle of the range of "small" effects; fully 85% of these effects classified as "very small" or "small", and fewer than 3% classified as "large" or "very large". 

  • The largest effect size was observed in  -- wait for it! -- "masculine vs. feminine traits", with an average d score of 0.73 -- right in the middle of the "large" range.
    • Actually, no other effect sizes classified as "large", and none classified as "very large".
  • The gender difference for aggression, favoring (if that's the proper word) males, yielded an average d of 0.45, classifying as a "medium" effect. 

Zell et al. conclude, "We utilized data from over 20,000 individual studies and over 12 million participants to reevaluate the gender similarities hypothesis and found that its core proposition receives strong support" (p. 18).

So, males and females are "far more similar than different" (Zell et al., p. 18) -- except on a few variables, the most salient being gender role: masculinity and femininity.  So now let's ask where these differences come from.

The debate over gender differences just won't go away.  In 2017, James Damore, a software engineer at Google, took a page from Lawrence Summers's book and argued that calls for increased representation of women in silicon Valley ignored scientific evidence of gender differences in cognitive abilities relevant to software engineering.  His manifesto quickly went viral.  Damore was subsequently fired on grounds of incompatibility with Google's culture, but the editors of The Economist argued (08/12/2017) that Larry Page, the co-founder of Google and the chief executive officer of Alphabet, Google's parent company, should have written a "ringing, detailed rebuttal" to Damore's manifesto instead.  Page didn't do that, but The Economist did it for him (08/19/2017), essentially echoing the points made here.  

Heredity and Environment in Gender Role

A number of twin studies have been conducted to determine the various sources of individual differences in gender role. Unfortunately, these studies have construed masculinity and femininity as polar opposites, not independent dimensions of gender role, but their results are still interesting.

Components of Variance in Gender RoleIn a pioneering study, Irving Gottesman Components of Variance in Gender Role et al. (1965, 1966), administered the masculinity-femininity scales of the MMPI and CPI to a sample of adolescent twins. Robert Dworkin and his colleagues repeated this testing with the same sample some 10 years later, when the subjects were adults. although these investigators did not calculate components of variance, the fact that the correlations for MZ twins were consistently higher than those for DZ twins gives prima facie evidence for a genetic contribution to individual differences in gender role. This isn't terribly surprising. But the relatively low magnitude of the MZ correlations suggests that the environment also plays a role in shaping this aspect of personality. In fact, if you apply the formulas discussed earlier, it's clear that the nonshared environment is by far the most powerful determinant of gender role. So much for the easy equation of biological sex (which, of course, is almost completely determined by the genes) and psychosocial gender.

Twin Study of
              Gender RoleA more recent study by Loehlin Components of Variance in Gender Roleet al., with a larger, more representative sample of subjects, came to much the same conclusions (Loehlin et al. tested both Americans and Europeans, but for purposes of comparison I show only the American results here). MZ twins were more similar than DZ twins, in terms of masculinity-femininity, but the nonshared environment was a much more powerful

So, it seems that the biological and social imprimaturs interact to produce the individual's gender identity and gender role, but (setting aside the issue of the masculinization of the brain) the chief effects of the genes and hormones are anatomical and physiological, not psychological. They endow the developing fetus with reproductive anatomy that is more or less recognizably male or female, and that is just about it.

  • At birth, the physical appearance of the child's genitalia literally structures the environment. The child is identified as a little girl or a little boy, and raised accordingly. In the process, the social environment organizes itself so as to bring up a masculine boy or a feminine girl. In a classic example of the evocation mode of person-by-situation interaction, the appearance of the child's genitalia literally structures the environment, activating gender-role socialization processes by which the environment constrains and supports the child's development of the appropriate gender role.
  • A lot of this socialization is imposed on the child from outside forces:
    • Based on the external genitalia, parents and others (e.g., older siblings) perceive the child as a boy or girl and raise him or her in accordance with cultural concepts of masculinity and femininity.
    • Parents and others engage in differential modeling of gender roles.
    • Differential socialization continues outside the home, especially in the hands of peers (and their parents), teachers, and other authority figures. In the last several decades, television and other media have become increasingly important to gender-role socialization.

In these and other ways the environment constrains and supports the development of "appropriate" gender roles.

Child-Rearing PracticesEvidence with respects to gender comes from classic studies of child-rearing practices (summarized by Maccoby and Jacklin,The Psychology of Sex Differences, 1974).

  • Furnishing of Rooms: Even before age 6, boys' rooms have a wider variety of furnishings in them. Boys' furnishings tend to be directed away from the home (e.g., sports, cars, animals, the military). Girls' furnishings tended to be directed toward the home (e.g., dolls, dollhouses, toy kitchens).
  • Household Chores: Children are asked to help with those tasks that are performed by the parent of the same sex.
  • Differential Rewards and Punishments: Girls receive smiles, praise, and attention for dancing, dressing up, playing with dolls, asking for help, and following their parents; they receive criticism for running, jumping, and climbing. The opposite trend holds for boys, who receive praise for building with blocks, and criticism for playing with dolls and asking for help.
  • Differential Modeling by Parents and Other Authorities: This is especially the case for fathers, and especially for boys. Fathers make stronger discriminations between the sexes: They are more concerned about gender-role socialization, and more likely to issue differential rewards and punishments for gender-typed behavior. Siblings, peers, and teachers are also important in this process. Recently, television has become central to gender-role socialization, as characters on TV present additional models gender-role socialization.
  • Differential Socialization Outside the Home: The people and institutions whom children encounter outside the home also support their development of "appropriate" gender roles.

Link to an interview with Eleanor Maccoby.

In the wake of the women's liberation movement that began in the late 1960s, the social environment has become somewhat less rigidly structured according to gender. Many people would like to think that gender-role socialization has been loosened in these more "enlightened" times. But while traditional concepts of masculinity and femininity may have been loosened, they have not been abolished.

As an example, consider these toddlers' "training pants" bought in 2000 -- more than 30 years after the feminist revolution swept America. The girls' version, in pink, portrays Minnie Mouse singing songs with Daisy Duck; The boys' version, in blue, portrays Mickey Mouse driving a car and flying a plane with Donald Duck. These differences have nothing to do with the physical differences between the sexes (which might well make structural differences in training pants desirable), and nothing to do with behavioral differences either. They are a simple, and not too subtle, reminder of the social differences -- the differences in gender role -- between boys and girls.

A 2012 doctoral dissertation by Elizabeth Sweet, a sociology graduate student at UC Davis, surveyed advertisements for toys in the Sears catalog over the 20th century.  She summarized her findings as follows ("Guys and Dolls No More?", New York Times, 12/23/2012):

Gender has always played a role in the world of toys.  What's surprising is over the last generation, the gender segregation and stereotyping of tows have grown to unprecedented levels.  We've made great strides toward gender equity over the past 50 years, but the world of toys looks a lot more like 1952 than 2012.  Gender was remarkably absent from the two ads at the turn of the 20th century but played a much more prominent role in toy marketing during the pre- and post-World war II years.  However, by the early 1970s, the split between "boys' toys" and "girls' toys" seemed to be eroding....  I found that in 1975, very few toys were explicitly marketed according to gender, and nearly 70% showed no markings of gender whatsoever.  In the 1970s, toy ads often defied gender stereotypes by showing girls building and playing airplane captain, and boys cooking in the kitchen.  But by 1995, the gendered advertising of toys had crept back to mid-century levels, and it's even more extreme today.  In fact, finding a toy that is not marketed either explicitly or subtly (through the use of color, for example) by gender has become incredibly difficult...  For example, last year [2011] the ego Group, after two decades of marketing almost exclusively to boys, introduced the new "Friends" line for girls....  Critics pointed out that the girls' sets are more about beauty, domesticity and nurturing than building -- undermining the creative, constructive value that parents and children alike place in the toys.

A 2014 "Big Data" analysis of anonymous Google searches revealed persisting gender differences in parents' concerns and expectations ("Google, Tell me.  Is My Son a Genius?" by Seth Stevens-Davidowitz, New York Times, 01/19/2014).

  • Parents were about 2-1/2 times more likely to ask "Is my son gifted?" than 'Is my daughter gifted?".
    • There were similar disparities with other terms related to intellectual ability.
  • Parents were about 1-1/2 times more likely to ask whether their daughter is beautiful, and 3 times more likely to ask if their daughter was ugly.
    • There were similar disparities with other terms related to physical appearance. 

Other modes of the person-by-situation interaction require more than the mere presence and appearance of the person. In these modes, the person must do something:

  • either overtly, in terms of publicly observable behavior;
  • or covertly, in terms of privately experienced thought.

The Social Construction of Gender Role

Barbie and G.I. JoeSimone de Beauvoir was onto something. The environmental forces shaping children's gender roles are not by any means subtle. Consider, for example, the different dolls -- because that's what they are -- offered for girls and boys to play with.

  • Girls get Barbie.
  • Boys get GI Joe.
Barbie in particular raises concerns because her physical proportions may give girls (and, for that matter, boys) an idealized body image that is impossible to achieve (even if it were desirable).

'Gee, Math class is hard!" (1967) But the effects of Barbie on gender identity and role are not restricted to physical features of the body image. They also extend to mental and behavioral aspects of the gender role. Consider, for example, the controversy that arose in 1967 over the first Talking Barbie, who was famous for saying "Gee math class is hard". Talking Barbie plays directly into gender-role stereotypes about sex differences in mathematical ability, and may discourage girls from taking advanced courses in mathematics.

Although certain aspects of masculinity and femininity may seem "natural", many of them are socially constructed.

Consider, for example, how little boys and girls are dressed. Only a century ago, it was very common for boys to be clothed if dresses until they were six or seven years of age, at which time they would also get their first haircut. Up until that time, a causal observer might be forgiven for mistaking them for girls.

Ernest Hemingway Life Magazine For example, here is a photograph of Ernest Hemingway, the 20th-century American author, winner of the Nobel Prize for Literature, who might be considered a paragon of masculinity.
Ernest Hemingway And here is a photograph of Hemingway as a child. Some psychoanalytically inclined authors have suggested that Hemingway's rather aggressive adult masculinity -- he seems never to have seen a wild animal that he didn't shoot -- emerged as a kind of reaction formation to the fact that he was treated "like a girl" as a child. But the point is that all little boys were treated in the same way.

More generally, it is now commonplace, almost a cliche, for newborn boys to be swaddled in blue, and newborn girls in pink. This somehow seems "natural". But it's not, and the proof of this is that, until well into the 20th century, the colors were reversed. For most of history, male and female infants were both dressed in white. In 1927, an article in time magazine actually recommended pink for boys and blue for girls. Only later did the color preferences reverse. Later, under the influence of the feminist revolution of the 1960s and 1970s, the clothing industry stopped promoting gender-specific colors, but the rule of "blue for boys, pink for girls" began to make a comeback in the 1980s. For more details, see "When Did Girls Start Wearing Pink?" by Jeanne Maglaty,Smithsonian, April 2011.

Growing Up Male and Female

For authoritative reviews of the literature on gender-role socialization, see:

  • John Money & Anke Ehrhardt,Man and Woman, Boy and Girl (Johns Hopkins University Press, 1972). Much of my treatment of gender dimorphism is drawn from this book.
  • Rebecca Jordan-Young,Brain Storm (Harvard, 2009).
  • Donald Pfaff,Man & Woman: An Inside Story (Oxford, 2010).
  • Eleanor E. Maccoby & Carol Nagy Jacklin,The Psychology of Sex Differences (Stanford, 1974).
  • Eleanor E. Maccoby,The Two Sexes: Growing Up Apart, Coming Together (Harvard, 1998).
  • Carol Tavris,The Mismeasure of Woman (Simon & Schuster, 1992).
  • Jo B. Paoletti,Pink and Blue: Telling the Girls From the Boys in America (2011)
  • Alice H. Eagly & Wendy Wood, "The Nature-Nurture Debates: 25 Years of Challenges in Understanding the Psychology of Gender" [with commentary] (Perspectives on Psychological Science, 2013).

Erotic Orientation

Aspects of erotic or sexual orientationThe last aspect of gender dimorphism is erotic or sexual orientation -- whom one is attracted to as a sexual partner. Again, the simpleminded story is that genetic males identify themselves as boys, grow up to become masculine men, and are sexually attracted to women; and genetic females identify themselves as girls, grow up to become feminine women, and are sexually attracted to men. But everything we've discussed about about gender dimorphism so far has proved to be more complicated than that. Genetic males don't always grow up with the corresponding reproductive anatomy; they don't always identify themselves as little boys; and they don't always become stereotypically masculine. Genetic females are no different in these respects. And so, as we might expect, erotic orientation is also pretty complicated.

  • Most men and women are, indeed, heterosexual in their erotic orientation: these men are attracted to women but not men, and these women are attracted to men and not women.
  • But some men and women, about 10% of the population are homosexual in their orientation: gay men attracted to men but not women, and lesbian women attracted to women but not men.
  • Some individuals are bisexual -- for example, some men are attracted to both women and men, and vice-versa for women.
  • And other individuals are, frankly, asexual. They may be biologically normal, masculine men or feminine women, but just don't feel that spark -- for anyone.

Are Bisexuals Really BI?

The existence of bisexuality has been a subject of considerable controversy. Manicheanism is very attractive, and the same Manichean view that people are either male or female, boys or girls or men or women, or masculine or feminine, extends to the view that people are either heterosexual or homosexual. Certainly, a nontrivial number of men and women claim to be attracted to, and sexually aroused by, partners of both sexes. But it's taken a surprisingly long time to put these self-reports to empirical test.

A pioneering study reported in 2005 by Rieger, Chivers, and Bailey used a penile plethysmograph -- a psychophysiological device that uses a strain gauge to quantify penile erections, which occur when the penis becomes engorged with blood -- to monitor the sexual arousal of a number of self-identified homosexual men. The finding was that 75% of the subjects were aroused exclusively by images of male homosexual sexual activity, while 25% were aroused exclusively by heterosexual images. of heterosexual activity. These investigators concluded, controversially, that male bisexuality wasn't a distinct pattern of sexual arousal, but rather an interpretation that some men placed on their homosexuality -- that they weren't really homosexual, or weren't homosexual exclusively.

However, later research, using the same sorts of procedures, appears to have identified genuine bisexuality after all. Another study from Bailey's laboratory, by Rosenthal et al. (2011), found that some bisexual men were, indeed, sexually aroused by both homosexual and heterosexual imagery. These later findings were supported by a second study by Cerny et al. (2011).

What made the difference between the earlier and later studies? Apparently, differences in subject selection. The subjects in Rieger's 2005 report were selected based on their self-reported patterns of sexual arousal; but in Rosenthal's 2011 report, the subjects were required to have had sexual experiences with at least two partners of each sex, and a romantic relationship with at least one person of each sex.

To date (2011), there have been no comparable studies of bisexual women -- although, to answer your question, there does exist a vaginal version of the plethysmograph, technically a photoplethysmograph, shaped like a tampon, capable of recording blood flow in the vagina as an index of female sexual arousal.

By the way, there's no comparable research on asexuals -- apparently this topic doesn't arouse much scientific interest.

For an article about the controversy over bisexuality, see "The Scientific Quest to Prove -- Once and For All -- That Someone Can be Truly attracted to Both a Man and a Woman" by Benoit Denizet-Lewis, New York Times Magazine, 03/23/2014.

There was a time when homosexuality was classified as abnormal behavior, a sign of mental illness, and homosexuals were frequently sent to psychiatrists and psychologists in an attempt to "cure" them of their deviance. In 1970, following a vote of the members of the American Psychiatric Association, homosexuality was removed from the listing of "sexual disorders" in the 3rd edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM-III). Individuals can still seek psychotherapy if they are bothered by their homosexual orientation (or by their heterosexual orientation, for that matter). But that is not the same thing as considering homosexuality per se to be a mental illness.

Transsexualism, or transgender, has undergone a similar evolution.  In DSM-II, published in 1968, transgender identity was listed as a "sexual deviation".  In DSM-III (1980), the edition which eliminated homosexuality as a category of mental illness, transsexualism was listed as a "psychosexual disorder".  In DSM-IV (1994) it was listed as a "sexual and gender identity disorder".  In DSM-5 (2013), the listing was changed to "gender dysphoria" and limited to individuals who distress or dysfunction with respect to their gender identity, male or female, trans or not.  In 2017 the World Health Organization proposed to remove transsexualism, or transgender identity from its International Classification of Diseases (ICD), the worldwide equivalent of the DSM.

Homosexuality was also commonly considered to be criminal behavior -- which is why so many homosexuals, especially homosexual men, stayed "in the closet" until relatively recently. As recently as 2003, 14 states still had "sodomy laws" on their books, proscribing such "unnatural" sexual behaviors as oral or anal sex -- although, frankly, these had rarely been enforced when practiced by heterosexual couples). These remaining laws were invalidated in a landmark Supreme Court ruling in Lawrence v. Texas (2003).

The origins of homosexuality are a persisting puzzle for psychologists and biologists. At first blush, homosexuality would appear to be maladaptive, because homosexuals do not procreate, the trait doesn't contribute to the survival of the species, and one would think it would have been erased from the human genome by now. For that reason, evolutionary psychologists twist themselves inside out trying to concoct "just so stories" to explain how homosexuality is adaptive after all.

Evolution of Homosexuality

Homosexuality poses problems for evolutionary psychology, because, like altruism, it seems maladaptive. If evolution favors traits that increase reproductive fitness, how could a trait evolve that doesn't lead to reproduction at all? Over the years, a number of hypotheses have been offered to explain how a genetic basis for homosexuality might have evolved:

  • Based on the observation that bonobos, a subspecies of chimpanzees (and thus closely related to humans) engage in homosexual and bisexual behavior in order to form strategic alliances, it has been suggested that homosexuality might enhance fitness in a similar way for humans.
  • Homosexuals may have such strong sex drives that they engage in heterosexual as well as homosexual activity, and thus pass genes for homosexuality to the next generation in the usual way.
  • It may be that homosexuality is determined by not one gene but many, and that the genes for homosexuality are only activated in a particular intrauterine environment (such as low levels of androgens). Thus, some heterosexuals could carry genes for homosexuality the same way some brown-eyed individuals carry genes for blue eyes. Homosexuality would only occur if the person inherited a critical number of homosexuality genes, and if these genes encountered the "right" intrauterine hormonal environment.
  • While homosexuals might not produce many children themselves, they may gain an adaptive advantage by serving as guardians of their kin. Because genes are shared among family members, anything that homosexuals do to increase the reproductive fitness of their heterosexual siblings and cousins will also pass a genetic tendency toward homosexuality into the next generation (a similar argument from kin-selection has been offered for altruism).
    • A similar explanation has been offered for the existence of grandmothers, who, being post-menopausal, can't procreate anymore either. Which, in my view, just goes to show you how stupid both ideas are.

These hypotheses are all very interesting, but they all appear to be predicated on the same adaptationist fallacy -- the notion that traits must be adaptive to evolve, and that traits evolve by virtue of their adaptive value.

The answer to the mystery of homosexuality may be simply that it is a mystery. If you think about it, setting reproductive issues aside, the sexual attraction that two people of the same sex feel for each other may be no different, no more mysterious, than the sexual attraction that two people of opposite sex feel for each other.

As Hanne Blank puts in in her book,Straight: The Surprisingly Short History of Heterosexuality (2012):

We don't know much about heterosexuality. No one knows whether heterosexuality is the result of nature or nurture, caused by inaccessible subconscious developments, or just what happens when impressionable young people come under the influence of older heterosexuals".

Development of Homosexuality

In many cases, the precursors of homosexuality can be seen long before adulthood, or even adolescence, in childhood behavior patterns (Bailey & Zucker, 1995).  As a group, both male and female homosexuals tend to show cross-sex-typed role behaviors fairly early in childhood.  These children have sex-appropriate gender identities, in that chromosomal boys identify themselves as boys and chromosomal girls identify themselves as girls.  It's just that they tend to display characteristics associated with the other gender role -- the boys more "feminine", the girls more "masculine" in sports, play, toy choice, and "pretend" play.  However, not too much should be made of these signs of "prehomosexuality".  Children also engage in a fair amount of role-experimentation, as they begin the process of figuring out who they are. 

Let's look at our tried-and-true way of examining the origins of some trait, which is the twin study.  Michael Bailey and his colleagues reviewed 12 large twin studies, looking at the concordance rate for homosexuality.  The figures differ a little depending on whether the study specifically recruited homosexual subjects, which injects some bias into the sample, or rather was based on more representative samples of the population.  In either case, the MZ concordance rate is higher than the DZ concordance rate, which again provides prima facie evidence for a genetic contribution to homosexuality.  When the researchers calculated an overall estimate, correcting for sampling bias, it's very clear that when it comes to erotic orientation, as with almost everything else we know about personality and attitudes, genes are important but not decisive: the environment, and particularly the nonshared environment, makes a big difference.

More recently, Andrea Ganna and her colleagues documented genetic influences on homosexuality -- or, at least, same-sex sexual behavior -- using the GWAS methods described earlier (Science, 08/30/2019, from which the graphics are taken).  Based on two large samples from the UK (the Biobank database of 408,995 individuals) and the US (68,527 individuals who subscribed to the "23and Me" service), as well as three smaller samples, they looked for specific gene loci (technically, SNPs) associated with subjects reporting that they had ever had sex with a partner of the same sex.  But the investigators also had information about the proportion of same-sex to total sexual partners, sexual attraction, and sexual identity.  After all this, they found 5 such SNPs two for men (on chromosomes 11 and 15), one for women (on chromosome 4), and two for both men and women (on chromosomes 7 and 12).  Of course, having sex with a partner of the same sex isn't quite the same thing as being homosexual -- there's a fair amount of same-sex sexual experimentation that never gets beyond that.  Interestingly, none of these loci were on the X chromosome -- a region known as Xq28 famously identified as the "gay gene" by Dean Hamer, a researcher at the National Institutes of Health, in 1993 (one wonders, then, how these 5 SNPs will fare in the next study). That's five -- 5 -- genes out of 23,500 in the entire human genome.  And taken together, these genes account for only 1% of population variance in same-sex behavior.  As the authors put it, "same-sex sexual behavior, like most complex human traits, is influenced by the small, additive effects of very many genetic variants, most of which cannot be detected at the current sample size". 

The researchers also examined the "genetic correlations" between same-sex sexual behavior and other personal characteristics -- that is, the amount of variance that two characteristics share due to genetic influences.  Among the strongest associations were marijuana use, openness to experience (one of The Big Five personality traits) and the number of sexual partners.  Interestingly, the genetic correlation between same-sex sexual behavior and the ratio of the lengths of the subjects' 2nd and 4th fingers -- the "2D:4D digit ratio", famously reported to be a physical trait correlated with androgen levels in men (leading perhaps millions of men to get out their rulers and measure themselves -- again).

It is possible that hundreds or thousands of genes make a contribution to same-sex behavior, but only these five -- 5! -- turned up significant in a GWAS involving half a million people.  And again, they account for only 1% of the variance.  That's a long way from "the gay gene".  And when the investigators took account of all the genome-wide correlations, even the ones that didn't reach statistical significance, they obtained a "SNP-based" heritability coefficient ranging between 8 and 25%.  That is, they estimated that all genetic influences, when aggregated, accounted for as little as 8%, or as much as 25%, of population variance in same-sex sexual behavior, broadly construed to include fantasies, attraction, and identity as well as actual behavior.  That's about the same as for other personality characteristics, as estimated by more conventional twin studies (and without all the high-tech hoopla).  And, it leaves the vast bulk of population variance to be explained by environmental influences, both shared and nonshared.  Some of the nonshared environment may, in turn, be influenced by genetic tendencies.  For example, there is a genetic contribution to the tendency to have multiple sexual partners -- and the more sexual partners one has, arguably, the more likely at least one of them will be of the same sex.  And there is a genetic contribution to openness to experience, and one of those experiences might be same-sex sex.

The influence of genetics has been used by some advocates to argue that homosexuality is not a choice -- it's given by nature, just like our other physical characteristics; and therefore, homosexuality should not be criminalized, and homosexuals should not be discriminated against (for example, they should have the same rights to marry as heterosexual individuals). And that's a point. But does the role of the (nonshared) environment indicate that there's a role for personal choice as well -- that, in the final analysis, homosexuals choose to be that way? Not necessarily.

At the same time, it's pretty clear that nature and nurture interact to create an individual's erotic orientation. How could this happen? We have no idea, but think about how, as we've shown nature and nurture interact to produce other aspects of gender dimorphism -- biological sex, gender identity, gender role -- and erotic orientation as well.

Here are two very interesting theories that suggest how nature and nurture could interact to determine whether one is heterosexual or homosexual. Even if these theories are wrong in detail, they are important because they suggest a way of thinking about the origins of homosexuality that escapes the rigid confines of thinking of it as a biologically determined trait on the one hand, or as a personal choice on the other.

For example, Michael Storms (1981) based his theory on the phenomenon of imprinting, and the notion of a critical period, discussed in the lectures on Learning. Storms began with the assumption, for which there is some supporting data, that gay men tended to reach puberty earlier than heterosexual men (this claim is controversial, but it doesn't matter for the purposes of this example). That is to say, some boys reach sexual maturity, start getting sexually attracted to anything, at a time in their lives when they're hanging around with other boys (because, at that age, girls are still yucky creatures to be avoided). Just as a gosling follows the first thing that moves after it hatches, so (the theory goes) a boy who enters puberty when there are mostly other boys in his environment will become erotically oriented to other males. A similar explanation would hold for lesbian women.

Does the
              Exotic Become the EroticMore recently, Daryl Bem (1996, 1998) proposed a theory of erotic orientation that is similar to Storms' in form, but differing in detail. Bem began with the assumption that we are sexually attracted to people who are very different from ourselves. Yes, similarity breeds liking, but -- to quote Bem's phrase -- "the exotic becomes erotic". So, a boy with stereotypically masculine personality characteristics will, when he reaches puberty, begin to be attracted to someone with stereotypically feminine personality characteristics -- which is, in all likelihood, a female. But a boy with stereotypically feminine personality characteristics will, when he reaches puberty, be attracted to someone with stereotypically masculine personality characteristics -- which will, in all likelihood, be a male. the same kind of process unwinds for girls. You can work out other possibilities for yourself.

Note that both Storms' and Bem's theories are compatible with a genetic contribution to homosexuality -- but, critically, neither one assumes that there is anything like a "gene for homosexuality"; and, thus, neither gets caught up in the Darwinian paradox of same-sex attraction.

  • Rather, for Storms, it might be that certain genes code for age of sexual maturity: a boy who gets a genetic endowment that leads to an early puberty might be more likely to develop homosexuality.
  • And similarly, for Bem, it might be that certain genes code for stereotypically masculine or feminine personality traits -- for agency and instrumentality or communality and expressiveness -- or, at least, for activity level and aggressiveness.

And also note that both theories are compatible with a large contribution of the nonshared environment.

  • For Storms, whether a precociously pubescent boy becomes homosexual will depend on whether his environment is full of boys or has some girls in it.
  • And for Bem, whether a stereotypically feminine boy becomes homosexual will depend on whether there are some stereotypically "masculine" girls around to be perceived as exotic, and thus eroticized.

It should be noted that, in part, the development of homosexuality has a cultural component to it.  It's been argued that gay people, much like deaf people -- for that matter, much like Catholics and Jews and New Yorkers, inhabit a distinctive culture that is different from the one inhabited by straights (Protestants, Muslims, San Franciscans).  Part of identifying yourself as a member of a group involves learning and partaking of that culture through the process of social learning.  This would be true even if homosexuality were completely determined by genetic endowment.  This is not to say that vulnerable children are recruited into the "gay lifestyle" by scoutmasters and gym teachers.  Wherever homosexuality, or any other part of one's identity and personality comes from, you have to learn how to be who you are.  It's also to say, paraphrasing David M. Halperin, an English professor at U. Michigan who has taught a course entitled "How to be Gay: Male Homosexuality and Initiation", which became infamous among conservative pundits, that "gay [people] acquire a conscious identity, a common culture, a particular outlook on the world, a distinctive sensibility....Queer [people] are different, and we should hold on to our culture" ("How to Be Gay", Chronicle of Higher Education, 09/07/2012; see also his 2012 book, How to Be Gay).

It should be noted that this general framework -- nature and nurture interacting, through genes and the nonshared environment -- is not specific to aspects of gender dimorphism. Other aspects of personality are probably acquired in much the same way. In other words, the development of gender differences in identity and role (not to mention erotic orientation) serve as models for the development of personality in general.

For a comprehensive overview of the development of homosexuality, and the implications of this research for public policy, see "Sexual Orientation, Controversy, and Science" by J. Michael Bailey et al., published in Psychological Science in the Public Interest (2016).

Gender Polymorphism

320 (+) Gender-Related Categories?All of this should have made clear that, when it comes to issues of sex and gender, the story is not merely one of straightforward genetic determinism. Contrary to what Freud said, anatomy isn't destiny after all.

  • There may be as many as five categories of biological sex.
  • And, if you include transgender individuals, there are at least four categories of gender identity.
  • And if you include androgynous and undifferentiated individuals, there are at least four categories of gender role.
  • And if you include bisexuality and asexuality, there are at least four categories of sexual orientation.

So, if you do the math, that's 5 x 4 x 4 x 4 = 320 different gender-related categories. And that's a minimum estimate -- especially when you consider that these aspects of gender dimorphism may not be organized as discrete categories at all, but rather as continuous dimensions, so that there is an infinite number of locations in the four-dimensional "gender space". Never mind that some children and adults are gender fluid, moving back and forth between male and female gender identity and masculine and feminine gender role.

And, to make things even more interesting, very recent research adds a fifth set of gender-related categories, what you might call erotic or sexual identity, analogous to gender identity, but having to do specifically with sexual attraction and behavior. So, just as you can have a person who is biologically male but has a feminine gender identity, or someone with a male gender identity who adopts a feminine gender role, so there appear to be individuals who are, say, homosexual in erotic orientation but who identify themselves as heterosexual (Haldeman, 2003, 2004;. That is, they acknowledge their homosexual leanings, but for some reason wish to identify themselves as heterosexuals -- and sometimes seek treatment to help them conduct themselves accordingly. This is not the same as forcing a homosexual to undergo "conversion therapy". Rather, this appears to be a matter of how individuals choose to identify themselves -- often for religious reasons, but sometimes for nonreligious personal reasons.

  • So, if we add four categories of erotic identity to the categories listed above, that makes 320 x 4 =1,280 different gender-related categories. As I said at the beginning, it's complicated.

But it's complicated in very interesting ways. And these complications have implications for how we think about development, and especially personality development, in general.  For example, Hyde et al. (Am. Psych., 2018) have argued that emphasis on the "gender binary" -- the idea that there are only two types of people, male and female, has thoroughly distorted scientific research on all aspects of gender.  They identify five sets of research outcomes that challenge the gender binary, and should lead us to think differently about gender in the future (quoting from their abstract):

  1. neuroscience findings that refute sexual dimorphism of the human brain;
  2. behavioral neuroendocrinology findings that challenge the notion of genetically fixed, non-overlapping, sexually dimorphic hormonal systems;
  3. psychological findings that highlight the similarities between men and women;
  4. psychological research on transgender and nonbinary individuals’ identities and experiences; and
  5. developmental research suggesting that the tendency to view gender/sex as a meaningful, binary category is culturally determined and malleable.

The Latest on Sex and Gender

For more on biological, psychological, and sociocultural aspects of sex and gender, see the Special Issue on Sex and Gender published by Scientific American, 09/2017.  As the editors say on the cover, "It's Not a Women's Issue: Everybody has a stake in the new science of sex and gender".  For example:

  • "Promiscuous Men, Chaste Women, and Other Gender Myths", by Cordelia Fine and Mark A. Elgar, takes issue with the classic position of evolutionary psychology that behavioral differences between males and females are hard-wired into the nervous (and endocrine) system by natural selection.  On the contrary, they show that environmental and experiential factors play a major role in many of these differences, that many of these differences, besides being small, are not immutable.  In their view, :progressive cultural shifts" "rewrite" nature.
  • "Is There a 'Female' Brain?", by Lydia Denworth, answers its own question with a fairly vigorous "No", and that "most brains are a mosaic of male and female characteristics".  As with behavioral and psychological sex differences, their is considerable overlap between males and females in the distribution of various features of the brain (e.g., the volume of the left hippocampus).
  • "When Sex and Gender Collide", by Kristina R. Olsen, summarizes the results of the TransYouth Project, a study which has followed more than 300 transgender and gender-nonconforming children for 20 years.  A major finding of the study is that a fairly firm "trans" identity develops fairly early in these children.
  • "Beyond XX and XY", by Amanda Montanez, consists of a fabulous chart, much expanded from Money and Ehrhardt's charting of the phyletic and social imprimatur, showing all the different biological factors that can affect biological sex (click on the image below for a full-size reproduction). 
  • "The Brilliance Trap", by Andrei Cimpian and Sara-Jane Leslie", takes on the question of whether the (small) sex differences in math ability justify the under-representation of women in STEM fields.  It doesn't -- not least because a whole host of psychosocial factors, including gender stereotyping (and the sex discrimination it leads to) and stereotype threat (and the self-handicapping it leads to) are probably more important.  They also make the interesting  argument that stereotypes about scientific or artistic "brilliance", coupled with the myth of gender difference (and the stereotype threat that comes with it) discourage girls and women from even entering some fields -- even though most of those who work successfully in these fields are not "brilliant" -- only very smart.  The bottom line is that there are plenty of women soldiers, pilots, and engineers to go around, if only the psychosocial context were more favorable.
In their introduction, the Editors note that "Sex is supposed to be simple -- at least at the molecular level.  The biological explanations that appear in textbooks amount to X+X=[female\ and X+Y=[male], Venus or Mars, pink or blue.  As science looks more closely, however, it becomes increasingly clear that a pair of chromosomes do not always suffice to distinguish girl/boy -- either from the standpoint of sex (biological traits) or gender (social identity)."

They're right.  The biology of sex and gender, taken alone, is complication enough.  Add the psychology, not to mention the anthropology, sociology, and all the other social sciences, and you've got plenty more.  

Maturation: Development as Quantitative Change

The earliest theories of psychological development focused on maturation and learning. In general, these theories offered a view of the child as a short, stupid adult who grows smarter as he or she grows bigger. Viewed in this way, there is a continuum between childhood and adulthood, with no abrupt, qualitative changes.

Maturation may be defined as the progressive, inevitable unfolding of certain patterns of behavior under genetic control. These behavior patterns occur in a regular sequence, unaffected by practice or environmental change.

walkingMaturation is a good description of certain developmental processes, such as walking. We speak of children "learning" to walk, but we know from the stepping reflex in infants (see the lecture supplement on Learning) that walking occurs naturally, requiring only that the child be able to support itself. Thus, walking occurs as soon as the skeletal musculature develops sufficiently to provide that support.

Development as Quantitative Change A classic study of maturation involved traditional Hopi and Navajo children, who are swaddled and bound to a cradle for the first year of life. This severely restricts motor behavior, but once released from the cradle there is little retardation in the emergence of walking.

              Developmental Schedules In another classic study, by Arnold Gesell, some children were trained in walking and climbing stairs. They did, in fact, show these behaviors earlier than untrained children. But the untrained controls quickly caught up, and the further progress of the experimental group was not accelerated by their training. Both groups advanced beyond walking at the same pace.

The Growth of IntelligenceThe continuous view of development is exemplified by the measurement of intelligence in terms of IQ:

  • Alfred Binet simply estimated the individual's mental age.
  • William Stern calculated IQ as the ratio of the individual's mental age to his or her chronological age, imposing an artificial ceiling of 18 years on both ages. An individual with an IQ of 100 is exactly as old mentally as he is chronologically.
    • This ratio IQ was also adopted by Louis Terman.
  • David Wechsler substituted the deviation IQ for Terman's "ratio IQ", so that an individual with an IQ has the same IQ test score as the average person in his age group -- even if he's older than 18.

The implication is either method of measuring intelligence is that children continuously grow smarter as they grow older.

Despite debates over whether IQ is heritable, the classical "continuity" view is that the child gradually acquires knowledge through learning, where learning was construed as tantamount to classical or instrumental conditioning. John Locke, an English philosopher of the 18th century, famously argued that the infant is a tabula rasa, or "blank slate", which is "written on" by experience. In the Lockean view, development is a matter of learning more than you already know.

Whether the theoretical focus is on maturation or learning, the process of development is viewed as a matter of continuous, quantitative change: the infant starts out small, physically and mentally, and gets bigger, physically and mentally, as he or she grows up.

Is Childhood a Recent Cultural Invention?

In his book,Centuries of Childhood (1960), Philippe Aries, a French social historian, created quite a stir by arguing that what we call "childhood" did not exist for most of history, and instead was a creation by modern liberal thought (by which he meant the liberalism of the Enlightenment of the 17th and 18th centuries). In medieval times, Aries argued, children joined the adult world pretty much as soon as they could walk, talk, and eat solid food. They married early (think of Romeo and Juliet, and they went to war (think of the Children's Crusade), and they earned money (in the fields or as domestic servants or apprentices). They were, in fact, short, stupid adults. It was only in the modern era that children were seen as different, as innocents who were sent to school, or protected at home (in what became the nuclear family), as opposed to going out to work with the rest of their extended family. The prime mover in this shift, according to Aries, was Jean-Jacques Rousseau, who promoted a sentimental view of children. Other historians quickly picked up the general thrust of his argument, particularly Edward Shorter (in The Making of the Modern Family, 1975) and Lawrence Stone (in The Family, Sex, and Marriage in England, 1500-1900, 1977).

Relatedly, it has been argued that adolescence is largely an invention of the 20th century, with its laws against child labor and compulsory schooling.

However, there were also dissenters, such as Steven Ozment (in Ancestors, 2001), who argued that children have always been children, pretty much as they are today.

Eventually, the dispute spawned a great deal of historical research on childhood and the family. Joan Acocella, reviewing the three-volume History of the European Family (2001, 2003, 2004), shows how family life did indeed change radically in the 17th and especially the 18th centuries. But it turns out that Aries generalized way too far beyond his limited data, which he generally selected in ways that would support his theory. The actual picture is pretty complex, but in the end Acocella generally sides with Ozment: "there had been a culture of childhood ever since there were documentable children" ("Little People",New Yorker, 08/18-25/03, from which the illustrations, by Saul Steinberg, are also taken).

How About Adolescence?

At least with adolescence, there's a physical marker -- the onset of puberty.  But even so, in pre-industrial societies -- including Western societies -- "adolescents" were often engaged on adult-like work behavior -- for example, as apprentices.  The concept of adolescence was introduced by G. Stanley Hall, a pioneer of developmental psychology, in 1904.  Hall thought that adolescence was universal, but cultural anthropologists find that it is far from that: many cultures don't have anything like it -- when puberty hits, "adolescents" take up adult roles in work and family life. 

Or "Emerging Adulthood"?

For better or worse, adolescence is now firmly entrenched in Western culture.  But some theorists, taking their inspiration from Erik Erikson (whose theory is detailed below), have argued for an intermediate stage between adolescence and adulthood, covering 18-25 years of age.  Jeffrey Jensen Arnett (2000) has vigorously promoted this view in a series of books, such as Adolescence and Emerging Adulthood: A Cultural Approach (2009) and Debating Emerging Adulthood: Stage or Process" (2011).  Arnett argues that, just as adolescence delays the transition from childhood to adulthood, so "emerging adulthood" delays the transition from adolescence to adulthood.  For example, the median age of first marriage increased by 4-5 years between 1970 and 1996; so has the median age of entry into the workforce.  The concept brings to mind the remark of Paul Ryan, Republican vice-presidential candidate in 2012, that “College graduates should not have to live out their 20s in their childhood bedrooms, staring up at fading Obama posters and wondering when they can move out and get going with life”.  On the other hand, a 2012 study by the Pew research Center indicates that most American twenty-somethings don't live with their parents; they have jobs and, in many cases, have formed stable relationships.  But still, as with adolescence, there may be something to the notion.  Just because a stage of life is, to some extent, a cultural artifact doesn't make it any less real.

As Opposed to "Established Adulthood"

After "emerging" adulthood, then what? Mehta et al. (Am Psych 2020) have argued for a new lifespan "stage", as it were, covering roughly 25-40 years of age.  During this period, in their view, adults must deal with the "intersecting demands of progressing in a chosen career, maintaining an intimate partnership, and caring for children" -- and often, one might add, aging parents as well.  Individuals at this stage of life often face what Mehta et al. call the "career-and-care crunch" of conflicting obligations, responsibilities, and desires.  The crunch especially effects women who work outside the home, but men aren't immune to it.


Development as Qualitative Change

Later theories of development focused on qualitative changes. The child is not just a short, stupid adult, but rather the young child is held to think differently than older children and adults. Thus, developmental differences are qualitative, differences in kind, not just quantitative, differences in amount. Children are not stupid, compared to adults, but their intelligence needs to be appreciated on its own terms. D

Despite debates over the heritability of IQ, the classical continuity view is that the child gradually acquires knowledge through learning, where learning was construed as tantamount to the sorts of things we studied in the lectures on classical and instrumental conditioning.  John Locke, an English philosopher of the 18th century, famously argued that the infant is a “tabula rasa”, or blank slate, which is written on by experience.  In the Lockean view, development is a matter of learning more than you already know.  This contrasted with the earlier view of Descartes, who assumed that some knowledge was innate, as a gift from God.  Whether the theoretical focus is on biological maturation or on learning, in either case the process of development is viewed as a matter of continuous, quantitative change.  The infant starts out small, physically and mentally, and gets bigger, physically and mentally, as he or she grows up.

Piaget's Stage Theory of Cognitive Development

This was the view propounded by Jean Piaget, who argued that the development of intelligence proceeds through a sequence of stages, each defined by cognitive landmarks:

  • sensory-motor intelligence, running from birth to about 2 years of age;
  • preoperational thought, encompassing approximately ages 2-7.
  • concrete operations, approximately ages 7-12, and
  • formal operations, the last stage of cognitive development, beginning about age 12.

              Stages of Intellectual DevelopmentAn important concept in Piaget's theory is the schema, a term which should be familiar from Bartlett's work on memory reconstruction and Neisser's concept of the perceptual cycle. For Piaget, a schema is an organized mental structure that produces a particular coordinated response to a certain class (or category) of stimulation. Thus, a schema is a kind of concept, that renders diverse objects functionally equivalent -- because they belong in the same class.

In a very real sense, cognitive development is the development of schemata (the plural of schema, although schemas is also an acceptable alternative) -- their elaboration and differentiation. Schemata are like categories, and guide the perception of (and thus response to) particular objects and events. The interaction between schemata and the objects with which they come into contact is characterized by the twin processes of assimilation and accommodation. By virtue of assimilation, the mental representation of the stimulus is altered so that it will fit into the schemata employed to process it; by virtue of accommodation, the schema itself is altered so that it can receive the stimulus. Thus, the final representation of the stimulus is a sort of compromise between what was expected and the actual stimulus itself. The neonate confronts the world with a primitive set of innate schemata; through assimilation and accommodation, the nature of these schemata gradually change. At certain points, however, the changes in mental schemata are so dramatic that they appear qualitative rather than merely quantitative; these shifts mark the child's move from one stage of cognitive development to another.

Sensory-Motor IntelligenceThe first of Piaget's stages is sensory-motor intelligence (also known as the sensory-motor period). In this stage, which encompasses the first two years of life, the world of the child is one of unrelated sensory experiences and reflex-like motor reactions to them. The child has no ability to connect the present with the past or the future, and no ability to distinguish between self and other. At least, that is how the child starts out. To take a phrase from William James, for the sensory-motor infant the world is a "blooming, buzzing confusion". Over the first two years of life, these capacities develop; according to Piaget, their complete acquisition terminates this stage of cognitive development.

One of the major accomplishments of the sensory-motor period is the development of object permanence. The newborn's behavior is tied to what comes through its senses, which are processed by sensory-motor schemata. Out of sight is out of mind. Eventually, however, the child comes to behave as if they have internal representations of objects that are not actually present in their sensory environment. Early on, if a toy is hidden they will turn their attention to something else. Later, if a toy is hidden they will search for it. This searching behavior shows that the child has an idea of the object that persists despite its physical disappearance -- at this point, the child has acquired the capacity for forming internal, mental representations --memories -- of the outside world.

              Pre-Operational PeriodAt this point, about 24 months after birth, the infant moves fully into the next stage of cognitive development, the preoperational period. At this point, the child is able to form and retain internal representations of objects and events, but these representations exist as individual mental units, unrelated to each other. The major achievement of the preoperational period is the ability to relate one representation to another, through higher-order schemata called operations. This takes about the next five years.

The development of operational modes of thought is marked by the emergence of conservation, which occurs by about the time the child is seven years of age. In the earliest portion of the preoperational period, the child does not conserve at all. If a short, wide cup of liquid is poured into a tall, thin glass, he or she may well say that there is more liquid in the former than in the latter. This is because the child can track changes in either height or diameter (actually, if you want to be technical, radius), but not both simultaneously. Thus, the child cannot understand that volume, which is a product of height and radius, remains constant when the liquid is poured from one container into the other. Similarly, if a young child is shown five objects lined up over a short distance, and later the same five objects arrayed over a longer line, he or she is likely to say that there are more objects in the latter case than in the former. Thus, the child confuses the number of the objects with the linear distance over which they are arrayed. At some point, however, the child no longer makes these mistakes: he or she has acquired the ability to consider height and radius, number and distance, simultaneously, and compensate for one with the other. At that point, the child has acquired the ability of conservation.

              "Three Mountains" TaskAn ge and
              Egocentrism analogue of conservation failure in the interpersonal domain is egocentrism, which is not to be confused with selfishness. From Piaget's point of view, egocentrism reflects the child's inability to take another's point of view, or to appreciate the viewpoints of other peoples. Thus, an egocentric child will say that other people view the world from the same perspective as he or she does; and that others will react to events as he or she does. By the age of seven, however, the child has abandoned the egocentric attitude, and is able to represent the world as others see it, as well as he or she sees it.

Concrete OperationsAt about age seven, the child enters the stage of concrete operations, in which children are capable of thinking and reasoning about objects and events which they have actually experienced. Concrete-operational children are actually pretty powerful thinkers. They conserve, they can pay attention to objects other than the most salient ones, they can take another's point of view, they can take account of transformations in state, they can classify objects into groups based on shared properties, and they can generate and use hierarchical classification schemes. You can get along pretty well in life with nothing more than concrete operations, so long as you are reasoning about familiar problems involving familiar objects and events.

Formal OperationsUnfortunately, concrete operations aren't always enough. It is often useful to go beyond our own past experiences, and to reason about things we haven't seen or touched, and about things that aren't visible or touchable. Concrete operations don't suffice for this purpose. Good thing, then, that around age twelve the child enters into the last of Piaget's stages,formal operations. In formal operations, the thinking can be purely symbolic, without referring to anything at all by way of concrete objects and events. It is necessary, for example, for the transition from arithmetic to algebra -- where x, for example, is just an abstract symbol and doesn't refer to anything at all.

The hallmark of formal operations is scientific thinking, which is what lies behind Piaget's notion of the child as a naive scientist. This is marked by four different qualities: (1) hypothetico-deductive reasoning, in which we can hypothesize about a certain state of affairs, and then reason deductively from that hypothesis (that is, we can assume that something is true without requiring that it actually be true); (2) inductive reasoning, in which the person generalizes from specific observations to general principles; (3) reflective abstraction, in which the child is able to reflect on their own thoughts to arrive a novel ideas; and (4) propositional logic, in which the child can reason about two or more abstract entities represented by statements such as "If there is a P, then there is a Q". Children who have developed the capacity for formal operations are able to deal with several abstract variables at the same time.

If concrete operations are analogous to arithmetic, formal operations are analogous to algebra.

Piaget traced cognitive development only up to adolescence. One aspect of the debate about his theory is whether cognitive development does in fact end with the acquisition of formal operations, or whether there are other, more advanced, stages of thought.

Piaget and the "Science of Creative Intelligence"

One proposal for further stages of cognitive development, beyond Piagetian formal operations, has come from Maharishi Mahesh Yogi, founder of the Transcendental Meditation movement and guru to the Beatles. The Maharishi now promotes a science of creative intelligence in which meditation -- Transcendental Meditation, of course -- moves the practitioner to higher stages of cognitive development.

Another question is whether Piaget's stages are stages at all -- that is, whether it is true that children progress from sensory-motor intelligence through preoperational logic and concrete operations to formal operations in the way that he thought. An important part of this question concerns the lower boundaries of Piaget's stages. Is it really true that children younger than age 7 are generally incapable of abstract thought? A very large research tradition has developed out of questions like these, the general conclusion of which is that cognitive development is probably more continuous than Piaget thought, and that even very young children have amazing powers of thought, at least in limited domains. For example, it has been shown that five-month-old infants are capable of rudimentary arithmetic operations of addition and subtraction, so long as they are only asked to deal with very small number of objects. 

Of course, Piaget was not the only theorist to offer a conception of development in terms of stages.  Long before Piaget, Sigmund Freud had offered a stage theory of psychosexual development running essentially from birth to adolescence.  And in the 1960s, Erik Erikson, a follower of Freud's, offered a stage theory of psychosocial development that encompassed the entire life cycle from birth to death.  Piaget, Freud before him, and Erikson afterward had an enormous influence on thinking in developmental psychology.  Their idea of development as a progression through a succession of qualitatively different stages was received quite enthusiastically.  And you can see the legacy of these theorists in the proliferation of stage theories of everything in developmental psychology.

Kohlberg's Theory of Moral Development

Stages of Moral DevelopmentFollowing in Piaget's footsteps, Lawrence Kohlberg developed a theory of the stages of moral development -- by which he meant moral reasoning, not moral behavior. For Kohlberg, moral development followed a strict trajectory of 3 stages, each consisting of two sub-stages, through which the child proceeds from a heteronymous (other) to autonomous (self) orientation..

  • In the preconventional stage, which runs from birth to about age 9, moral reasoning is based on what is rewarded and what is punished. Think of this as analogous to Piaget's sensory-motor and pre-operational stages.
    • In the first sub-stage, moral choices are based on a principle of simple obedience and avoidance of punishment.
    • In the second sub-stage, moral choices are based on a self-interested desire to gain reward.
  • In the conventional stage, moral reasoning is based on rule-following. The child knows the rule, and follows it, without taking into account the reasoning behind the rule, or whether the rule itself is reasonable. This is analogous to Piaget's stage of concrete operations.
    • In the third sub-stage, the child is primarily concerned with interpersonal accord, and in gaining approval and avoiding disapproval through conformity to conventional rules.
    • In the fourth sub-stage, the child is guided by notions of duty, and obedience to authority, and desire to avoid feelings of guilt.
  • In the post-conventional stage, moral reasoning is based on more abstract principles, like the Golden Rule. The child now understands the reasons behind the rules, can make rational choices between alternative rules, and can reason to his own rules.
    • In the fifth sub-stage, moral reasoning is based upon notions of agreed-upon rights, such as the social contract.
    • In the sixth and final sub-stage, moral reasoning is based on universal ethical principles.
    • There may be a seventh sub-stage, of transcendental morality, but the nature of this stage isn't entirely clear, and I think that Kohlberg made a place for it simply on the chance that there really was some fifth Piagetian stage of cognitive development, beyond formal operations.

Kohlberg assessed an subjects' stage of moral reasoning by coding their responses to the Heinz dilemma -- a story about a man, named Heinz, who has a sick wife:

A woman was near death from a special kind of cancer. There was one drug that the doctors thought might save her. It was a form of radium that a druggist in the same town had recently discovered. The drug was expensive to make, but the druggist was charging ten times what the drug cost him to produce. He paid $200 for the radium and charged $2,000 for a small dose of the drug. The sick woman's husband, Heinz, went to everyone he knew to borrow the money, but he could only get together about $1,000 which is half of what it cost. He told the druggist that his wife was dying and asked him to sell it cheaper or let him pay later. But the druggist said: "No, I discovered the drug and I'm going to make money from it." So Heinz got desperate and broke into the man's store to steal the drug for his wife.

Should Heinz have broken into the store to steal the drug for his wife? Why or why not?

Like Piaget, Kohlberg believed that his stages were universal, but in fact his studies were largely confined to male subjects -- leading him to conclude, for example, that women generally failed to reach the highest stages of moral development (roughly equivalent to the views of a liberal Democrat).Later, Carol Gilligan -- who was first Kohlberg's student, and later his colleague at Harvard's Graduate School of Education -- argued that Kohlberg's procedures were flawed, and that women went through stages of moral development that were qualitatively different from those of men. In this way, Gilligan laid the foundation for "difference feminism", based on the "essentialist" view that women's mental lives follow different principles than those of men.

In any event, Gilligan revised Kohlberg's scheme, based on the idea that women are primarily motivated by a concern for others, rather than independence, and an ethic of care rather than an ethic of justice. Unlike Kohlberg (and Piaget), Gilligan did not assign particular age ranges to the stages, but she retained the assumption that girls and women moved through these stages in an invariant sequence. Whereas Piaget and Kohlberg assumed that the transition from one stage to the other is based on changes in cognitive capacity, Gilligan argues that the transition is mediated by changes in the person's sense of self.

  • In her view, the preconventional stage is concerned with individual survival, and is dominated by what can only be called selfishness. At some point, however, the person makes the transition from selfishness to a concern for others.
  • In this way, the conventional stage is oriented toward self-sacrifice as the manifestation of goodness. Of course, an ethic of self-sacrifice can be pretty detrimental too, so at some point the girl or woman makes the transition to the truth that she is a person too, and worthy both of self-care and the sacrifice of others.
  • So, finally, in the post-conventional stage, moral reasoning is based on a principle of nonviolence, and a goal to hurt neither others or oneself.

Freud's Theory of Psychosexual Development

Stages of
              Psychosexual Development In the domain of personality, another theory postulating qualitatively different developmental stages is Freud's theory of psychosexual development. The fundamental assertion of Freud's psychoanalytic theory is that personality is rooted in conflict between certain biological instincts (sex, aggression) and environmental constraints and demands. This conflict must be resolved in some way. The child's adaptation to conflict interacts with other developmental events, and personality is formed from the resulting habitual adaptation.

For Freud, sexual and aggressive motives are at the center of personality. These instincts arise from the id and are controlled by the ego and superego. They are the urges that the defense mechanisms defend us against. Now, nobody would argue that sexual issues are not important for personality. Many of the issues that confront adolescents and adults are sexual in nature. Once you reach puberty, if not before, sex is an issue. But Freud went further, by stating that sex is the paramount issue, from birth. He believed that sexual impulses were present in the newborn child, and that they continued to seek expression and gratification until death. The theory of infantile sexuality was Freud's most radical hypothesis. However, the student should understand that for Freud, sexuality was not confined to intercourse and orgasm. Rather, Freud defined sex as anything that lead to pleasure. Thus, the theory of infantile sexuality is a portrait of the infant as an active seeker of pleasure. The precise form that this pleasure takes is determined by the child's stage of development.

For Freud, all instincts have their origins in some somatic irritation -- metaphorically, some itch that must be scratched. At any time, a particular portion of the body is the focus of that instinct -- the place where arousal and gratification occur. These somatic loci change systematically through childhood, and stabilize in adolescence. These systematic changes comprise the stages in psychosexual development, and the child's progress through these stages is decisive for the development of personality.

  • The oral stage comprises the period from birth to approximately 12 months of age. According to Freud, the newborn child begins as "all id", "no ego", experiencing only pleasure and pain. In utero, nourishment was provided automatically. After birth, with feeding, the child must begin to respond to the demands of the external world -- what it provides, and the schedule with which it provides it. Thus, instinct gratification is initially centered on the mouth: sucking at breast or bottle. This sucking activity has obvious nutritive value, in that is satisfied hunger and thirst. But Freud also asserted that it also had sexual value, because the child gained pleasure from sucking. In addition, aggressive instincts can be displayed through the mouth, as in biting.
    • The legacy of the oral stage is a complex of dependency and separation anxiety. The child needs its mother for instinct-gratification, and her absence leads to feelings of frustration and anxiety. It also leads to the development of the ego, the mental structure whose job it is to separate fantasy from reality.
  • The anal stage lasts from about 1 to 3 years of age. Toilet training provides the child with his or her first experience of the regulation of impulses -- the child must learn to postpone the pleasure that comes from the relief of anal tension.
    • The legacy of the anal stage is the sense that one can acquire desirable goods (e.g., praise) by giving or retaining; the first pangs of loss; and, especially, the first sense of self-control.
  • The phallic stage lasts from about 3 to 5 years of age. In this period, Freud believed the child, boy or girl, was preoccupied with sexual pleasure derived from the genital area -- curiosity, exhibitionism, and masturbation. Why is this stage called phallic, when only boys have a penis? His idea is that in different ways, both males and females are interested in the penis. How this is so leads us to one of Freud's most startling theories, the Oedipus complex.
    • During the phallic period, Freud thought there occurred an intensification of sexual interest in the parent of the opposite sex. In his terms, there is a sexual cathexis (e.g., heightened attention) toward the parent of the opposite sex, and an aggressive cathexis toward the parent of the same sex. This is the Oedipus complex, named after the Greek myth about the man who unknowingly killed his father and married his mother, and the phallic stage revolves around its resolution.
    • The beginnings of the Oedipus complex are the same for males and females. They love their mother because she satisfies their needs, and they hate their father because he competes for the mother's attention and love.
      • In the male, the Oedipus complex occurs as the jealousy of the father combines with castration anxiety. The child, remember, is engaging in autoerotic activity, which is often punished by a threat to remove the penis. This threat is reinforced by observation of the female genitalia, which obviously lack a penis. So the child gets the idea that this threat is real. Nevertheless, the boy's love for his mother intensifies, and these incestuous desires increase the risk of being harmed by the father. The father is too powerful, and must be appeased. Thus, the boy represses his hostility and fear, and converts it by means of reaction formation into expressions of love. At the same time, mother must be given up, though she remains desirable. Thus, the child also represses his sexual longings. The final solution of the boy's problem is identification with his father. His father is now an ally instead of an enemy, and through this identification the boy can gain vicarious satisfaction of his desire for his mother.
      • In the female, the same sort of process works itself out in the Electra complex, named after the Greek myth of the princess, daughter of Agamemnon and Clytemnestra, who conspires with her brother Orestes to murder her mother and her mother's lover, in order to avenge their father's death. But the Electra complex is not the mirror-image of the Oedipus complex. For example, the girl's problem is not castration anxiety, since there is no penis to injure, but resentment at deprivation. Initially, the girl loves her mother for her role as caretaker, and has no particular feelings toward her father. Nor is she punished for autoerotic activity -- perhaps because it doesn't occur, perhaps because it isn't discovered. Eventually, however, the girl discovers that she lacks the phallic equipment of the male: this leads to disappointment and feelings of castration -- what Freud called penis envy. She blames her mother for her fate, which weakens her cathexis toward her; and she envies her father's equipment, which strengthens her cathexis toward him. The result is that the girl feels love for her father, and hatred and jealousy for her mother. She wants her father to give her a penis, and accepts a baby -- represented by a doll -- as a substitute. Thus, there is no clear-cut resolution of the Electra complex in girls. Castration is a fact, not a threat. In the end, however, the girl identifies with her mother in order to gain vicarious satisfaction of her love for her father.
    • In any event, the first legacy of the phallic stage is the superego -- the child internalizes social prohibitions against certain object-choices, as well as parental rewards and punishments.
    • The second legacy is psychosexual identification: the boy identifies with his father, the girl with her mother, and take on the characteristic roles, and personality, of the same-sex parent.
  • During the latency period, which lasts from about age 5 to age 11, Freud thought that the child's instinctual impulses subsided, with a slowing of the rate of physical growth, and the effects of the defenses brought to bear in the resolution of the Oedipus/Electra complex. During this period the child is not actively interested in sex and aggression, but works on the task of learning about the world, society, and his or her peers.
  • During the genital period, which lasts from age 12 to death, the person moves into another period of sexual interest. Sexual maturity reawakens the sexual instincts which have been dormant. But with a difference. There is a shift away from primary narcissism, in which the child takes pleasure in stimulating his or her own body, to secondary narcissism, who takes pleasure from identifying with an ideal. Sexuality shifts from an orientation toward pleasure to an orientation toward reproduction, in which pleasure is secondary. There is strong attraction to the opposite sex, and an interest in romance, marriage, and children. There is a continued focus on socialization. However, the earlier stages of psychosexual development can influence the nature of the individual's genital sexuality, in terms of the preferred locus of sexual foreplay, and the bodily focus of erotic interest.

Freud believed that the individual's passage through these stages left its imprint on adult personality. If all goes well, the person develops a genital character, as reflected in full sexual satisfaction through orgasm. The genital character is able to effectively regulate his (or her) sexual impulses for the first time. The person need no longer adopt primitive defenses such as repression, though certain adaptive defenses are still operative. The person's emotions are no longer threatening, and can be expressed. The person is no longer ambivalent, and is able to love.

However, all doesn't usually go well -- else there wouldn't be a need for psychoanalysts! Freud believed that people rarely, if ever, passed through the psychosexual stages without incident, and people rarely develop the genital character spontaneously. Usually, the person experiences some sort of developmental crisis at an earlier stage -- a crisis that prevents growth, fulfillment, and the final achievement of genital sexuality.

  • These difficulties are resolved through the aid of additional defense mechanisms.For example:
    • In fixation, for example, the anxiety and frustration experienced while advancing to a new stage cause growth to halt, so that the individual remains at the earlier stage.
    • In regression, the anxiety and frustration occur after the advance is completed; growth is lost as the person defensively reverts to an earlier stage of adjustment. The point at which fixation or regression occur determines the adult character.

By virtue of fixation and/or regression, the person -- that means you, and me, and everyone -- develops a particular neurotic character, depending on the developmental stage at which the person has fixated, or to which he or she has regressed.

  • The oral character develops through the resolution of conflicts over feeding and weaning. The oral-dependent type relies on others for self-esteem and relief of anxiety; he or she manifests oral preoccupations -- smoking, eating, drinking -- to overcome anxiety. The oral-aggressive type expresses hostility towards those who are responsible for his or her frustrations; this is expressed not through physical biting, but rather through "biting" sarcasm.
  • The anal character develops through the resolution of conflicts over toilet training. The anal-expulsive type engages in retaliation towards those responsible for his or her suffering. The person is messy, irresponsible, disorderly, and wasteful; alternatively, by virtue of reaction-formation, the person appears to be neat, meticulous, frugal, and orderly; but, Freud asserted, somewhere, something is messy -- revealing the person's essential anal-expulsive character. The anal-creative type produces things to please others, and also oneself. He or she displays generosity, charity, and philanthropy. The anal-retentive type shows a marked interest in saving and collecting things. The basic traits are parsimony and frugality; alternatively, again via reaction-formation, a record of foolish investments, reckless gambling, and spending.
  • The phallic character reflects an overvaluing of the penis. The male, compelled to demonstrate that he has not been castrated, is reckless, vain, and exhibitionistic ("Look what I've got!"). The female, resentful at having been castrated, is sullen, provocative, and promiscuous ("Look what I've lost!").

As you can see, this is the kind of theory that only a particular kind of man could conjure up. In discussing Freud's psychoanalysis, the student is warned that there is hardly a shred of clinical or experimental evidence in support of the theory. And a lot of it will strike a contemporary reader as quaint, if not downright silly. Nevertheless, psychoanalysis has had such an enormous impact on our culture -- literature, cinema, and art -- that it would be criminal to ignore it entirely. So you get some introduction to psychoanalysis in this course, but this is it -- a couple of crummy paragraphs buried in a lecture supplement on development.

Erickson's Theory of Psychosocial Development

Traditionally, mental development was thought to stop with adolescence -- or, at latest, the entry to adulthood.

  • In the earliest method of calculating IQ (mental age / chronological age), a ceiling of 18 years was established for both variables. Thus, mental growth was assumed to be complete in late adolescence.
  • In Freud's stage theory of psychosexual development, the oral, anal, and phallic stages were all negotiated before age 5, and the person was held to have achieved full adult psychosexual development -- the genital stage -- at adolescence.
  • In Piaget's theory, the highest level of thought -- formal operations -- is also achieved at adolescence.
  • Same for Kohlberg's and Gilligan's theories of moral development.

Even so, there has always been some sense that development was not complete at adolescence -- that change and growth were still possible in young adulthood, middle age, and old age.

              Eight ages of Man"This concern with development throughout the lifespan, from birth to death, is expressed in the psychosocial theory of Erik Erikson, a disciple of Freud's. Erikson argued that personality was the product of the social environment as well as of biology. He de-emphasized the instincts, and especially, infantile sexuality, and focused instead on the social conditions of child development and adult life. Mostly, Erikson focused on the issue of ego identity -- one's awareness of oneself, and one's meaning for other people. He also expanded the notion of development by arguing that there is, indeed, life after puberty. Not only did he propose stages of growth beyond the genital, but he also introduced a social reinterpretation of the classic Freudian stages -- hence the label "psychosocial", rather than "psychosexual".

A Self-Made Man

Erickson was personally consumed by issues of identity. He described himself as a "child of the borders". His adopted name was Erik Homburger, but he changed it to Erik Erikson -- literally, "Erik Son of Erik". As a self-made man, Erikson remade himself as his own father and his own son.

In the end, Erikson gave us an epigenetic conception of development similar to Freud's. That is, the individual progresses through an inevitable sequence of stages; and, at each, meets and resolves some crisis. Each stage builds on the one(s) that went before. Each stage has several elements, including a crisis that must be met and a strength that develops during the crisis. The resulting stage theory is sometimes known as the "Eight Ages of Man".

  1. In the oral-sensory stage, from birth to 1 year, the child comes to recognize some objects as familiar, experiences hunger for nourishment and stimulation, and deals with teething and grasping. The crisis is basic trust vs. mistrust: the child must trust that his or her wants will be frequently satisfied, while others must trust that the child will learn to cope with its impulses (to cry or bite). The legacy of the oral-sensory stage is hope, the enduring belief in the attainability of wishes.
  2. In the muscular-anal stage, from 1 to 3 years, the child learns to walk and talk, dress and feed itself, and control elimination. The crisis is autonomy vs. shame and doubt: the child learns to rely on its own abilities, or that its efforts will be ineffectual and criticized. The relevant strength is will, the determination to exercise free choice and self-restraint
  3. In the locomotor-genital stage, from 3 to 6 years, the child really begins to move about, find its place in the group, and approach objects of desire. The crisis is initiative vs, guilt: the child learns to approach what seems desirable, and experiences the contradiction between desires and restrictions. The relevant strength is purpose, the courage to envisage and pursue valued goals.
  4. In the latency period, from 6 to 11, the child makes the transition to school life, and begins to learn about the world. The crisis is industry vs. inferiority: the child learns and practices adult roles, but may conclude that it cannot operate the things of the world. The strength is competence, the free exercise of dexterity and intelligence.
  5. In the stage of puberty-adolescence, from 11 to 18, the young person experiences physiological growth, sexual maturity, adolescent love, and involvement with cliques and crowds. The crisis is identity vs. role confusion: the idea that one's past has prepared one for the future, as opposed to the failure to differentiate oneself from others and find one's place in the world. The strength is fidelity, the ability to sustain loyalties.

In the previous stages, Erikson reinterpreted Freud (as some of their names imply). In the following stages, emerging after adolescence, Erikson added to the basic Freudian conception.

  1. In the stage of young (or early) adulthood, from 18-30, the person leaves school for the outside world of work and marriage. The crisis is intimacy vs. isolation: the ability to share oneself in an intense, long-term relationship, as opposed to an avoidance of sharing because of the threat of ego loss. The strength if love, or mutuality of devotion.
  2. In the stage of (middle) adulthood, from 30 to 50, the person invests in the future through work and home. The crisis is generativity vs. stagnation: the ability to establish and guide the next generation, as opposed to a concern only for one's personal needs and comfort. The strength is care, a widening concern for what has been generated by love, necessity, or accident.
  3. In the stage of maturity (or late adulthood), from 50 into the 70s or so, death enters one's thoughts on a daily basis. The crisis is ego integrity vs. despair: a strong sense of self and of the value of one's past life, as opposed to a lack of satisfaction, coupled with the sense that it is too late to start all over again. The strength is wisdom, a detached concern with life itself.

As he (actually, his wife and collaborator, Joan Erikson) entered his (her) 9th decade, Erikson (in The Life Cycle Completed, 1998) postulated a ninth stage, in which the developments of the previous eight stages come together at the end of life:

  1. The stage of very old age, beginning in the late 80s, brings outcomes of the previous eight stages together.  The crisis is despair vs. hope and faith, as the person confronts a failing body and mind.  If the previous stages have been successfully resolved, will be able to transcend these inevitable infirmities. 

Observational studies have provided some evidence for this fourth stage, but Erikson's original "eight-stage" view is the classic theory of personality and social development across the life cycle. 

It should be noted that Erikson's theory, like Freud's, is highly impressionistic, and not necessarily based on a proper scientific analysis of lifespan development. Also, like Freud's, it is strongly based on a particular cultural experience, and a particular view of what is important in life. Erikson's theory is not presented as established scientific fact, but rather as a good example of how a stage concept of development can be applied throughout the life span. There are some pearls of wisdom here, but take the whole thing with a grain of salt. The theory has been extremely influential in popular culture, and it has fostered an entire new discipline of life-span developmental psychology.

Stage Theories of Everything

Piaget, Erikson, and Kohlberg, not to mention Freud, popularized stages (rather than continuities) as a framework for understanding psychological development, and pretty soon stage theories began to appear in lots of different domains, including of everything.

James Fowler proposed a stage theory of the development of religious faith.

  1. Primal or Undifferentiated Faith, seen from birth to about age 2, is focused on personal safety -- that is, the development of faith that the environment is safe and secure as opposed to neglectful and abusive.  This faith in other people (or not) sets the stage for the development of faith in God (or gods).
  2. Intuitive-Projective Faith, from 3-7 years of age, in which the child has difficulty distinguishing reality from imagination, such that discussion of angels and devils is taken very literally.
  3. Mythic-Literal Faith, in which God (or gods) is (are) viewed in highly anthropomorphic terms.
  4. Synthetic-conventional Faith, in adolescence, is characterized by conformity to religious rules and rituals.
  5. Individual-Reflective Faith, in the 20s and 30s, is a period of struggle with religious belief.
  6. Conjunctive Faith entails a "mid-life crisis" elicited by awareness of the contradictions and paradoxes of religious belief.
  7. Universalizing Faith, and resolution of these paradoxes and contradictions into an "enlightened" faith in God (or gods).

Elisabeth Kubler-Ross proposed a stage theory of death and dying.

  • Denial, either of illness or of death as a consequence;
  • Anger, feelings of betrayal and envy;
  • Bargaining, an attempt to postpone or delay the inevitable;
  • Depression, in the face of the certainty of death;
  • and finally Acceptance, and the inner peace that comes with it.

And, famously, "Bill W." and "Dr. Bob", the founders of Alcoholics Anonymous, proposed the "12 Steps" that people must take, in order, in order to recovery from alcoholism -- a "self-help" approach that has since been generalized to other forms of substance abuse.

  1. Admit you are powerless over X (where X is alcohol, drug addiction, sex addiction, whatever).
  2. Believe that a "higher power" is necessary to restore proper function.
  3. Decide to turn your life over to God "as you understand him".
  4. Make a moral inventory of yourself.
  5. Admit our wrongs to God, ourselves and someone else.
  6. Ready yourself for God to remove character defects.
  7. Humbly ask God to remove your shortcomings.
  8. List all the people you have harmed.
  9. Make direct amends to those people.
  10. Continue to take personal inventory and promptly admit new wrongs.
  11. Meditate to improve contact with God "as you understand him". 
  12. Carry the 12-Step message to others.

As intuitively appealing as these stage theories may be, they share the problems which beset Piaget's theory (and, for that matter, Freud's and Erikson's and Kohlberg's).  So where does the intuitive appeal come from?  Mostly, I think, stage theories appeal to us because we like stories, and stage theories provide a kind of narrative structure that organizes what goes on in development.  Development proceeds from starting point (like Piaget's sensory-motor period) and proceeds to some endpoint (like formal operations).  The story has a beginning, a middle, and an end, with plot-points along the way like the acquisition of object permanence and the loss of egocentrism. 

The only problem is that stage theories aren't right.  Development just doesn't proceed in the lockstep stages envisioned by Piaget and Freud.

Cognitive Development After Piaget

In all stage theories, the stages of development are universal,obligatory,stereotyped, and irreversible.

  • All normal individuals must pass through them.
  • The stages are passed in the same sequence for all individuals;
  • Once a stage has been successfully negotiated, there is no going back;
  • The achievement of one stage is a necessary condition for advancement to the other.

These are essentially hypotheses about the nature of development, and when they are tested, research has usually failed to confirm them. In particular, research testing Piaget's theory revealed a number of anomalies, which led investigators to refocus their theories on continuities in mental development.

Critique of
              the Piagetian StagesThe first of these, one noted by Piaget himself, was the phenomenon of decalage (pardon my French). The implication of Piaget's stage theory of development is that the child makes a wholesale, quantum leap from one stage to the next. But Piaget and others recognized that this transition wasn't as abrupt as initially believed. A child moving from the pre-operational period to concrete operations, for example, would conserve on some tasks but not others.

The second, and by far more critical, had to do with the lower boundaries of the various stages. Again, the implication of Piaget's theory is that there are certain kinds of tasks that a child of a certain age just cannot do. And so developmental psychologists began to ask whether, in fact, 3- and 4-year-olds might conserve -- if only they were tested in the right way.

"Counting Principles" in Pre-Operational
              ChildrenAmong the first to ask this question was Rochel Gelman, who examined the child's conception of number. Piaget had argued that pre-operational children just didn't have any concept of number, or quantity, they couldn't count, and they certainly couldn't do arithmetic. But Gelman, working with Randy Gallistel (a mathematical psychologist who was also her husband), analyzed the concept of number into three principles:

  • One-to-one correspondence means that each number term corresponds to a particular quantity.
  • Stable order refers to the child's organization of these terms into a consistent order, from lowest to highest.
  • Cardinality means that, when counting a group of objects, the final number used refers to the number of objects in the group.

So, suppose you show a 4-year-old a set of 3 objects, and ask her how many there are, and she says "five". On first blush, it looks like she doesn't know how to count. But Gelman went further, and asked the child to count the objects out loud. A child might say "One, four, five -- five things!"; asked again, she repeats her count, "One, four, five -- five things". What this indicates is that the child understands one-to-one correspondence, has a stable order, and understands cardinality. She just doesn't assign the conventional words to various quantities. But she's clear got a concept of number, and she clearly has a grasp of counting principles. Gelman's study showed that "pre-operational" children typically displayed an understanding of these counting principles, even if they didn't count like the adults did.

Infant ArithmeticKaren Wynn pushed the envelope even further, by asking if infants also had the ability to count. This is clearly impossible, according to Piaget, because numbers represent quantities, and children in the sensory-motor period don't have representations. Wynn's research was based on the idea that looking time -- the amount of time that children (and adults, for that matter) spend looking at something -- is an index of surprise and attention. she showed 4-5-month-old infants displays of the following sort:

  • First one object is placed in a tube, and then another; when the tube is emptied, either one or two objects fall out.
  • First two objects are placed in a tube, and then one is removed; when the tube is emptied, either one or two objects fall out.

Looking TimesWynn found that infants were surprised -- they looked longer at the display -- when it gave the "wrong" answer -- when 1+1=1 rather than 2, or (especially) when 2-1=2 rather than 1. While the infants probably couldn't do differential calculus, they did seem to have at least a rudimentary ability to add and subtract. Not bad for babies who haven't even learned to talk yet!

These are just two examples of a huge body of post-Piagetian research that undermined Piaget's stage theory of cognitive development. Young children simply had greater cognitive skills than Piaget's theory allowed them. These empirical findings, in turn, led developmentalists to propose alternative theories of cognitive development.

Link to an interview with Annette Karmiloff-Smith.

Similar findings have been obtained in other areas, such as logical thinking.  Piaget argued that formal logic was a relatively late cognitive accomplishment, but some psychologists have found evidence of logical thinking even in very young infants.  For example, Visual behaviors, such as a shift in one’s gaze or a prolonged stare, can be diagnostic of internal thoughts. Cesana-Arlotti et al. ("Precursors of Logical Thinking in Preverbal Human Infants", Science, 03/16/2018) asked whether infants (aged 1 to 1-1/2 years) can entertain a disjunctive syllogism of the form Either A or B is true; A is false; Therefore B must be true.  When the infants were presented with scenes depicting the syllogism, they looked longer, expressing surprise, when B proved to be false. 

Novices and Experts

Development as the Acquisition of ExpertiseOne post-Piagetian approach construes development as the development of cognitive skills. According to this view, the infant starts out as a novice in all domains of problem-solving, and acquires expertise through learning - through experience and practice. However, in this view expertise is not just a quantitative difference of "knowing more" than one did before. Rather, the argument is that experts represent problems differently than novices do:

  • Expert knowledge is cross-referenced, promoting easy access to it in a variety of situations.
  • Expert knowledge focuses on higher-order patterns, so that experts think in bigger "chunks", and take larger steps in problem-solving, than novices do.

One model for the development of expertise is the difference between novice and expert chess players. Both kinds of players know the rules, but experts represent the game differently, and play differently, than novices do.

Of course, all of this is not very far from the theory of development as learning. Recall the definition of learning as "a relatively permanent change in behavior that results from experience"; in the same way, expertise develops with experience and practice. However, there are at least two important differences between the theory of expertise and the theory of learning.

  • The acquisition of expertise involves qualitative leaps in skill that represent the individual's successive reorganizations of task performance. These qualitative leaps are somewhat analogous to Piaget's stages, but they aren't the same as Piaget's stages, because even young children can attain more expertise than adults (as an example, consider young children's expertise in dinosaurs).
  • The theory of expertise does not consider the infant as a blank slate to be written on by experience. Instead, the child is viewed as bringing a rudimentary cognitive apparatus into the world, such that learning experiences modify his or her innate propensities.

These differences between the theory of expertise and the theory of learning reflect the lasting contribution of Piaget to developmental theory -- despite the fact that his theory appears to be wrong in many salient details.

  • Theories of development are no longer so concerned with the cognitive "starting point" -- as in the debate between nativism and empiricism.
  • Now, the focus of developmental theory is on the cognitive "end point" -- the outcome of psychological development. The cognitive starting-point is viewed in light of where the child is going.


Theories of expertise seem to imply some degree of continuity over the course of development, but studies of expertise reveal one big difference between younger and older children: Put bluntly, older children know what they're doing, while younger children don't. Older children are not simply more expert than younger ones: they're also more reflective, more deliberate, and more strategic in their thought and action. In other words, what older children possess, and younger children lack, is metacognition.

Metacognition was defined by John Flavell (1979) as, literally,cognition about cognition -- or, put another way, our ability to monitor and control our own cognitive processes. Metacognition is one's "knowledge about cognition and cognitive phenomena", including:

  • Knowledge of what is going on in your own mind:
    • Whether you're perceiving something or just imagining it;
    • What knowledge you have stored in your memory, and how your memory works (this fund of meta-knowledge is sometimes called metamemory);
    • Whether you actually understand something that you've learned, or that has been explained to you.
  • Appreciation of the rules governing mental processes:
    • How to deploy attention effectively;
    • How to use strategies for encoding and retrieving memories (another aspect of metamemory);
    • How to break down large problems into sub-problems.

Flavell has argued that there are several different aspects of cognitive monitoring, including:

  • Goals or Tasks: knowing the objectives of a cognitive enterprise.
  • Actions or Strategies: cognitions and/or actions employed to attain goals and complete tasks.
  • Metacognitive Knowledge: knowledge about factors influencing your own and others' cognition.
  • Metacognitive Experiences: Conscious thoughts and feelings pertaining to cognition.

His general idea is that with development, children make more conscious, deliberate use of their mental faculties.

The Theory of Mind

The Theory
              of MindMore broadly, we might say that as they develop children come into possession of a theory of mind -- a term coined by Premack and Woodruff (1978; see also Premack, 1988) based on their comparative study of cognition in humans and chimpanzees. Put briefly, the theory of mind is the ability to impute mental states to oneself and to others. It includes:

  • Knowledge of Our Own Minds: understanding that we have mental states, and realizing that our experiences are our own -- that, somehow, our experiences are separate from the world outside the mind, and that we can control our own beliefs, feelings, and desires. Knowledge of our own minds entails phenomenal awareness -- introspection -- of what we think, feel, and want.
  • Knowledge of Other Minds: understanding that our mental states may differ from those of other people -- that different people have different minds, and thus different experiences. Knowledge of other minds entails an ability to make inferences about what others think, feel, and want.

"False Belief" Task ExampleThe development of a theory of mind is commonly indexed by what is known as the false belief task. This task, typically, involves an experimenter, a child, and a puppet. The puppet hides a ball in an oatmeal container. After the puppet is put away in a cupboard, the experimenter and the child together switch the ball from the oatmeal container to a box. Then the puppet is brought out of the cupboard, and the child is asked where the puppet will look for the ball.

  • Children younger than 4 years of age typically answer that the puppet will look in the box, "because that's where it is".
  • Children older than 5 years of age typically answer that the puppet will look in the oatmeal container, "because that's where he thinks it is.

Age and
              False Belief Test PerformanceAn early study by Wellman et al. found that children younger than 40 months typically failed the false-belief test, while children older than 50 months typically passed it. somewhere between the ages of 4 and 5, children get a theory of mind -- they understand that our minds are our own, and that different people will have different percepts, memories, knowledge and beliefs.

Of course, the moment researchers established a landmark like this, other researchers began to try to push the envelope back -- to determine whether even younger children have a theory of mind, too, if only we tested them the right way. One important feature of the standard false-belief test is that it is verbal -- the experimenter asks questions, and the child has to answer. So what would happen if we concocted a nonverbal version of the false-belief task?

              Infants Have a Theory of MindTo make a long story short, Onishi and Baillargeon (2005) tested 15-month-old infants on a nonverbal version of the false-belief task. The experiment itself is a thing of wondrous beauty, they devised a totally nonverbal version of the FB task, relying on the general finding that even infants look longer at events that violate their expectations.

First, the children were familiarized with the test situation, in order to build up certain expectations.

For this purpose, the infants were given three familiarization trials.

On Trial 1, they saw an actor hide a (plastic) slice of watermelon in a green box.

On Trials 2 and 3 the actor returned and reached into the green box for the watermelon.

e a Theory of Mind?
Then the infants were divided into four groups for the belief-induction trial. The infants, saw,for just one trial:

In the True Belief Green condition, the actor watched as the yellow box moved toward the green box.

In the True Belief Yellow condition, the actor watched as the watermelon moved from the green box to the yellow box.

In the False Belief Green condition, the actor was no longer present when the watermelon moved to the yellow box.

In the False Belief Yellow condition, the actor watched as the watermelon moved from the green box to the yellow one; but was no longer present when the watermelon moved back to the green box.

Belief-Induction Trial
On the Test Trial, the infants watched as the actor opened the door and reached into the green box or the yellow box. Test trial

And, in fact, the infants looked longer on trials where an actor behaved in a way that seemed to contradict her (the actor's) understanding of where a plastic watermelon slice had been hidden. Apparently, even infants have some sense of what others believe, that their beliefs might be different from their own, and that their beliefs might be incorrect. If infants expect others to behave in accordance with their beliefs, and are surprised (and pay extra attention) when they do not do so, then it can be said that even infants, long before age 4, have a rudimentary theory of mind.

Actually, it's not always the case that infants look longer at events that violate their expectations. In some case, they look for less time at counter-expectational events. Nobody quite knows why. But in either case, a difference in looking times indicates that, from the infant's point of view, something has gone wrong. And that's all the logic of the experiment requires.

Looking Times in the Non-Verbal False-Belief

So even very young children have some rudimentary theory of mind, even if they can't express it verbally.

I know what you're thinking: if we have a nonverbal test of the theory of mind, maybe we can use it to see if non-human animals have a theory of mind, too. In fact we can, and Call and Tomasello did, and they found that chimpanzees utterly failed the test. But that's a study for another course (I discuss it in my courses on "Scientific Approaches to Consciousness" and "Social Cognition" -- so if you're really interested, go to those websites and click on "Development").

Egocentrism and the Theory of Mind

In some ways, the theory of mind revives Piaget's notion of egocentrism. For Piaget, the preoperational child thinks that his experience is universal. But the older child, having entered concrete or formal operations, understands that others may not think the way he does -- that others' percepts, memories, and knowledge might not be the same as his own.

In fact, the theory of mind usually emerges between 5 and 7 years of age -- exactly the same point as Piaget's shift from preoperational thought to concrete operations.

The "Theory" Theory

In some respects, cognitive development is the development of social cognition -- the ability to think about oneself and other people. The acquisition of a theory of mind is a qualitative change, like the shift from one of Piaget's stages to another. But it is a qualitative shift that takes place against a continuous acquisition of knowledge.

But it turns out that the child isn't just developing a theory of self and others. The child is developing a theory of the whole world -- of physics and biology as well as psychology -- and testing it out in much the same way that a scientist would (the metaphor of child as scientist is another legacy of Piaget's theory). This is the essential proposition of what has come to be known as the "theory theory" of development -- that the developing child is engaged in a continuous process of proposing, testing, revising, and rejecting theories of how the world works -- including theories of how minds work, as part of the world.

The "theory theory" of cognitive development takes seriously Piaget's notion that the child is operating as a naive scientist, actively exploring the world and experimenting with it -- formulating hypotheses ('I wonder if it works this way"), gathering evidence ("Let's see what happens if I do this"), and revising hypotheses based on the outcome ("OK, that didn't work, maybe it works this way, instead"). In this way, the child develops theories of the world -- abstract, coherent systems of knowledge that enable him or her to predict or control events, and also to interpret and explain events.

Like the view of development as the acquisition of expertise, the "theory theory" emphasizes continuities rather than changes in cognitive development. The child is constantly experimenting, right from birth. Put another way, the child is constantly learning -- learning to predict and control the world around him. Learning, in fact, is central to theory-formation.

  • The child learns about the conditional probabilities linking events, through something like classical conditioning.
  • And the child learns about the outcomes of interventions, through something like instrumental conditioning.

But while some conditioning theorists viewed the learning organism as a tabula rasa, or blank slate --what the 18th-century French philosopher called a "perfect idiot" -- which is written on by experience, that's not the view of the "theory theorists". Instead, they believe that the child comes into the world with an innate theoretical capacity -- a rudimentary ability to form, test, and revise its understanding of the world. So, like Piaget, they believe that the child comes into the world already prepared with a rudimentary cognitive schema. This position is known as starting-state nativism, and it holds that in some nontrivial sense the child comes into the world with "substantive innate theories" of various domains -- of physics, biology, a theory of the mind, and, possibly, a theory of society as well. The child develops as it puts these innate theories into action, testing them in the real world, and revising them when the test results prove to be surprising.  Just as in science, what starts out as a very narrow, primitive theory of some domain becomes progressively more expansive, refined, and robust -- a theory that actually predicts, and explains, what goes on in the world outside and inside the child's mind.

Much of what we know about what infants know comes from studies using the "looking" paradigms of the sort employed to investigate the infant's concept of number and understanding of other minds.  Infants tend to look longer at novel, unexpected events -- as if they're surprised by them, and are trying to figure out what's going on.  So, by tracking what infant look at, we can figure out what they already expect -- what they already know.  Other studies observe the infants' actions -- what they reach for, what they crawl to, what they imitate. 

Using these sorts of paradigms, we now understand that even infants have some rudimentary understanding of basic physical principles:

  • the trajectory of movement;
  • the pull of gravity on objects;
  • how one object can contain or enclose another.

In much the same way, infants have some understanding of basic biology:

  • the distinction between animate and inanimate objects;
  • plants and animals have an essential core that does not change, even when their outward appearances change.
  • that living things grow and non-living things do not;
  • that living things can inherit properties from their parents;
  • that living things can get sick.

They haven't been taught these things, and they seem to know them before they've had any opportunity to learn them.  In fact, it seems that this innate knowledge about the physical and social world makes it possible for them to learn new things.

Probability Learning

According to the "theory theory", infants then build on this innate theoretical knowledge to develop a more refined understanding of themselves and the world around them -- all without benefit of language or much deliberate teaching on the part of their parents.

As discussed in the lectures on Language, infants quickly begin dividing up the sound world to which they are exposed.  They learn to expect which syllables are likely to follow other syllables, and which musical tones are likely to follow other tones (e.g., Saffran, Aslin, & Newport, 1996).  Of course, as discussed in the lectures on Learning, nonhuman animals do much the same thing -- picking up on conditional probabilities in the course of classical and instrumental conditioning.  So it's not a particular surprise that human infants can do this, too.  But the point is that infants already "know" something about probabilities.

They already now something about sampling, too.  UCB's Fei Xu (2008) showed 8-month-olds a container full of colored ping-pong balls -- e.g., 80% white, and 20% red.  The experimenter then dipped into the container, and removed five balls.  The infants showed sins of surprise if the color distribution of her selection departed markedly from the color distribution in the container as a whole -- showing that they already had some idea that a sample should resemble its parent population. 

Given these rudimentary conceptions of probability, infants can begin acquiring knowledge about their world -- what goes with what, and what causes what to happen. 

And this can take a while.  In phylogenetic terms, those species with the most intelligent and flexible adults also have the longest periods of infancy. 

  • Precocial species, like chickens and most birds, mature quickly; and they are also highly dependent on innate, instinctual routines.
  • Altricial species, like most mammals, especially humans and other primates, as well as crows, mature slowly; and their behavior is much more dependent on learning, including social learning.

For more on this, see "How Babies Think" by UCB's own Allison Gopnik (Scientific American, July 2010), from which these examples are drawn. 
See also Gopnik's books intended for a popular audience, including

  • The Scientist in the Crib: Minds, Brains, and How Children Learn (1999; reviewed by Jerome Bruner, who was Gopnik's mentor in graduate school, in "Tot Thought", New York Review of Books, 03/09/2000);  and 
  • The Philosophical Baby: What Children's Minds Tell Us About Truth, Love, and the Meaning of Life (2009).

Ethics and Morality

What an innate theory might look like is illustrated by research concerning infants' conceptions of ethics and morality. Long before they've gone to Sunday School, and long before they've received any serious rewards or punishments, infants apparently have some rudimentary sense of good and bad, right and wrong. Early on, of course, they have some of the prerequisites for making moral judgments.  For example, they understand the difference between animate and inanimate objects -- a logical prerequisite to understanding the concept of agency.

An experiment by Kuhlmeier, Wynn, and Bloom (2003) shows that even infants have a rudimentary concept of agency.  They watched an animated film of a ball climbed a hill: another figure, a triangle, pushed the ball up, while another figure, a square, kept him down.  Later, they saw a test film in which the ball approached either the square or the triangle on level ground.  Five-month-old infants didn't discriminate between the two films, but 12-month-old infants did.  Apparently, the older infants expected the triangle to approach the "good" object that "helped" it up the hill, but not the "bad" object that hindered its progress.

  • Later experiments by Hamlin in the same lab, acted out the same scenario with actual objects, and then gave the infants a choice.  Both 6- and 10-month-old infants actively preferred the "helper".  Even 3-month-olds, who were too young to reach for anything, preferred to look at the "helper" rather than the "hinderer".

In another series of experiments, Felix Warneken and his colleagues arranged accidents, like a dropping a pen.  Infants as young as 18 months of age will actually help the experimenter -- but not if the experimenter intentionally throws the pen on the floor. 

  • Similar human-helping behavior has been observed in both lab-reared and wild-reared chimpanzees.

The age of the infants in these studies, and the behavior of the chimpanzees, suggests to some theorists that children are born with an innate, if rudimentary, sense of right and wrong -- in innate moral knowledge that is further amplified through learning.  Others deny this, but claim that infants are very fast learners in this domain.  Perhaps humans and other primates are "prepared" to learn about right and wrong, even if they're not actually born with this knowledge.

The Pendulum of Developmental Theory

In some sense, theories of development cycle back and forth between continuity and change, and between qualitative and quantitative changes. Developmental psychologists aren't just on a swinging pendulum, however. Every turn in the cycle represents an increasingly sophisticated shift in our understanding of the nature of mental development -- and, for that matter, in our understanding of the mind.

Cognitive Aging

Erikson was clear that social development continued throughout the lifecycle: roughly half of his "Eight Ages of Man" occur after, in Piaget's view, the child acquires formal operations -- and certainly after children acquire a theory of mind. The pimplication of Piaget's theory, however, is that cognitive development stops with the acquisition of formal operations.  After that, intelligence assumes a kind of steady state.

Or, worse, it's often said that the story after early adolescence is one of steady cognitive decline (never mind the effects of age-related dementias such as Alzheimer's disease).  But it turns out that even that story is complicated. 

  • Recall, for example, from the lectures on Thinking, that -- at least in Cattell's view -- fluid intelligence may decline with age, but crystallized intelligence stays steady, or increases, as the individual acquires new knowledge. 
  • Similarly, recalling (sorry!) the lectures on Memory, procedural knowledge may decline, especially if we no longer practice a particular skill (I used to be a pretty good French horn player, but that was more than 50 years ago, and I don't think I could manage more than the C-major scale now).  
  • One function that seems to consistently decline with age is processing speed, as measured by reaction times on various cognitive tasks; but even in this case there are complications, depending on what's being processed.  And, in any event, this would just disadvantage older individuals on "speed" as opposed to "power" tasks.  The elderly may not win Jeopardy!, with its signalling devices, but they will do just fine at Trivial Pursuit.
A review of performance on various standardized tests of intelligence and memory (such as the WAIS) by Joshua Hartshorne and Lura Germine (Psychological Science, 2015) shows a clear decline in performance with age.  What's even more interesting, however, is that the peak of performance varies, depending on the function being tested.  Of course, as with all studies such as this, there is considerable variability around each of these data points, meaning that there are even some very old individuals who continue to perform reasonably well. 

One important principle in cognitive aging, articulated most forcefully by Nancy Denney at the University of Wisconsin (Developmental Psychology, 1984), is "use it or lose it".  That is, individuals who continue to practice a cognitive skill show less decline in that skill than those who do not.  So, if you can, keep playing that French horn.

A related point is that it is possible for middle-aged and elderly individuals to acquire new skills.  A social trend toward lifelong learning is documented in several recent books, such as Beginners: The Joy and Transformative Power of Lifelong Learning by Tom Vanderbilt (reviewed in "The Joys of Approaching Life san Amateur" by Cal Newport, New York Times, 01/31/2021), or Late Bloomers: The Hidden Strengths of Learnng and Succeeding at Your Own Pace by Rich Karalgaard (both reviewed in "Starting Fresh" by Margaret Talbot, New Yorker, 01/18/2021).  A spectacular example is that of Nell Irvin Painter who, after a distinguished career at Princeton as a historian of the 19th-century American South, decided to become a painter, completing both BFA and MFA programs in retirement.  It's not that Painter turned to a talent or interest that she already possessed, but had set aside during her academic career.  She just decided that she was going to become a real painter, not a dilettante like Winston Churchill or George W. Bush '43, good as these amateurs were and are (see her memoir, Old in Art School: A Memoir of Starting Over).

Of course, it's may be easier to regain a skill that had once been laid down.  That's what savings in relearning is all about.  From time to time, I think of going on the market for a used French horn.  But I don't think my family would like it.

Whether it's learning something for the first time or relearning something you used to know how to do, Rachel Wu of UC Riverside and her colleagues have identified a number of factors that promote success (Human Development, 2016):
  1. Open-minded input-driven learning, which relies on new observations rather than prior knowledge.
  2. Individualized scaffolding, such that the steps in skill-acquisition are arranged in increasing order of difficulty.
  3. A growth mindset (the term comes from Carol Dweck, as discussed in the lectures on Motivation) that abilities are not fixed and innate, but can improve with practice and effort;
  4. A serious commitment to learning.
  5. A forgiving environment that supports the learning activity, even through initial failure;
  6. A practice of learning several skills simultaneously, so what what you learn in one domain may help you learn in another.

Wu et al. note that these six factors are present when children learn, and decline in the environments in which adults learn.  The implication, is that lifelong learning requires not just what the Buddhists would call a "beginner's mind" (in which habitual modes of thought disappear, making the world seem new and unfamiliar), but also a "beginner's environment".

The Phylogenetic View of Development

One important perspective on the development of mind is provided by evolution. The phylogenetic point of view on development traces the evolution of mind in the human species as a whole, often by comparing mental processes in subjects of different species. This is the field known as comparative psychology. It is an interesting challenge to develop tests of perception, memory, learning, categorization, problem-solving, and even language that can be reasonably applied to nonhuman animals, and comparative psychologists often exercise great ingenuity in their work.

Three Cheers for Evolution!

Darwin's theory of evolution by natural selection is based on four principles (summarized by Richard Lewontin in "Not So Natural Selection",New York Review of Books, 05/27/2010):

  • Variation: "Among individuals in a population there is variation in form, physiology, and behavior."
  • Heredity: "Offspring resemble their parents more than they resemble unrelated individuals", by virtue of some biological characteristic that they have inherited from their parents.
  • Differential Reproduction: "In a given environment, some forms are more likely to survive and produce more offspring than other forms."

"Evolutionary change is then the mechanical consequence of variation in heritable differences between individuals whenever those differences are accompanied by differences in survival and reproduction. The evolution that can occur [in this manner] is limited by the available genetic variation, so in order to explain long-term continued evolution of quite new forms we must also add a fourth principle."

  • Mutation: "New [more or less random] heritable variation is constantly occurring".

The usual view of natural selection is that some inherited traits are passed on to the next generation because they facilitate the species' adaptation to their environmental niche -- "a preexistent way of making a living into which organisms must fit or die". But Lewontin points out that adaptation is actually a two-way street. "Organisms do not 'fit into' niches, they construct them...." The organism affects its environment, even at the level of physics and biology, at the same time that the organism is affected by its environment. And then he offers a great example:

The most remarkable feature of terrestrial organisms is that each one of them manufactures the immediate atmosphere in which it lives.... By use of a special kind of optical arrangement (Schlieren optics) on a motion picture camera it is possible to see that individual organisms are surrounded by a moving layer of warm moist air. Even trees are surrounded by such a layer. It is produced by the metabolism of the individual tree, creating heat and water, and this production is a feature of all living creatures. In humans the layer s constantly moving upward over the body and off the top of the head. Thus, organisms do not live directly in the general atmosphere but in a shell produced by their own life activity. It is, for example, the explanation of wind-chill factor. The wind is not colder than the still air, but blows away the metabolically produced layer around our bodies, exposing us to the real world out there.

So even at the level of physics and biology, not just the building of structures and the like, organisms change the environment that they live in. They're not passive recipients of environmental stimulation.

Somewhere Steven Jay Gould, the late paleontologist and evolutionary biologist (and close colleague of Lewontin's), cited three classes of evidence for evolution:

  • Evolution Around Us: Although Darwinian evolution transpires over millions of years, we can see similar sorts of changes, on the smaller scale of micro-evolution, occurring over the course of just a few generations or even a single lifetime. Examples include the domestication of dogs and of crop plants, and the emergence of DDT-resistance in agricultural pests, and of antibiotic-resistance in human pathogens.
  • Intermediate Forms: Although there are definite gaps, the fossil record contains ample evidence of extinct species that mark the transition between one species and another. Examples include the shift from reptiles to mammals, the origins of whales in cow-like land creatures, and "Neanderthals" and other species of hominoids (see below).
  • Oddities and Imperfections: Various physical traits that serve no current adaptive purpose, or that reveal the "attempts" of evolution to solve some problem of adaptation. Gould's favorite example was the panda's thumb, which also provided the title for one of his best popular-science books.

The classic view of evolution, known as the Modern Synthesis, combined natural selection with genetics (Mendel published his work on inheritance in peas in 1866, after Darwin published the Origin of Species, and his work on the laws of inheritance was not really recognized and appreciated until the early 20th century).  The Modern Synthesis basically argued that randomly varying genes cause phenotypic characteristics which are then selected for (or against) by the environment.  And the Modern Synthesis reached its apex in the discovery of the discovery of the structure of DNA by Watson and Crick, and the development of techniques for mapping the human genome (and the genomes of other organisms).

This story remains valid, but some theorists (e.g., Jablonka and Lamb, in Evolution in Four Dimensions, 2005) have argued that the genome itself is responsive to environmental pressures, and that there are other transmissible differences besides genetic ones. They have suggested that there are, in fact, four different types of influence on evolution:

  • Genetic, basically following the lines of the Modern Synthesis.
  • Epigenetic, meaning the transmission of phenotypic variations that do not, themselves, depend on differences in DNA.
  • Behavioral, meaning inadvertent transmission by observational learning. 
  • Symbolic, meaning the deliberate transmission by means of language.

Ordinarily, evolution occurs over extremely long periods of time, but the intervals involved can be much shorter.  Consider, for example, the case of the peppered moth, an insect whose white wings are "peppered" with black spots.  Before the Industrial Revolution, most peppered moths resembled the image on the left, with individual variation in the density of the "peppering".  But by the end of the 19th century, the dark-colored variant depicted on the right vastly outnumbered the light-colored variant.  The evolutionary explanation is that, in the heavily polluted environment of industrial England, the denser peppering was highly adaptive, because the darker-colored variant was harder for birds to see against soot-darkened trees, and therefore less likely to be caught and eaten.  This evolutionary change, occurring over less than 100 years, is probably the clearest evidence of Darwinian evolution by natural selection.

But evolution doesn't only affect body morphology.  It also affects behavior, as discussed in the lectures on Learning.  And moths provide evidence of this, too. Moths are famous for their "flight to light" behavior a taxis (also discussed in the lectures on Learning) in which moths fly toward sources of light ("as they say, "like a moth to a flame"), with frequently fatal consequences.   Florian Altermatt and Dieter Ebert, two Swiss evolutionary biologists, (Biology Letters, 2016) found that ermine moths collected from light-polluted environments were significantly less attracted to light than those collected from dark-sky environments.  The researchers speculate that this behavioral change increases reproductive fitness, in terms of both survival rate and reproductive capacity.  Maybe there's hope for those sea turtles after all!

It's worth pointing out how powerful natural selection is.  Consider a calculations by JBS Haldane, a British evolutionary theorist, which assumes that one gene (A) is found in 99.9% of the population, and another gene, B, is found in only the remaining 0.1%. If B has a merely a 1% reproductive advantage, producing 101 offspring for every 100 offspring produced by A, after only 4,000 generations the numbers will be reversed: B will be found in 99.9% of the population, and A in only 0.1%!

For more on evolution, see UCB's 'Understanding Evolution" website at Also he January 2009 issue of Scientific American, which celebrated the 150th anniversary of the publication of Darwin's Origin of Species (1859).

Similarly, for the 150th anniversary of the publication of Darwin's The Descent of Man (1871), Science published an extensive review of advances in our understanding of human evolution since Darwin's book "Modern Theories of Human Evolution Foreshadowed b Darwin's Descent of Man by Petetr J. Richerson, Sergey Gavrilets, and Frans B.M. de Waal, 05/21/2021).  Given the status of both the authors and the publication venue, this overview maybe considered authoritative.  Herewith, some quotations from the "Review Summary" (images also from the main article).

Modern research shows that we share many developmental, physiological, morphological, cognitive, and psychological characteristics as well as about 96% of our DNA with the anthropoid apes. We now know that since our last common ancestor with the other apes 6 million to 8 million years ago, human evolution followed the path common for other species with diversification into closely related species and some subsequent hybridization between them. Since Darwin, a long series of unbridgeable gaps have been proposed between humans and other animals. They focused on tool-making, cultural learning and imitation, empathy, prosociality and cooperation, planning and foresight, episodic memory, metacognition, and theory of mind. However, new insights from neurobiology, genetics, primatology, and behavioral biology only reinforce Darwin’s view that most differences between humans and higher animals are “of degree and not of kind.” What makes us different is that our ancestors evolved greatly enhanced abilities for (and reliance on) cooperation, social learning, and cumulative culture—traits emphasized already by Darwin. Cooperation allowed for environmental risk buffering, cost reduction, and the access to new resources and benefits through the “economy of scale.” Learning and cumulative culture allowed for the accumulation and rapid spread of beneficial innovations between individuals and groups. The enhanced abilities to learn from and cooperate with others became a universal tool, removing the need to evolve specific biological organs for specific environmental challenges. These human traits likely evolved as a response to increasing high-frequency climate changes on the millennial and submillennial scales during the Pleistocene. Once the abilities for cumulative culture and extended cooperation were in place, a suite of subsequent evolutionary changes became possible and likely unavoidable. In particular, human social systems evolved to support mothers through the recruitment of males and nonreproductive females. The most distinctive feature of our species, language, appeared arguably driven by selection for simplifying cooperation. Reliance on social learning and conformity led to the emergence of new factors constraining and driving human behavior, such as morality, social norms, and social institutions. These forces often act against the immediate biological or material interests of individuals, promoting instead the interests of the society as a whole or of its powerful segments. Continuous engagement in cooperation has led to the evolution of strong coalitionary psychology, which can bring us together whenever we perceive that our identity group faces outside threats. Coalitionary psychology also has an undesirable byproduct: often negative or even hostile reaction to others who differ from us in their looks, behaviors, beliefs, caste, or class (p. 806).

In Descent, Darwin remarked in a few passages on the origin and antiquity of humans, but he and his contemporaries had almost no relevant fossils to work with and very underdeveloped archaeological and paleoecological records. We now know that the human lineage has undergone a rather dramatic series of changes since our last common ancestor with the other apes 6 million to 8 million years ago. Human evolution followed the path common for other species, with diversification into closely related species and some subsequent hybridization between them. DNA and fossil remains suggest that our ancestors diverged from Neanderthals and Denisovans more than half a million years ago. Anatomically modern humans were present in Africa 200,000 years ago. Around 70,000 years ago, up to six highly distinctive subspecies of humans coexisted. Since then, we have been a single species that emerged fromAfrica about 50,000 years ago. Some of our derived features, especially bipedal locomotion, are fairly ancient; others, especially stone tool knapping, evolved a little before the first fossils attributable to our genus Homo appears in the fossil record around 2 million years ago; and still others appeared after 250,000 years ago. Human behavior was substantially modern by 30,000 years ago, but both biological and especially cultural changes have been dramatic right up to the present. In the Holocene, cultures evolved a whole series of new ecological niches based on cultural adaptations and symbolic markers of tribes and tribe-like social units that partially isolate ecologically different populations (p. 808).

One thing that Richeson et al. make clear is that some of the traits that evolved through the Darwinian principle of natural selection -- namely, general intelligence, language, and consciousness -- set the stage for cultural evolution through discovery and social learning.  And cultural evolution is quite different from organic evolution -- not least because it is much faster.  In their "Review Summary", they illustrate the differences with this image, derived from Anthropology: Cultural Patterns and Processes (1923) by UCB's pioneering anthropologist, Alfred Kroeber.  "Biological inheritance is rigid from parents to offspring..., and [different] species mostly do not exchange genes."  The result is a tree with very distinctly separated branches.  By contrast, cultural traits "are potentially acquired from anyone in  person's social network, and ideas spread rather readily from one culture to another".  This results in a much more tangled tree.

In a lead editorial in the same issue ("The Descent of Man, 150 Years On), Agustin Fuentes, an anthropologist at Princeton, reflects on the racism and sexism that permeates Darwin's book (he was not immune to the attitudes of his time).  The bottom line, says Fuentes, is that "Descent is a text from which to learn, but not to venerate".  As the "father" of evolutionary theory, Darwin got a lot right.  But, Fuentes writes, students "should also be taught Darwin as an English man with injurious and unfounded prejudices that warped his view of data and experience".

Another expression of the phylogenetic point of view is evolutionary psychology, an offshoot of the sociobiology proposed by E.O. Wilson in his book by that title. Sociobiology assumed that patterns of social behavior evolved in the service of adaptation. Put another way, large segments of social behavior are instinctual in nature. Similarly, evolutionary psychology assumes that mental functions also evolved to serve adaptive purposes. Put another way, these modes of experience, thought, and action are also instinctual in nature -- they are part of our innate biological endowment, a product of evolution.

Kinds of Selection

Most discussions of biological evolution focus on Darwin's first principle of natural selection -- that those traits, resulting from natural, random variation, that enhance the organism's ability to survive in a particular environment, and thus pass its genes to its offspring. In the case of Darwin's finches,

Heikegani DetailDarwin's concept of natural selection was deliberately modeled on the "artificial" selection by which farmers and ranchers have "improved" their livestock since time immemorial. Thus, consider Carl Sagan's example (in Cosmos: A Personal Voyage) of the samurai crab (Heikea japonica). The Japanese "Tale of the Heike" tells the story of Heike warriors in a naval battle in the 12th century, who committed suicide by jumping overboard rather than face defeat. Later, fishermen working in the vicinity of the battle caught some crabs whose shells resembled human faces. Believing that these crabs were the reincarnations of the drowned warriors, they returned them to the sea. So, the face-like appearance of the shell became an adaptive trait, which was passed on to subsequent generations.

Heikegani Ghost Taira TomomoriDetail from illustration: The ghost of Taira Tomomori and heikegani with faces of fallen soldiers,ukiyo-e print by Utagawa Kuniyoshi (1797-1861).

Photograph of a demon-faced crab, found in the waters surrounding Japan (Smithsonian Magazine).

But it turns out that natural selection is not the only possible form of selection. There are certain traits that do not necessarily promote the survival of the individual organism, but which are adaptive in other ways, and thus also likely to be passed from one generation to another.

              Bird NestIn sexual selection, certain traits enhance the likelihood that the organism possessing them will be able to mate -- even if that trait is maladaptive in other respects. The classic example are strength and size (in males), elaborate plumage in birds (again, males), and various kinds of courtship displays. the idea actually was originated by Darwin's grandfather, Erasmus Darwin, but Charles Darwin elaborated on the concept in The Descent of Man and Selection in Relation to Sex (1871). Sexual selection is theoretically important, because it makes clear that the basis for evolution is not merely "survival of the fittest", as the stereotype goes, but rather reproductive success. Fitness doesn't mean mere survival. It means the ability to pass on one's genes. The classic example of a trait subject to sexual selection, one noted by Darwin himself, is the male peacock's display of tail-feathers. Another, also noted by Darwin, is the bower bird of Australia and New Guinea, which adorns its nest with all sorts of colorful objects, including fruits and flowers but also found objects like pieces of glass, in an attempt to attract a mate. Richard Milner, in the Encyclopedia of Evolution: Humanity's Search for Its Origins (1990) characterizes sexual selection as "survival of the flamboyant".

A further principle of group selection is often invoked to show how certain group behaviors, like cooperation, might be adaptive. The classic examples are found in social insects: beehives often function as if they were a single organism. Individuals contribute to the reproductive success of the group, and so pass on certain genes that are common to their group -- even if they (and their immediate family members) do not reproduce. But it's not the group's survival that's at issue. Rather, the selection favors genes that the group members have in common. Milner (1990) calls group selection the "survival of the social unit". Evolutionary psychologists often invoke group selection as the biological basis of religion.

In kin selection, organisms pass on traits that do not necessarily promote their own. This concept has its origins in the work of William Hamilton, who was interested in problems of altruism. For example, why do worker ants and bees, themselves sterile, sacrifice themselves to serve their queen? Hamilton's answer is that such activities incidentally increase the chances that one's own genes will be passed on. Even if the individual dies, the survival of his genetic relatives insures that the family's genes are passed on. Consider that, by virtue of sexual reproduction, each individual organism shares about 50% of its genes with first-degree relatives. We share an average of 50% of our genes with our parents, siblings, and children; an average of 25% of our genes with our first cousins, and an average of 12.5% of our genes with our second cousins. If an organism has a trait, like working instead of reproducing, that increases the likelihood that its (many) relatives will reproduce successfully, then it thereby increases the likelihood that it's own genes will be passed on as well. Instead of the individual's reproductive fitness, kin selection enhances inclusive fitness. J.B.S. Haldane, a British geneticist, once remarked that he would be willing to sacrifice himself if he could be assured that his genes would live on: when pressed further, he announced his willingness to die for "two brothers, four uncles, or eight cousins" (as quoted by Milner, 1990; see also Lehrer, 2012). Kin selection is the basis of Richard Dawkins's theory of "the selfish gene" -- that it's not species that are struggling to reproduce themselves, nor individuals, nor group members -- but, rather, genes themselves.

According to William Hamilton, a British mathematical biologist who undertook to express Haldane's theory precisely, rB >C, meaning that a gene for altruism could evolve if the benefit of the genetically controlled behavior exceeded its cost, provided that genetic relatedness was taken into account. This formula became the basis of inclusive fitness theory which gets its name because the reproductive value of the trait includes the individual's close genetic relatives as well as the individual itself. Hamilton's theory, in turn, was popularized by E.O. Wilson, the Harvard entomologist who, in such treatises as Sociobiology: The New Synthesis (1975) and On Human Nature (1979), was among the first to apply evolutionary theory to human social behavior -- a foreshadowing of today's evolutionary psychology.

Inclusive fitness theory, in turn, has been challenged by other theorists, including now Wilson himself, who have proposed a principle of group fitness instead. According to this theory, the reproductive value of a trait includes other members of the individual's social group (think: tribe), regardless of how genetically related the group members are. In some sense, group fitness reverses inclusive fitness. In inclusive fitness, the effect of the gene determines he composition of the group, because it favors the survival of genetic relatives. But in group fitness, the group comes first, and the gene favors the survival of group members.

In fact, both principles, kin selection and group selection, operate, but at different levels: we have genes for both selfishness (which favors the survival of the individual and his genetic relatives) and cooperation (which favors the survival of unrelated group members), which tend to oppose each other. As Wilson (2007) has put it: "Selfishness beats altruism within groups. Altruistic groups beat selfish groups. Everything else is commentary."

For the record, there's at least one other form of selection, known as balancing selection.  Consider the distribution of the "Big Five" personality traits discussed in the lectures on "Personality and Social Interaction".  We know that each of these traits is, to some degree, under genetic control.  And we can assume, as well, that there is an optimal distribution for each of these traits -- let's say, for purposes of argument, a fair amount of extraversion, agreeableness, and openness to experience; lots of conscientiousness; and not too much neuroticism.  How come natural selection hasn't operated to give us all the optimum levels of these traits, just as it's given every normal human the capacity for language?   How come there are any neurotics, or psychopaths, at all?  Why don't we all just get along?  According to the principle of balancing selection, different combinations of traits are optimum for different environments, both physical and social.  If everyone were low on neuroticism, it would be adaptive for "neurotic" people to be aware of, and responsive to, threats that non-neurotic people are oblivious to.  If everyone were high on openness to experience, it would be adaptive for "closed" people not to go jumping out of planes without a parachute.  So, over time, evolution establishes a sort of equilibrium.  And because the genetic contribution to neuroticism or openness isn't a single gene, but rather a large number of genes, as more-neurotic and less-neurotic people mate and produce offspring, you tend to get something that looks like a normal distribution.

Genes, Culture, and Altruism

Evolutionary psychologists get twisted up in knots trying to explain altruism: How could it possibly be adaptive for an organism to sacrifice itself for others?  How could such a tendency be built into the genes?  It's to solve this problem that ideas like kin selection and group fitness were put forth.  But these principles have an ad-hoc quality to them.  In scientific terms, they're not very parsimonious, and appear (to me) to have been proposed in acts of desperation to make sure that there is a genetic, biological, Darwinian explanation for everything

The first thing to ask is: how often does true altruism occur?  How often do individuals sacrifice themselves for others who are not their genetic offspring?  I bet a statistically valid survey would answer: not very often.   One place where it does occur is on the battlefield, where soldiers often (but  not always -- which is not a criticism) often sacrifice themselves to save others.  Consider, to take a nonrandom example, Humayun Khan, son of Khizer and Ghazala Khan, who famously reprimanded soon-to-be-President Donald Trump for his attitudes toward Islam and Muslims.  In 2004, while serving as an  Army captain in Iraq, he ordered his company to stay sheltered while he confronted a suspected car-bomber alone.  His suspicions proved valid, and the car bomb went off: Khan is now buried at Arlington National Cemetery, but the rest of his company lived.  This happens with some frequency in the military and first-responders at home (consider the firefighters who went up the Twin Towers on 9/11, when everyone else was coming down.  These examples suggest that altruism isn't built into the genes.  It's built into the culture -- more precisely, the particular sub-culture that is the military, or the fire department, or similar organization.

So altruism does occur, and it has to be explained. 

One way of studying altruism has been through the Prisoner's Dilemma, a game which I discussed briefly in the lectures on "Personality and Social Interaction".  In the game, two players take the roles of criminal suspects, A and B, who face a prison sentence for committing a crime (Poundstone, 1992).  The prosecutor offers each of them the following deal:

  • If A and B both confess, saving the prosecution the expense of a trial,
  • they will each be sentenced to 2 years in prison.
  • If A confesses and implicates B, A will go free while B will serve 3 years.
  • If B confesses and implicates implicates A, B will go free while A will serve 3 years.
  • If both stay silent, each will serve 1 year in prison on a lesser charge.

Here's a depiction of the payoff matrix in the classic Prisoner's Dilemma: 

Prisoner A
Prisoner B
A -- 1 Year
B -- 1 Year
A -- 3 Years
B -- 0 Year
A -- 0 Year
B -- 3 Years
A -- 2 Years
B -- 2 Years

The two prisoners cannot communicate with each other, so each has a choice of whether to cooperate by remaining silent or to defect (compete) by confessing.  Obviously, defection is the rational choice for the one who defects; but cooperation is the altruistic outcome, because is minimizes the loss to the other person as well as to oneself.  But cooperation requires each prisoner to trust that the other will not defect.  

There are two basic versions of PD.  In the standard PD, there is just one round of the game.  In iterated PD, there are several rounds, and subjects' behavior changes over time.  On the first round, as in the standard game, many, perhaps most, subjects compete by defecting.  On the second round, an initially cooperative subject may retaliate by competing in turn.  In this "tit for tat" strategy, each player mirrors the other's behavior.  Eventually, however, both players come to understand that they can maximize their joint outcomes by cooperating. 

In addition to this process, known as direct reciprocity, there are other processes involved in the emergence of cooperative behavior (for details, see "Five Rules for the Evolution of Cooperation" by Martin A. Nowack, Science, 2006; also "Why We Help" by Nowak, Scientific American, 07/2012):

  1. Spatial selection: neighbors tend to cooperate with each other, and the network of cooperators gradually expands.
  2. Kin selection, as discussed earlier.
  3. Indirect reciprocity, in which people help others who have already established a reputation for helping.
  4. Group selection, also as discussed earlier, in which individuals help others for the "greater good".

Nowack argues that evidence for all these mechanisms can be found in many different species of animals, suggesting that they have in common some basis in Darwinian evolution by natural selection.  But, as he also points out, some of these mechanisms aren't carried on the genes.  In particular, direct and indirect reciprocity depend on experience, and the sharing of information (e.g., about reputation) via language.  This is especially the case for direct reciprocity, which depends on players' ability to learn from experience; and indirect reciprocity, which depends on players' ability to learn from others (i.e., social learning).  The mechanism for Darwinian evolution by natural selection is based on genetic variability in innate traits.  But we also know that there's no inheritance of acquired characteristics.  So, helping behavior isn't all encoded in the genes.  It's also encoded in knowledge, beliefs, and attitudes -- it's encoded in the mind.

Our Phylogenetic Heritage

Humans have a particular place in the phylogenetic scheme of things. The earliest analyses of the human place in nature were based on morphological similarity -- the similarity in form and structure between humans and other animals. From this perspective, humans are warm-blooded vertebrates, primates related to the hominoid apes: orangutans, chimpanzees, gorillas, and gibbons. One popular morphological analysis holds that we are most closely related to orangutans.

The Cosmological Backdrop

The earth, and the rest of the physical universe, evolved too -- just not through such processes as natural selection. Like our understanding of human evolution, our knowledge of the evolution of the universe changes constantly, with new scientific discoveries. However, the rough outlines can be drawn below, beginning with the "Big Bang", about 14 billion ya.

The point is that the universe has not always existed, in a sort of steady state. As far as we can tell, the universe had its origins in the (extremely brief) era known as "Quantum Gravity", shortly before the Big Bang -- very shortly before, about 10-43 second into the existence of the universe (that's a zero, a decimal point, and a 1, with 42 zeroes in between). The entire universe was essentially a point in space, measuring 10-33 centimeters in diameter (so it wasn't exactly a point, but that's close enough for government work). Space and time were discontinuous, and all the physical forces in the universe were unified. Time began at this point, but there was not yet any space -- that is, there weren't any meaningful dimensions of length, width, and depth.

The "Era of Unification"

  • At 10-39 seconds, the "strong force" split from the "weak force" and from electromagnetism, beginning the "era of unification".
  • At 10-34 seconds, the Big Bang caused the cosmos to swell.
  • At 10-11 seconds, the "weak force" split from electromagnetism.
  • The period before the "Big Bang" was defined by physicist Alex Vilenkin as "a closed spherical spacetime of zero radius".

"Quark Soup"

  • At 10-5 seconds, quarks combined into protons and neutrons, beginning the era of "quark soup". All this so far, and we're still less than a second into the age of the universe!

The "Primordial Fireball"

  • From 10-2 seconds to 3 minutes, nucleosynthesis occurred. Protons and neutrons formed the nuclei of atoms, yielding the light elements -- helium, lithium, and deuterium.
  • In about 400 thousand years, atomic nuclei began capturing electrons. The universe became transparent, and cosmic radiation was released.

The "Dark Ages"

  • After about 1 million years, the cosmic background radiation faded, leaving the universe empty and dark.

"First Structures"

  • At about 500 million years of age, the dark ages ended with the formation of the first stars. These stars then exploded, filling the universe with heavy elements. At this point, the era of "first structures" began.
  • Beginning at about 1 billion years, the first galaxies formed, with black holes at their centers. These were the quasars, the farthest objects that can be seen from the earth today.
  • From 2 to 6 billion years of age, other galaxies formed, including our own Milky Way.
  • At 7 billion years, "dark energy", began to accelerate the expansion of the universe.
  • At 9.5 billion years, or about 4.5 billion ya, our solar system, including the Sun and the Earth, were born, essentially completing the universe essentially as we know it -- a universe built from about an ounce of primordial stuff that exploded in the Big Bang.
  • Interestingly, though, it appears that most of the universe appears to be composed of unseeable "dark matter" of subatomic particles left over from the Big Bang.
The Modern Universe
  • Some 3.5 billion ya, life began on earth. The universe was about 10.5 billion years old.
  • About 3 billion ya, dark energy outweighed matter in the universe.
  • And here we are today, about 14 billion years after the birth of the universe, about 4 billion years after the origin of the Earth, and about 3.5 billion years after the first emergence of life on Earth.

Source: "In the Beginning", by Dennis Overbye (New York Times, 07/23/2002)

The Future Universe
  • Cosmologists tell us that the evolution of the universe isn't over yet. In about 2 billion years, the warming Sun will make Earth uninhabitable. About 3 billion years after that, the Sun will swell into a red giant, burning the Earth to a crisp. As if that weren't enough, the Milky Way will collide with our nearest galactic neighbor, Andromeda.
  • About 131 billion years later,if the universe keeps expanding, the galaxies will be moving away from each other at such a high speed that they will outpace the speed of light, and stars will no longer be visible If there were anyone here to see them).
  • Forever, that is, unless the expansion accelerates, in which case the universe might literally shred itself in what some cosmologists call the "Big Tear" or "Big Rip", leaving no particles remaining, and thus no space either. This possibility was satirized by the cartoonist Roz Chast in the New Yorker magazine (12/07/2020).
  • Or a flaw in the structure of the Universe might create a "Quantum Bubble of Death" that moves through the universe, destroying everything in its path.The universe might continue expanding this way forever. 
  • If the Universe just keeps expanding, it may eventually suffer a "Heat Death", as all its energy dissipates into cold darkness (or dark coldness).
  • Alternatively, the expansion of the universe might come to a stop, in what is known as a "Flat State", and just stay that way forever.
  • Of course, there's also the possibility that the universe will fall back on itself, in what is called the "Big Crunch". If so, this will likely be followed by another "Big Bang" (or "Big Bounce", if you will), and the evolution of the universe will start all over again. Given the contingent nature of evolution, with details depending on accidents of circumstance, it's not clear that the Milky Way (and other galaxies), or our solar system (and other solar systems), would appear again in the form(s) we know them.  Sort of like the cosmological equivalent of Joni Mitchell's "Circle Game"
And the seasons they go round and round
And the painted ponies go up and down
We're captive on the carousel of time
We can't return we can only look behind
From where we came
And go round and round and round
In the circle game

  • Or, as proposed by several prominent cosmologists, it may be that there are already several alternative universes, each produced by the same process that ignited the Big Bang that created our own universe, and each reflecting the operation of different contingencies in the process just described -- and each outside the boundaries of the only universe we can know.  Of course, because this is the only universe we can know, there's no way of proving (or disproving) the multiverse hypothesis.  The fact that distinguished physicists can seriously entertain a hypothesis that is completely untestable should put an end to "physics envy" among psychologists, once and for all. 

See The End of Everything (Astrophysically Speaking) by Katie Mack (2020), which reviews various cosmological scenarios for the end of the Universe; also her article, "Tearing Apart the Universe, American Scientist, 11-12/2020. Reviewing the book in the New York Times ("This Is How It All Ends", 09/06/2020), James Gleick, quotes Robert Frost ("Some say the world will end in fire,/Some say in ice") and T.S. Eliot ("This is the way the world ends/Not with a bang but a whimper"), but ends with this:

...I found it helpful -- not reassuring, but mind-expanding to be reminded of our place in a vast cosmos.  Mack puts it this way: "When we ask the question, 'Can this all really go on forever?', we are implicitly validating our own existence, extending it indefinitely into the future, taking stock and examining our legacy."

It seems safe to say, though, that any meaning and purpose will have to be found in ourselves, not in the stars.  The cosmic end times will bring no day of judgment, no redemption.  All we can expect is the total obliteration of whatever universe remains and any intelligence that still abides there.

Or, as Priyamvada Natarajan, writes, in another review of Mack's book ("All Things Great and Small", New York Review of Books, 07/01/2021, which also contains a lovely update of both cosmological and subatomic theory),

As Mack points out, only one thing is certain: the universe will end.  It simply cannot persist unchanged forever.

Mammals, Primates, Hominoids, Hominins, Hominids, Humans

The fossil evidence suggests a gradual divergence among the hominoids. Life on earth began about 3 billion ya, during the Precambrian era of geologic time, in a probiotic soup of organic molecules (i.e., molecules containing the element carbon). For the next 2 billion years, only very simple life forms -- bacteria and algae -- existed. About 500 million ya, during the Paleozoic era,complex invertebrates began to evolve. Then, especially in the Mesozoic era, about 250 million ya, came the vertebrate species -- fish, then amphibians, then reptiles. Finally, in the cretaceous period of the Mesozoic era, beginning about 145 million years, ago, and especially in the tertiary period of the Cenozoic era, beginning about 65 million ya, birds, and mammals.

Morganucodon oehleri SmithsonianThis is what our earliest mammal ancestor, the morganucodon (Morganucodon oehlieri, to be exact) looked like, as depicted in a bronze sculpture at the Smithsonian Museum of Natural History in Washington, D.C.

Among mammals,primates are a relatively recent development. All primates have a set of morphological features in common, that tend to distinguish them from other mammals:

  • grasping hands and feet, with opposable thumbs and big toes;
  • nails (rather than claws) on the digits;
  • converging eye sockets (i.e., eyes that face forward);
  • postorbital bars (bony rings around the orbits);
  • other physical characteristics that enable the animal to leap from branch to branch and tree to tree;
  • large brains.

The earliest primates, emerging more than 60 million ya, were tree-dwelling ancestors of present-day tree-shrews and lemurs. About 40 million ya, the "higher primates" -- or, more correctly, the anthropoid primates -- began to emerge. These came in two groups, evolving in distinct areas of the globe: the "New World" monkeys, first appearing in North America, but then colonizing Central and South America; and the "Old World" monkeys and great apes, in Eurasia, diversifying into Africa.

Hominoids and Hominids

The great apes -- present-day gibbons, gorillas, chimpanzees, and orangutans -- are also known as hominid primates, and they share ancestors in common with modern-day humans.

The fossil evidence further indicates that the gibbons split off from the rest of the primates about 25 million ya. Then, about 19 million ya there was a big split, dividing chimps and gorillas from humans and orangutans. Finally, about 18 million ya, hominins -- the ancestors of modern humans -- split from orangutans. Thus, by the morphological and fossil evidence, humans are most closely related to orangutans.

The Human
              GenomeFor many years this was the standard view in the field. More recently, however, this view has been revised by genetic evidence. The human genetic endowment consists of 23 pairs of chromosomes, which are contained in the nucleus of each cell in the human body. (There is one exception to this rule: the sperm cells of men, and the egg cells in women, contain only one randomly selected member of each pair. When the egg is fertilized by the sperm, the chromosomes in one cell are matched with their counterparts in the other, yielding a one-celled embryo that contains all 23 of the required pairs, one chromosome in each pair contributed by each parent.

Each chromosome consists of thousands of genes. These are the basic units of heredity, and affect the organism's physical features, and the course of its development. The genes themselves are composed of DNA (deoxyribonucleic acid), a chain of four chemical bases (adenine, guanine, thymine, and cytosine). Every gene is located at a particular place on a specific chromosome. According to the theory of evolution, closely related species have closely similar sequences of bases. Thus, examining the similarity in DNA molecules between modern humans and modern nonhuman hominoids indicates the evolutionary relations among these species. And with knowledge of the rate of DNA change, we can determine how early, or how recently, these species diverged.

It turns out, somewhat paradoxically, that morphological change is not necessarily related to genetic change. Comparisons of the structure of DNA in the blood (see material on genetics, below) indicate that humans are most closely related to chimpanzees -- in fact, we have about 98.5% of our genetic material in common what that species. Further, the genetic evidence indicates that the split between chimpanzees and hominids occurred only about 6 to 8 million ya.

So, our evolutionary history is encoded in our genes, and our genes tell us that we are most closely related to chimpanzees. At the same time, those 1.5% of genes that are not held in common by the two species can make quite a difference. Our characteristically hominid features include:

  • The ability to walk upright on two legs, freeing our hands to manipulate objects.
  • Opposing thumbs on both hands, giving us a unique manual dexterity, and ability to grasp. All mammalian species tend toward pentadactylism, or having five digits per limb (the hooves of horses and deer evolved from the five-digit limbs of earlier species, not the other way around). The digits of primates have nails rather than claws, sensitive tactile pads on the tips, and the ability to grasp. The opposable thumb (meaning that it can face the other fingers) permits both a precision grip and a power grip.
  • Binocular vision, in which two eyes focus on objects, giving a different image in combination than either eye could give alone;
  • A uniquely structured vocal tract, tongue, and mouth cavity, that permits highly flexible vocal communication;
  • An extremely large cerebral cortex, much larger than would be expected given our body mass, and much larger than would be expected given the size of the rest of our brains. 

The last two features,vocal apparatus and brain mass, are the most characteristically human. And, of course, the brain provides the biological substrate of cognition -- it supports our learning, thinking, and problem-solving. It also contains specific cortical structures specialized for language, permitting symbolic representation of objects and events, and flexible, creative communication with other humans. Thus, the most important legacy of evolution is the human mind -- a particularly powerful cognitive apparatus coupled with a capacity for linguistic representation and communication. This feature, the human mind, sharply divides the dullest human from the smartest chimpanzee -- it is the difference that 1% makes.

The special features of the human brain doesn't mean that other animals don't have minds. For example, pigeons have a high capacity for abstracting concepts. And chimpanzees and dolphins have some limited linguistic abilities (symbolic representation, some degree of flexibility and creativity) -- though no capacity for speech because of the different configuration of their vocal tracts. The mental abilities of nonhuman animals are interesting in their own right, and we can learn a great about ourselves from studying other species.

But this does mean that the human mind is something quite special, and that we should focus on its development in the life of the individual -- that is, move from the phylogenetic perspective on development to the ontogenetic perspective.

"The March of Progress"

"The March of Progress" (TMOP), perhaps the most famous (and most frequently parodied) scientific illustration of all time, appeared in Early Man by F. Clark Howell, an anthropologist at UC Berkeley and generally regarded as the father of paleo-anthropology.  Published in 1965 by Time-Life books, Early Man was intended as a non-scholarly introduction to the field (there's a copy in the UCB Education-Psychology Library).  TMOP itself was drawn by Rudolph Zellinger, an important scientific illustrator. 

Reading from left to right, TMOP traces 15 milestones in human evolution, as it was understood at the time:
  • Pliopithecus, a gibbon-like primate.
  • Proconsul, possibly an early ape.
  • Dryopithecus, an early ape.
  • Oreopithecus.
  • Ramapithecus, an early orangutan.
  • Australopithecus, the earliest hominid
  • Paranthropus.
  • Advanced Australopithecus.
  • Homo erectus, an early member of the genus Hom.
  • Early Homo sapiens
  • Solo Man, a sub-species of H. erectus.
  • Rhodesian Man, an early H. sapiens.
  • Neanderthal Man.
  • Cro-Magnon Man.
  • Modern Man.

Of course, the illustration -- wonderful as it is --  an oversimplification: 

  • In the first place, there's been a lot of progress in paleo-anthropology since 1965, and we know that human evolution was more complicated than that.  For example, there may have been at least six different kinds of early H. sapiens, and they may have even lived in close proximity to each other, fought, and interbred.
  • More important, there was no linear "march" from early primates to modern man (and Howell didn't think there had been -- he knew better).  As Stephen Jay Gould tirelessly pointed out, and even Darwin agreed, evolution is better depicted as a bush, with lots of branches, than as a ladder, or even as a tree.
  • As Gould also was at pains to point out, even the notion of "progress" is suspect.  Evolution isn't just a progression from simple to complex, or from less good to better.  Even very simple organisms are perfectly adapted to the environment in which they evolved.  Humans are in no sense an ideal form, towards which evolution has been progressing for millions of years.  We're animals with a high degree of general intelligence, consciousness, and language, and that gives us a special place in nature -- which is why we call the era of modern humans the Anthropocene. 
  • But from a biological point of view, we're just another species of animal.
For more on TMOP, see:
  • From an evolutionary point of view, "The Iconography of an Expectation" by Stephen Jay Gould, in Wonderful Life (1989).
  • From a design standpoint, see There's Nothing Funny About Design by David Barringer (2009).
Click on the thumbnail to the right for a larger view of the image.

"The March of Progress"

The Evolution of Hominins

The precise manner in which modern humans evolved from ancient hominids is not known, and there is considerable controversy over this matter among physical anthropologists. To make life even more interesting, every so often a new discovery will shake up the field of paleontology. Still, the basic outlines are known. The account that follows is based on my understanding of the theory of Donald Johanson, discoverer of the skeleton of Lucy, one of our earliest ancestors (the rival theory is by Louis and Mary Leakey). According to Johanson's view, the earliest hominids split off from the ancestors of African apes (gibbons, gorillas, orangutans, and chimpanzees) about 6-8 million ya.

About 4.4 million ya, in the midst of the Ice Age, the genus Australopithecus ("Southern ape") emerged. Australopiths have human-like teeth, but in other respects they resembled terrestrial apes: short body, long arms, small brain. Perhaps their most important physical feature was that they walked upright on two feet, thus freeing their arms and hands to make and use tools -- something which began about 2.5 million ya, and coincided with a period of increasing brain size.

The earliest known example of Australopithecus is, in fact, Lucy -- discovered in the Afar Triangle of northern Ethiopia, and thus named Australopithecus afarensis ("Selam", an infant A. afarensis also dubbed "Lucy's baby", although the fossil was about 100,000 years older than was also discovered nearby). A. afarensis lived from about 4 million to about 2.5 million ya. Another ancestor,Australopithecus africanus, lived from 3 to 1 million ya: specimens have been found in eastern and southern Africa; the most famous sites are in Kenya and Tanzania. Yet another ancestor,Australopithecus robustus, bigger and more powerful than the others of its kind (hence the name), lived from 2.5 to 1.5 million ya: specimens have been found in southern Africa. A fourth ancestor,Australopithecus boisei (named for Charles Boise, a benefactor of many fossil hunts) lived from 2.5 to 1 million ya.

Another hominid line,Paranthropus, lived from about 2.8 million to about 1.4 million ya.There's also a Paranthropus boise, with the same namesake.  There is evidence that P. boise lived in East Africa  alongside early early Homo species, which is how its got its name: para from the Greek for "beside" and anthropus, of course, for the Greek for "human being"  . 

For a summary of what we know about P. boisei, see "Meet Your Exotic, Extinct Close Relative" by Bernard Wood and Alexis Williams, American Scientist, 11-12/2020.

Actually, according to Johanson, neither Australopithecus nor Paranthropus is not a direct ancestor of humans. That honor belongs to another hominid entirely:Homo habilis ("handy Man", the initial capital indicating that it refers to both males and females of the species), discovered in eastern Africa by the Leakeys. H. habilis had a much bigger brain than any Australopith. It made and used tools, while the Australopiths probably did not. It lived as a community, building shelters and surrounding its camps with fences or windbreaks. H. habilis emerged in eastern, southeastern, and southern Africa about 2 million ya, and lived alongside several genera of Australopiths for about 500 thousand years. Homo habilis apparently gave direct rise to another ancestor,Homo erectus ("upright Man"), which lived from about 1.6 million ya to about 200 thousand ya. H. erectus has been found everywhere in the Old World, including Europe, Asia ("Peking Man"), and Southeast Asia ("Java Man"). It had an even bigger brain, a better toolkit and building materials, hunted, and used fire.

The Origins of Fire

Anthropologists and paleontologists generally date the control of fire to about 250,000 years BCE. However, deposits of burned wood and flint discovered in the Gesher Benot Ya'aqov site in northern Israel strongly suggest that early humans may have controlled fire (as opposed to simply using it) as long as 800,000 ya. If validated, the discovery would help explain how early humans were able to migrate out of Africa and into the colder climates of Europe and Asia -- a migration that began at about this time (N. Goren-Inbar et al.,Science, 04/30/04).

About 300 thousand ya, yet a new subspecies,Homo sapiens ("Wise Man"), emerged. To put it bluntly, this is us. The archaic form of H. sapiens, popularly known as Neanderthal Man (from fossils found in the Neander Valley near Dusseldorf, Germany), controlled fire and made clothes; thus they were the first hominids to be able to survive in cold climates (naked humans cannot survive outside the tropics). They cared for the sick and buried their dead. They produced art. But they didn't last. They competed unsuccessfully with another subspecies,Homo sapiens sapiens ("very wise Man", I guess), popularly known as Cro-Magnon Man (from fossils found at Cro-Magnon, in southwest France). Neanderthals went extinct approximately 30,000 ya. Recently, Neanderthal Man has been renamed h. neanderthalensis, and modern man, simply,h. sapiens.  The illustration at left shows a reconstruction of a Neanderthal skeleton (foreground), with a skeleton of Homo sapiens in the background (from the American Museum of Natural History).

Neanderthals get a bad rap, because they were replaced by modern humans.  Then again, they lasted for about 350,000 years, and never once came even close to blowing themselves up with nuclear weapons, or threatening the planet with global warming.  And, for the record, the first modern humans to migrate out of Africa didn't last too long, either.  A more positive appreciation of Neanderthals is found in Kindred: Neanderthal Life, Love, Death, and Art (2020) by Rebecca Wragg Sykes.  Reviewing the book in Science (10/30/2020), Emma Pomeroy, an anthropologist at Cambridge University, wrote that "Wragg Sykes evaluates the available evidence on Neanderthals with empathy and even-handedness, revealing the group to be less 'them' and more 'us'".  (For another, more extensive review, see "Why Did They Vanish?" by Tm Flannery, New York Review of Books, 05/13/2021.).

The ancestors of Neanderthals emigrated from Africa roughly 500-250 thousand ya, turned north and west, and ended up in Europe.  Another archaic form of homo sapiens, known as the Denisovans, turned east, and ended up in Asia (the first Denisovan bones were discovered in Denisova Cave, in the Altay Mountains of southern Siberia).  Unfortunately, the only fossils definitively known to be Denisovan consisted of a single finger and a couple of teeth from a single individual ("Denisova Girl")found in Siberia, and a jawbone found in China.  However, Carmel, Gokhman, and their colleagues were able to use advanced DNA analyses (don't ask) to generate a guess at what Denisova Girl might have looked like.  The image, published in Cell (2019), turns out to be consistent with the independently discovered jawbone -- but the researchers caution that this is only a reconstruction of a single individual who may or may not be representative of her species as a whole.

About 50,000 ya, the ancestors of modern humans also left Africa, encountered and interbred with both Neanderthals and Denisovans (depending on which turn they took), competed with them, and became dominant, after which the Neanderthal line died out.  As a result of this cross-breeding, however, modern humans still carry some Neanderthal genetic material -- amounting to as much as 5% of the human genome.

Here's one version of the story, based mostly on DNA evidence (Akey et al., Science, 2016):

  1. The first encounter with Neanderthals occurred soon after modern humans left Africa, somewhere in West Asia, leaving traces of Neanderthal DNA among the ancestors of modern Europeans, East Asians, and Melanesia.  (At this point, the ancestors of modern Melanesians split off from the Europeans and Asians.)
  2. A second encounter with Neanderthals, somewhere in the Middle East resulted in further interbreeding with the ancestors of modern Europeans and East Asians, and South Asians.
  3. An encounter with Denisovans resulted in interbreeding with the ancestors of modern Melanesians.
  4. And a third encounter involved only Neanderthals and the ancestors of modern East Asians.
  5. It was long thought that the ancestors of Neanderthals migrated out of Africa, turned left or right, but never turned around.  In 2020, researchers from the Max Planck Institute for the Science of Human History, found that some Neanderthal DNA can be found in the genomes of modern Africans, providing evidence of 'backmigration" from Eurasia into Africa.
  6. Recent DNA modeling by Alan Rogers, a population geneticist at the University of Utah, suggests that there may have been interbreeding between Neanderthals, and Denisovans and "archaic", "ghost" lineages, such as h. erectus, who also migrated out of Africa (Science, 02/21/2020, from which the image at the right is taken).  Some of this interbreeding may also have involved the ancestors of modern humans.  If so, we don't just carry a little Neanderthal DNA with us -- the "ghosts" in the human family tree go back way further than that.
  7. Actually, recent DNA sequencing studies (e.g., Petr et al., Science, 09/25/2020) indicate that there may have been two encounters between Neanderthals and h. sapiens.  The first migration of h. sapiens out of Africa and into Europe may have occurred as long as 200-300,000 years ago.  These first modern humans in Europe died out, but not before transferring some of their genes (ahem) to Neanderthals who were already living there.  Then there was a second migration about 40-80,000 years ago, after which the Neanderthals went extinct, and the modern humans thrived (this is the standard story).

There may have been other, isolated encounters, some of which did not result in the passing of DNA to descendants. But the four interminglings documented in the DNA evidence means that the ancestors of modern Melanesians got only one "pulse" of Neanderthal DNA -- most of their archaic DNA comes from Denisovans.  Europeans and South Asians got two "pulses" of Neanderthal DNA, while the ancestors of modern East Asians got three.  And of course, the ancestors of modern Africans, who never migrated out of Africa and so never encountered Neanderthals or Denisovans, got none at all.

This whole story is told by Svante Paabo, the Swedish biologist who first sequenced the Neanderthal genome, in Neanderthal Man: In search of Lost Genomes (2014), a book which has been compared with James Watson's The Double Helix (1968).

For a shorter version of the story, see "Neanderthal Minds" by Kate Wong, Scientific American, 02/2015.

Neanderthals and Cro-Magnons were closely related in genetic terms. In 2006, two different teams of researchers reported the first steps in reconstructing the Neanderthal genome, based on samples from preserved bone tissue: preliminary results indicate that the two genomes are about 99.5% identical.  So, that's 1.5% genetic difference between us and chimpanzees, and 0.5% difference between us and Neanderthals. It was also determined that Neanderthals had the FOXP2 gene that is considered critical (though probably not sufficient) for speech, if not language, suggesting that they may have had the ability to use language as well.  However, it's not really possible to make inferences about the mental and behavioral capabilities of Neanderthals from knowledge of their genomes.  For that, we have to rely on the physical evidence they left behind to be studied by archeologists

For a summary of what we know about Neanderthal psychology, see How to Think Like a Neanderthal by Thomas Wynn and Frederick L. Coolidge (2011).

The traditional theory is that modern humans invaded Neanderthal territory in Europe and eliminated them -- and that similar scenarios played out between modern humans and other bands of "archaic" humans.  However, the close similarity in DNA, and other evidence,now suggests that modern humans actually interbred with Neanderthals, and perhaps other archaic forms with whom they shared close genetic resemblance.  (See "Human Hybrids" by Michael F. Hammer, Scientific American May 2013).

These are the broad outlines.  It's clear where we came from, but the precise path is unclear, and it's certainly not a straight line from Pan troglodytes to Homo sapiens. 

For a recent overview of the complexities of human evolution, see "Shattered: New Fossil Discoveries Complicate the Already Devilish Task of Identifying Our Most Ancient Progenitors" by Katherine Harmon, Scientific American, February 2013.

The Geological "Ages"

The geological timescale is a chronology of the history of the earth, divided into

  • eons, which in turn are divided into
    • eras, which in turn are divided into
      • periods, which in turn are divided into
        • epochs, which in turn are divided into
          • ages.

The origin of life on earth can be traced to at least 3.5 billion years ago, which is the date for the earliest known fossils.  Most biologists push back the actual origins to the formation of the oceans, about 4.41 billion years ago -- not too soon after the origin of the earth itself, roughly 4.54 billion years ago (give or take).  According to theory, life emerged from a primordial soup of organic compounds.  We needn't tarry over the details, much less argue about precise dates.  The general idea can be illustrated by the 1952 Miller-Urey Experiment in which a vessel of water (representing the Earth's oceans) and a vessel of gasses (representing the Earth's atmosphere), when stimulated electrically (lightning!), yielded a substance containing glycine, the simplest amino acid.  More recently, it's been suggested that life began in hydrothermal vents in the ocean, where chemical reactions could create amino acids.  For an excellent account of the evolution of theories concerning the origins of life, see The Genesis Quest: The Geniuses and Eccentrics on a Journey to Uncover the Origin of Life on Earth (2020) by by Michael Marshall (reviewed by Tim Flannery in "In the Soup", New York Review of Books, 12/03/2020). 

In the Cretaceous period, animal life was dominated by reptiles. According to Luis Alvarez, about 65 million ya an asteroid or comet struck the Earth, spreading dust into the atmosphere and suppressing photosynthesis, leading to the death of the dinosaurs and the emergence of mammals as the dominant animal species. The "Cretaceous-Tertiary Barrier (K-T barrier, after its abbreviation in German) is marked by a layer of iridium in the earth's crust, apparently the remains of the meteor.

Early humans began to emerge during the Miocene Epoch of the Cenozoic Era, and H. sapiens emerged during the Pleistocene Epoch -- the "Environment of Evolutionary Adaptedness" touted by evolutionary psychologists (the term comes from John Bowlby; it is also referred to as the "Environment of Early Adaptation", and other variants, and abbreviated EEA).

The last "Ice Age" (there have been others) occurs at this time as well, with a sheet of Arctic ice covering North America as far south as the Ohio and Missouri rivers, Europe as far south as the British Isles and northern Germany, and Asia as far south as the Himalayas; it permitted early emigrants to walk across the Bering Sea from Siberia to Alaska, beginning the population of the Americas (an Antarctic ice sheet covered South America as far north as Patagonia and the southern Andes).

Era Period Epoch Age Ended ya

600 million
Paleozoic Cambrian

500 million

425 million

405 million

345 million

280 million

225 million

190 million

136 million
Lower Cretaceous

Upper Cretaceous

65 million
Cenozoic Tertiary Paleocene
58 million
36 million
25 million
13 million
2 million
Quarternary Pleistocene
10 thousand
("Entirely Recent")

The Present

The Anthropocene

Recently, earth scientists have begun to refer to the Anthropocene era, defined as the epoch in which human beings began to have a palpable and permanent impact on the environment -- such as mass extinctions and global warming. The term was coined by Paul Crutzen (2000), an atmospheric chemist who won the Nobel Prize for his work documenting the hole in the ozone layer (in the 19th century, Antonio Stoppani, proposed the term anthropozoic, but it didn't catch on). Similarly, Andre Revkin, a journalist, coined the term Anthrocene in 1992.  But while the label is recent, the idea is very old. As long ago as 1960, Theodosius Dobzhansky wrote (in "The Present Evolution of Man",Scientific American, Sept. 1960):

Mutation, sexual recombination and natural selection led to the emergence of Homo sapiens. the creatures that preceded him had already developed the rudiments of tool-using, tool-making and cultural transmission. But the next evolutionary step was so great as to constitute a difference in kind from those before it. There now appeared an organism whose mastery of technology and of symbolic communication enabled it to create a supra-organic culture. Other organisms adapt to their environments by changing their genes in accordance with the demands of the surroundings. Man and man alone can also adapt by changing his environments to fit his genes. His genes enable him to invent new tools, to alter his opinions, his aims and his conduct, to acquire new knowledge and new wisdom.

Some geologists, however, object to adding a new era, because the evidence for it can't be found "in the rocks", as it were.  They want a definite date -- like the one about 12,000 ya, when the last Ice Age ended -- to identify the beginnings of any new era.  Proponents, though say that the critics shouldn't be "sticks in the mud" (sorry).  They say that the Anthropocene can be dated at least to the beginnings of the Atomic Age, which left traces of radiation in the soil; and maybe as far back as the beginnings of agriculture, at least in Europe.  Crutzen himself dates the Anthropocene to the industrial revolution of the 19th century.  All organisms affect the planet. We may be the first to do so permanently; we are certainly the first to do so deliberately.

By one account, the Anthropocene began about 8,000 ya, with the rise of the first cities, and consequent deforestation, which led to an increase of carbon dioxide in the atmosphere and prevented a new Ice Age from occurring. It's also been argued that the Anthropocene began much earlier, more than 100,000 ya and before the rise of cities, when neolithic hunter-gatherers first used fire to prepare land for cultivation -- thereby artificially changing the landscape for the first time.  Then too, control of fire enabled early humans to keep warm at night and in the winter -- permitting them to expand over, and thus have an effect on, more of the earth's surface -- not to mention burning forests to clear land for agriculture.  A recent worldwide collaborative study known as the ArchaeoGLOBE Project (Stephens et al., Science, 08/30/2019), based on a worldwide sample of archeological evidence, indicate that prehistoric human activity, such as deforestation and rice farming, and leading to an increase in atmospheric methane and carbon dioxide, had already substantially altered the landscape worldwide by about 3,000 ya. 

Crutzen himself suggests that the Anthropocene began in the late 18th century, with the beginnings of the Industrial Revolution, when atmospheric CO2 began to rise consistently.  Gradually, theorists have fixed on 1950 as a good starting date, because a lot of environmental trends began to change dramatically about then (graphic from "Nine Key Questions About the Future: 1. What Mark Will We Leave On the Planet?" by Jan Zalasiewicz, Scientific American, September 2016):

  • plastics and other polymers began to appear in soil deposits, and contaminating rivers and oceans;
  • concrete, invented by the ancient Romans, began to be used in abundance as a building material;
  • the concentration carbon and other byproducts of fossil fuels took a sharp turn upward;
  • plutonium isotopes 239 and 240 begin to appear in soil samples, the product of above-ground nuclear testing;
  • carbon dioxide and other greenhouse gases, rising since the beginning of the Industrial Revolution, increased markedly after World War II;
  • the concentration of methane emitted by livestock increases in the atmosphere;
  • as does the concentration of nitrous oxide, a product of fossil fuels and chemical fertilizers.

But the era really picked up steam with the scientific revolution of the 17th and 18th centuries. At least, that's the implication of David Deutsch's argument in the Beginning of Infinity: Explanations that Transform the World (2011). Before the scientific revolution, humans changed the environment through things like the invention of agriculture and the domestication of animals. After the scientific revolution, we acquired a whole new level of ability -- almost limitless, in Deutsch's view -- to understand the world around us, and so to shape it according to our inclinations.

A good example of the difference between the Anthropocene and earlier geological periods is provided by a famous wager between Paul Ehrlich, and Julian Simon, an economist, about the effects of overpopulation.  In his book, The Population Bomb (1968), Ehrlich drew on the theories of Thomas Malthus to predict that, with uncontrolled population growth, the earth would exhaust its natural resources, precipitating widespread famine.  Simon, for his part, believed that human rationality would triumph by creating innovative efficiencies and other conservation efforts that would permit a larger population to thrive on diminishing resources.  In 1980, Simon proposed the following bet: Ehrlich would select a "market basket" of five commodities to be purchased for $1,000; if, after 10 years, the price went up, reflecting increasing demand and decreasing supply, Simon would pay Ehrlich the difference between the 1980 and 1990 prices; if they went down, meaning that the commodities were not quite so precious any longer, Ehrlich would pay Simon.  As it happened, although the population of the world increased by about 800 million over that decade, the price of the "market basket" went down by more than 50%.  So, Ehrlich cut Simon a check for $576.07.  Now,you can argue that Ehrlich's "market basket", full of metal, wasn't quite right, and that something different would have happened with commodities like wheat and corn; but still, Ehrlich could have selected any commodities he wished.  The point, here, is that the biological environment (population, arable land, etc.) doesn't change independently of human activity: human ingenuity can find ways to cope with increasing population, climate cycles, and the like.  For example, the "green revolution" initiated by Norman Borlaug introduced high-yield crops to underdeveloped countries such as Mexico and India, greatly increasing food security in those countries (Borlaug won the 1970 Nobel Peace Prize for his efforts).  We're not at the mercy of the environment: rather, the environment is at the mercy of us.

The impact of man on the physical environment is suggested by a little piece of pseudo-mathematics:

I = P x A x T,

where I = impact; P = population; A = affluence; and T = technology. It's also been suggested that a fourth factor be added, E, for education, which can -- we can hope -- mitigate the others.

On the other hand, there are some writers who object to the term "Anthropocene" as well, too human-centered.  The idea is that we humans think we're at the center of everything, so we even name a geologic era after ourselves.  On the other hand, the changes in question were undoubtedly caused by us, rather than by autonomous geological processes like tectonic shifts, or random physical events like the asteroid that wiped out the dinosaurs.  Another critique is that the term "Anthropocene" blames humans in general for problems that are caused by only some of us -- namely, those of us who live, more or less comfortably, in more-or-less capitalist, colonialist, industrial or post-industrial societies.  And it's true that hunter-gatherers weren't responsible for wrecking the environment, and that if we still lived like that we wouldn't be talking about the Anthropocene.  But a lot of us do live like that, and even the last remaining hunter-gatherer tribes-people are going to suffer the consequences if we don't fix it and soon.

For more on the debate over the Anthropocene, see a series of articles by Elizabeth Kolbert:

  • "Enter the Anthropocene Age of Man" by Elizabeth Kolbert,National Geographic, March 2011.
  • "The Lost World", Parts 1 and 2, New Yorker, 12/16 and 12/23-30/2013.
  • "Nine Key Questions About the Future: 1. What Mark Will We Leave On the Planet?" by Jan Zalasiewicz, Scientific American, September 2016.
  • And Kolbert's book, The Sixth Extinction (2014).
Also, for other full-length treatments:
  • The God Species: Saving the Planet in the Age of Humans by Mark Lynas, (2011)
  • Adventures in the Anthropocene: A Journey to the Heart of the Planet We Made by Gaia Vince (2014).

For the story of the Ehrlich-Simon wager, see:

  • The Bet: Paul Erlich, Julian Simon, and Our Gamble Over Earth's Future by Paul Sabin (2013). 
  • For a synopsis, see "The Battle of Two Hedgehogs" by Cass R. Sunstein, New York Review of Books, 12/03/2013).

Whether the Neanderthals were killed by Cro-Magnon, or interbred with them, the line died out about 30 thousand ya. H. sapiens sapiens thrived, through the Old (Paleolithic), Middle (Mesolithic) and New (Neolithic) Stone Age.

Why the Cro-Magnons thrived, while the Neanderthals did not, will likely remain an anthropological mystery. The most likely hypothesis is that the Cro-Magnons had a capacity for language, and for symbolic thought, that gave them an advantage (as indeed it would). So far as we can tell from the scant evidence, Neanderthal culture was pretty primitive even by the standards of Early Man -- much more so than Cro-Magnon culture. But it's all pretty speculative -- pretty close to reasoning backward from the fact that we have a sophisticated capacity for language and symbolic thought.

Out of Africa... Into the Americas

For an overview of research on human migration to and through the Americas, see the First Americans: In Pursuit of Archaeology's Greatest Mystery by J.M. Adovasio (Random House, 2002).

For an overview of fossil and genetic evidence concerning the migration of early modern humans, see "The Greatest Journey" by James Shreeve, National Geographic, March 2006.

For an up-to-date account, based on the latest discoveries in population genetics, see Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past (2018) by David Reich, a leading figure in the application of population genetics to cultural anthropology.  In his chapter-by-chapter histories of Africans, Neanderthals, modern Europeans, South Asian Indians, East Asians, Polynesians, and Native Americans, Reich demonstrates how population genetics expands, and sometimes corrects, theories of early human migration based solely on the fossil evidence.

In 2013 Paul Salopek, a journalist, began to retrace the path of human migration out of Africa, on foot, beginning at Herto Bouri, in Ethiopia, where some of the earliest human remains have been found, and ending in Tierra del Fuego, Chile, where human migration ended.  The journey is traced in his forthcoming book, A Walk Through Time (scheduled for publication in 2016), as well as in a series of articles in National Geographic (December 2013, July 2014, December 2014, March 2015, April 2016, December 2017).

The path out of Africa looks something like this:

  • H. sapiens emerges in Africa about 160-200,000 ya.
  • About 55-70,000 ya, H. sapiens expands from Africa into what is now the Middle East.
  • From there they expand in two directions, arriving in both western Europe and Australia about 45,0000 ya.
  • And also in a third direction, arriving in Arctic Siberia about 35,000 ya.
  • About 14,000 ya, they crossed they arrived in the Americas, reaching South America about 13,500 ya.

Traditional theory, based on the discovery of an early human site in Clovis, New Mexico with distinctively shaped spear-points, suggests that the first humans crossed the Bering Strait from Asia into North America about 13,500 ya, and had settled as far south as Tierra del Fuego, at the tip of South America, within about 500 years.  In this view, these first hunters and foragers were the ancestors of the ancient Hohokam and Anasazi cultures who built great hamlets and towns (e.g., Montezuma Castle and Casa Grande by the Hohokam; Canyon de Chelly by the Anasazi), and gave rise, respectively to present-day Tohono O'Odham and Pueblo cultures in Arizona and New Mexico.

However, evidence from other sites, including Adovasio's own excavations near Meadowcroft, Pennsylvania, suggest that there were even earlier  migrations.  And other evidence indicates that humans were making tools (though not Clovis points), and killing mammoths, in Florida at least 14,500 ya. 

According to the coastal migration hypothesis, the first Americans didn't come across a land bridge from Siberia, but rather by boat along the Aleutian Islands, 15-16,000 ya.  There's even a theory that the Solutrean people migrated from Europe to the east coast of North America more than 20,000 ya.

In these revisionist views, Clovis people were not the pioneers, but rather later immigrants, whose culture, exemplified by Clovis points, spread widely throughout North America.

In 2015, two groups of investigators, examining a large set of genomic data, offered competing conclusions about the population of the Americas.

  • One group, including researchers from UCB, found evidence for the traditional hypothesis, that (for the most part) the Americas were populated in a single wave, crossing a land bridge over the Bering Straight somewhere between 15,000 and 23,000 ya.
  • Another group, including researchers from (wouldn't you know it?) Harvard, examining the same genomic evidence, saw signs of two separate waves -- one originating in Siberia (that's the traditional origin), and another originating in Australasia. 
  • Both groups agree that the Inuit people of Alaska represent yet another, independent wave of migration.

So that's two or three waves of prehistoric migration, depending on how you count.  But notice that these researchers are looking at the same set of data, with the most powerful tools available to modern physical anthropology and paleontology.  So this issue isn't likely to be settled, for good, anytime soon.

Especially because there's yet another data set -- one that is especially provocative, even though it doesn't contain any human remains.  This is the "Cerutti Mastodon Site" near San Diego, named for the staff member at the San Diego Natural History Museum who discovered it.  The site contains a mastodon skeleton, obviously, but it also contains some cobblestone-like rocks whose markings suggest that they may have been used as hammers and anvils.  The thing is, this site has been dated to about 130,000 BCE -- long before Clovis people.  And while the Clovis people were homo sapiens, stone tools similar to those found at the  Cerutti Site have been found in homo erectus sites in Africa.  We know that there were migrations out of Africa by species other than h. sapiens -- the Neanderthals, to take a clear example.  And, in a neat coincidence, there may well have been a land bridge between Asia and North America during a period of global warming which also occurred about 130,000 ya.  So, it's possible that h. erectus also got as far as North America.  But again, no erectus fossils have ever been found in the Western Hemisphere.  

Whatever the details, it's clear, from the high degree of genetic similarity between ancient remains found in Canada and the Northern United States, and other remains found in Chile and Brazil, and even between remains found in South America and remains found in Australia and New Guinea, that the migration to and through the Americas happened remarkably quickly (in historical terms, that is).

And, for that matter, anthropologists keep pushing back the date for the earliest migration out of Africa.  H. sapiens skulls have been found at the Skhul and Qafsez sites in Israel which have been dated to 90-120,000 ya, and a 2018 report that fossil evidence of h. sapiens, including stone tools, has been found in Israel's Misliya Cave dating to 180,000 ya. 

Then again, in 2019 a group of paleoanthropologists led by Katerina Harvati reported finding two hominid skull fragments in a Greek cave known as Apidima -- one with Neanderthal features, the other more closely resembling modern humans (Nature, 2019).  Uranium dating suggested that the H. sapiens skull was 210,000 years old, older than the skull found at Misliya Cave.  The find suggests that modern humans at least put their toes into Europe long before generally believed.  If so, they may have died out quickly, or they may have gone back where they came from, only to return later.  Now, the classification of the skull isn't completely solid, nor is the dating.  But it's not beyond the realm of possibility that modern humans ventured from the Middle East into Southeastern Europe at an early point in prehistory -- it's not that far. 

Here's our best understanding of early human migration, based on the paleological, archeological, and paleogenomic findings as of 2018 (for details, see "The Origin of Us" by Natalie O'Shea and Eric Delson, and "The Paleogenomic Revolution" by Robert DeSalle, both published in Natural History, 09/2018 -- an entire issue dedicated to our understanding of human origins). 

  • There's still agreement that human origins were in East Africa, but now those origins have been pushed back to about 300,000 ya.
    • A group of UC Berkeley biochemists, analyzing mitochondrial DNA (mtDNA), which is inherited maternally, suggested in 1987 that all known mtDNA could be traced to a single female, whom they dubbed "mitochondrial Eve", who lived about 143,000 ya.
    • Not to be outdone, in 1999 a group of Stanford geneticists identified a group of genes that are inherited paternally, and proposed the existence of a "Y Chromosomal Adam", who lived about 59,000 ya. 
    • How mitochondrial Eve and Y-chromosomal Adam ever got together is anyone's guess, but more recent analyses have put them much closer in time, if not in place (see below).  
  • Humans dispersed throughout Africa, but also moved out of Africa beginning about 180,000 ya.
    • There may have been earlier migrations, but these appear not to have been successful.
    • There were likely multiple migrations, some following a "northern" route through modern Egypt, and others following a "southern" route through modern Saudi Arabia.
    • Either way, migration reached Indonesia about 70,000 ya, and Australia about 65,000 ya.
    • At some point, some migrants "turned left", ending up in modern Europe about 30,000 ya.
    • Others crossed a land bridge across the modern Bering Strait from modern Siberia into North America, and then into South America.
    • And there may well have been a migration back into Africa.
  • Analyses of maternally inherited mtDNA, the paternally inherited Y chromosome, and also recombinant DNA have shed further light on population ancestry and dynamics, especially within the five habitable continents (This is mostly the work of a group at the Max Planck Institute in Leipzig, who focused on DNA sampled from specimens collected in the pre-colonial period, before Europeans really sent genetic diversity into high gear by, if you pardon the expression, contaminating DNA all over the place).  In the map on the right, color coding indicates genetically related groups of pre-colonial peoples.
    • The greatest of genomic diversity is found in Africa, with three major lineages: hunter-gatherers living in the rain forest of central Africa; Khoesan-speaking peoples of southern Africa; and the Hadza peoples of Tanzania.   
    • "Y-Chromosomal Adam" seems to have lived in West Africa, near modern Cameroon. 
    • The Tanzanian lineage was the source of much of the migration into Europe, but there were three additional sources: Paleolithic hunter-gatherers, Neolithic farmers from the Fertile Crescent, and late Neolithic and early Bronze Age Yamnaya people from the Caucasus.
    • There may have been two waves of migration into Asia, one occurring about 90-120,000 ya, and the other about 40-60,000 ya.  The first of these seems to have been the source of Aboriginal Australians, while the second was the source of East Asians.
    • There were at least two migrations across the Bering Strait, resulting in separate lineages for Athabaskan- and Amerindian-speaking populations of Native Americans.
    • And it wasn't just h. sapiens who migrated out of Africa.  Although their split with Neanderthals occurred 360,000-470,000 ya, the two populations of hominins interbred enough, as recently as 100,000 ya, to leave about 1-2% Neanderthal DNA in the genomes of Europeans and Asians.
For a timeline of the prehistoric population of the Americas, see "First Americans" by Glenn Hodges, National Geographic, January 2015.  See also "Journey into the Americas" by Jennifer Raff, Scientific American, 05/2021.

For an authoritative update, see "Late Pleistocene Exploration and Settlement of the Americas by Modern Humans" by M.R. Waters (Science 2019, from which these images are taken).  Waters concludes that the best evidence indicates that migration south of modern Canada began as long ago as 17,500 ya -- but no earlier.  This group then split into two branches of Native Americans, one remaining in the north (NNA) and one moving towards the south (SNA), and splitting into a number of related sub-branches. This review is a must-read for anyone interested in this subject.

About the only place we didn't get by walking was Antarctica, and now we're there, too, with permanent settlements since 1959.

Pushing Back Our Origins

The story of human evolution from Australopithecus through Paranthropus to Homo is fairly well accepted, though there are disagreements about the details. However, as noted earlier, occasionally a major discovery will occur that calls for major revisions in the story. The major consequence of these new discoveries is to push the origins of hominids back a little (or, in some cases,a lot) further back in time, or to add a branch to the main lines of human descent. (Image by Viktor Deak from "The Human Pedigree" by Kate Wong, Scientific American 01/2009).

              Georgia SkullsFor example, until recently it was generally thought that the earliest hominids, known as Australopithecus, dated back some 4.5 million ya, to an area in eastern Africa. But in just two short weeks in 2002, different teams of paleontologists reported discoveries that pushed the emergence of hominids back a lot in time and over a bit in space.

Sahelanthropus tchadensis.The first of these, and the most surprising, was the discovery by Dr. Michel Brunet of the University of Poitiers, in France, of the "Chad skull" in Chad (hence its name), in the region of the southern Sahara desert known as the Sahel (actually, the skull was discovered by Ahounta Djimdoumalbaye, an undergraduate at Chad's University of N'Djamena, who was a member of Brunet's research group). This skull apparently belongs to a new hominid species known as Sahelanthropus tchadensis ("Sahel man of Chad") -- or Toumai ("Hope of Life") in Goran, the language of the people in the area where it was discovered. Toumai lived as long as 6 or 7 million ya, about 1 million years earlier than the earliest hominid previously known, and more than 3 million years before the famous Australopithecus known as "Lucy" (see below). Moreover, previous hominid skulls were found in the eastern and southern portions of Africa.

Before Toumai, the earliest known hominid,Orrorin tugenensis, dating from about 6 million ya, was discovered in Kenya.

The fact that the Toumai skull was found in western Africa suggests that our ape and hominid ancestors were more widely dispersed on the continent than previously believed. The Toumai skull mixes features of apes, such as a small brain-case, with features characteristics of hominids, such as a flat rather than protruding face. For this reason, it has been suggested that it may be an ancestor of both chimpanzees and hominids, living before the two lines diverged. Another theory is that it represents another primate branch that has simply gone extinct.

The "Georgia Skull".The other discovery, known as the "Georgia skull" because it was found in Georgia, a country that was formerly part of the Soviet Union, in an area near Tbilisi between the Black and Caspian Seas, is about 1.75 million years old and belongs to one or the other of three hominid species already known --Homo habilis,H. ergaster, or H. erectus. But it and similar skulls were found with stone tools, such as choppers and scrapers, that seem much more primitive than those typically associated with H. erectus, so some theorists , So, unlike the Toumai skull, it doesn't suggest any sort of "missing link". Rather, the surprise here is that the skull was found outside of Africa. Previously, it was believed that the Homo species evolved in eastern Africa, and that H. erectus moved out of Africa sometime more recently than 1.6 million ya. But here's a homo skull well outside Africa, and more than 1.6 million years old. So, the migration out of Africa might have begun earlier than we previously thought.

              FloresiensisHomo floresiensis. In October 2004, a team of Australian anthropologists discovered a "downsized" or "hobbit" version of homo erectus, with adults standing only 3-31/2 feet high, in a cave on Flores, an island in the Indonesian archipelago near Bali. Individual members of this species, named homo floresiensis, have been dated between 95,000 and 13,000 ya -- meaning that they overlapped with h. sapiens, which arrived in Australia about 40,000 ya. Stone tools also found have been attributed to h. floresiensis rather than later Neanderthals -- which is interesting, because these humans had brains hardly larger than those of adult chimpanzees. So, it's not brain size that's related to intelligence, as brain structure -- including, perhaps, the sulci and gyri that feature so prominently in human brains, and that permit humans to pack a large amount of cortex into a relatively small space. The precise placement of h. floresiensis in the hominid family tree remains a matter of controversy. In some respects, they appear more primitive than either h. erectus or h. sapiens -- yet they made and used tools. Some anthropologists speculated that they might be genetically deformed h. sapiens dwarfs. As of 2009, the consensus was that they were, indeed, a distinct species of hominids, which split off from the main hominid line 1-2 million ya, more closely related to h. habilis than to h. sapiens..

Ardi ReconstructionArdipithecus ramidus. In 2009, paleontologists from UC Berkeley and their colleagues from Ethiopia discovered a new fossil hominid,ardipithecus ramidus (nicknamed "Ardi") that is a million years older than Lucy, dating to about 4.4 million years before the present.Ardipithecus was discovered in the Awash River valley of Ethiopia, not very far from where Lucy had been found -- but in an area that was, at the time, forest rather than savanna. What's really interesting about Ardi is that it walked upright (if not particularly gracefully, with short legs and lacking an arched foot), but still had the capacity -- like long arms and opposing thumbs on the feet as well as the hands -- to move about easily in trees. It had previously been thought that bipedalism had emerged on the Savannah, as an adaptation useful for living in an area that had no trees. But Ardi suggests that bipedalism first emerged in an arboreal area. Apparently, by the time of Lucy, a million years later, bipedalism had advanced to the point that the new species,Australopithecus, could walk rapidly over long distances.

As of its discovery, Ardi was the oldest known hominid skeleton. It is different enough from chimpanzees to suggest that the common ancestor of chimpanzees and humans was about 6-7 million years old.

A-Sediba FossilsAustralopithecus sediba. In 2011, a group of anthropologists at the University of Witwatersrand, in South Africa, led by Lee Berger, announced the discovery of a new pre-human species, named A. sediba, which appeared to comprise a "patchwork" of ape and human features. There is a hand that is able to grip tools as well as hold on to tree branches, and a foot that can support upright walking as well as climbing trees. The pelvis was human-like, able to accommodate big-brained fetuses; but the skull itself was small -- suggesting that the pelvis evolved, not to accommodate big-brained fetuses, but rather as a byproduct of bipedalism. The researchers dated the skeleton to about 1.977 million ya, and suggested that it is now the oldest "missing link" -- not between ape and man, exactly, but between the most recent known Australopith species,A. africanus, and the earliest known human species,H. habilis. This conclusion is, of course, vigorously disputed by other anthropologists. Perhaps more important, at least so far as the theory of evolution is concerned, is what it suggests about the emergence of complex human characteristics, such as the hand. Rather than evolving microscopically (or, for that matter, rather than being created instantaneously out of whole cloth), complex features like the hand might have evolved in a sort of modular fashion. There might be several different configurations of thumb, wrist, finger length, etc., all occurring more or less randomly, until one particular combination -- short, opposing thumb, long fingers, and a particular twist of the wrist, perhaps -- happened to appear, proved to be particularly adaptive, and so was passed down to subsequent generations while other combinations, perhaps less adaptive, simply (and literally) died out. [For more on A. sediba, see "First of Our Kind" by Kate Wong,Scientific American, 04/2012.]

Lucy's Grandfather(?)  In 2019, a group of Ethiopian researchers published a reconstruction of a skull of A. anamensis, the direct predecessor to "Lucy".  Previously, anamensis was known only from teeth and jaws.  But in 2016, Ali Bereino, an Ethiopian goat herder who had a side interest in fossil hunting, uncovered a complete anamensis skull while preparing a shelter for his animals.  The fossil, known as MRD for Mira Dora, the place where it was found, has been dated to 3.8 million ya.  The current theory is that Lucy's species, A. afarensis, branched off from A. anamensis about 3.7 to 3 million ya.  The researchers also promptly hired Bereino.

The Dmanisi Skulls.  In an interesting twist on the "Georgia skull" described earlier, a set of skulls uncovered at Dmanisi, Georgia, and reported in 2013, cast doubts on the traditional classification of early hominids into separate species -- Homo erectus, Homo ergaster, Homo habilis, and Homo rudolfensis.  The traditional story, remember, is that only H. erectus flourished, eventually evolving into H. sapiens.  The distinctions among the various species are based largely on features of the skull.  The Dmanisi skulls, which include the "Georgia skull"" are all dated to roughly 1.8 million ya, but they show amazing variability -- as much as the variability found in skulls from Africa, which led to the naming of several different species.  The implication is that there may have been just one early species of Homo, namely erectus, characterized by a high degree of within-species variability with respect to skull morphology.  Rather than there having been several different Homo species, all competing, as it were, for survival, there was just one -- H. erectus, which eventually evolved into H. sapiens.  Another implication is that, rather than waiting, H. erectus began to move out of Africa almost as soon as it appeared on the East African savanna.

In 2015, Lee Berger, an American anthropologist teaching at the University of Witwatersrand, in South Africa, reported the discovery of hominid skeletons in the Rising Star Cave near Pretoriain South Africa (the bones were accidentally discovered by spelunkers; Berger was also the discoverer of A. sediba, so he's one lucky paleontologist!).  Analysis strongly suggests that these bones come from an entirely new hominin species, Homo naledi ("naledi" means "star" in a local indigenous language).  Based on the geology of the cave and naledi's primitive anatomy, Berger has suggested that the skeletons date from more than 2.5-2.8 million ya -- that is, close to the dawn of H. sapiens.  As such, naledi might be close to the "missing link" between australopiths and homo.  A photo of a substantial portion of a naledi skeleton was published in Scientific American, 08/2017 ("Our Cousin Neo", by Kate Wong).

Link to a NOVA/National Geographic documentary on the discovery of H. naledi, broadcast in September 2015.

All Aboard for Marrakech?  Excavation at a mining site near Marrakech, Morocco, uncovered human and animal bones and stone tools, originally dated to 40,000 BP and identified as Neanderthal remains.  But more recent investigation (Hublin et al., Nature, 2017) has reclassified the human remains as H. sapiens and dated the site to 350,000-280,000 BPE -- which is pretty surprising, given that standard theory dates the emergence of H sapiens to East Africa "only" 60,000 ya!  So if correct, the Morocco H. sapiens discovery would push back the origins of our particular species back by about 100,000 years.  They also indicate that some H. sapiens, instead of simply migrating out of East Africa, populated the entire African continent as well.  On the other hand, there are enough morphological differences between the Moroccan specimens and H. Sapiens that they might represent a distinct, yet to be named, H. antecessor.

From Fossils to DNA.  Most of the story of human evolution is told through fossils like Lucy or the Dmanisi Skulls, but it's now possible to answer some questions about our origins from analyses of DNA extracted from fossilized remains.  For example, Matthias Meyer, Juan-Luis Arsuag, and their colleagues (2013) sequenced the DNA from a 'Neanderthal" skeleton found at a site in Spain known as Sima de los Huesos, dated to 400,000 before the present era (BPE).  Surprisingly, they found that it closely resembled DNA extracted from a "Denisovan" skeleton found in Siberia, and dated to 80,000 BPE.  Based on the fossil evidence alone, it has long been assumed that Neanderthals and Deniisovans represented distinct species of Homo, which left Africa about 300,000 BPE.  The Neanderthals turned left into Europe, while the Denisovans turned right into Asia. Later, about 200,000 BPE, H. sapiens migrated out of Africa, and replaced both the Neanderthals and the Denisovans.  Possibly, the Spanish fossils represent yet another Homo subspecies, yet to be named.  Or, the Denisovans migrated to Europe as well as Siberia, and the two interbred.  Or something. 

A series of papers published in Nature in 2016 may have settled the matter.  Comparing samples of DNA collected from all over the world, including many populations of indigenous people, three separate teams of researchers concluded that all non-Africans are descended from a single group of H. sapiens that emigrated from Africa 50-80,000 ya.  This doesn't mean that there weren't earlier waves of migration, such as the Neanderthals and Denisovans.  It's just that the descendants of any earlier migrations didn't last.  So far as modern Europeans, Asians, Australian aborigines, and Native Americans, it seems that we're all descended from that last migration out of Africa,

Ian Tattersall, an anthropologist at the American Museum of Natural History, has traced recent changes in our understanding of human evolution with two excellent geneological trees -- the one on the left, drawn in 1993, and the one on the right, drawn in 2011 (from "Human Evolution in Perspective" by I. Tattersall, Natural History, 06/2015).  Note that h. naledi isn't there -- it's that new a discovery.

For an excellent survey of human evolution, see Masters of the Planet, and the Strange Case of the Rickety Cossack: and Other Cautionary Tales from Human Evolution, both by Ian Tattersall (2012).


Fossiles of the Human FamilyStill, as I noted earlier, the main outlines of the story of human evolution remain intact.Here's the story as of Fall 2009, as depicted in the Science paper announcing Ardi.

Fossil Hominids

For a comprehensive, detailed, up-to-date overview of human evolution, check out Fossil Hominids: The Evidence for Human Evolution, a website maintained by Jim Foley that features a timeline of recent fossil

As H. sapiens sapiens proliferated around the world, adaptation to different climatic zones produced the physical differences associated with the different "races" of humans (much as h. floresiensis developed small stature in an. For example, pale skin is an advantage to those living in cool, cloudy lands because it facilitates the absorption of ultraviolet radiation, promoting the manufacture of vitamin D, a substance necessary for healthy bone growth. For people who live in the tropics, skin darkened by melanin protects against peeling and blistering, cancers caused by constant exposure to these same ultraviolet rays. In the tropics, a tall, slim physique radiates surplus heat, and keeps the body cool. In the cold climates of the far north, a short, squat body with high fat levels serves an insulating function. In areas with unreliable or inadequate supplies of food, short stature is adaptive, as are fatty buttocks. Improved diets increase stature and shrink teeth, muscles, and bones.

Much ink (and blood) has been spilled over the biological reality of various racial distinctions. Perhaps the best perspective on this issue is presented in an essay, "Human Equality is a Contingent Fact of History", by Stephen Jay Gould, a paleontologist at Harvard, who contributed a regular column, "This View of Life", to Natural History magazine.  The essay, written while Gould was giving a series of lectures on racism in South Africa (whose black majority was then suffering under the white-imposed apartheid regime) was published in the November 1984, issue of the magazine, and contains Gould's reflection on the evolution of our notions of race and racial differences.  In his article, Gould argues that "Human equality is a contingent fact of history" (italics original).  By this he means that it could have happened that different, and unequal, races evolved through human history; it just didn't happen that way.  Initially, Western thought was thoroughly imbued with the idea that the different races had separate origins (polygeny), and that racial inequality was somehow a "natural" consequence of this separate creation.  But with the gradual acceptance of Darwin's ideas about The Origin of Species, suggesting evolutionary links between humans and other animals, Western views of race have themselves evolved.

In addition to debunking the geneological and geographical arguments for racial inequality, Gould also presented what he called "positive" arguments for the equality of the races.

More recent research by Marc Feldman and associates, based at Stanford University, suggests that there are, after all, small genetic differences among the races. Surveying the DNA sequences of 1000 people sampled from each of 52 populations, he found that DNA differences between the groups fell into five clusters, or groups, roughly corresponding to their continents of origin: Africa, Eurasia (including Europe, the Middle East, and South Asia), East Asia, Oceania (including Australia), and North and South America. These, of course, correspond to the five geographical "races" of folk taxonomy: Negroid, Indo-European, Mongolian, Pacific Islander, and (American) Indian. 

For more details of the genetic differences between the races, see A Troublesome Inheritance by Nicholas Wade (2014).  But pay attention only to the first half of the book, which sets out the genomic differences among the geographical races.  There are some, but you should also bear in mind that these racial differences are minuscule when compared to the entire human genome, with its 22,500-some genes.  As Richard Lewontin, a prominent population geneticist, has pointed out, the differences between racial or ethnic groups are far smaller than the similarities.  Put another way -- and this is a general rule: the differences between groups are smaller than those within groups.  In the second half, Wade tries to argue that racial differences in social outcomes -- such as the "achievement gap" between Asian-, European-, and African-Americans -- are due to these differences -- and here, his science is on very shaky ground indeed.  Earthquake-sized shaky.  Magnitude 8 shaky.  Not least because the differences in social outcomes within the races are far greater than the differences between them.  Moreover, Wade offers no evidence that any of the genetic differences between the races, small as they are (and they are tiny), have anything to do with the behavioral differences that he is concerned with (or much of anything else, except skin color and lactose tolerance). 

For a counterargument, that race is essentially a social construction, see Fatal Invention: How Science, Politics, and Big Business Re-Create Race in the 21st Century by Dorothy Roberts (2014).

A more balanced view is offered by Adam Rutherford, a science journalist with a PhD in genetics, in A Brief History of Everyone Who Ever Lived (2017).  Rutherford argues that genetic studies can tell us a lot about where we came from -- it turns out, for example, that the British people generally known as "Celts" (Welsh, Irish, Scots, etc.) are not genetically related: "Celt" really is a social construction!  Neanderthals had the FOXP2 gene that is crucial for speech.  And it's now been demonstrated convincingly that every person now living on Earth, no mater what their "race", is descended from a very small group of H. sapiens who lived just a few thousand ya.  Outward appearances to the contrary notwithstanding, our genomes don't divide us into the usual racial categories.

See also Black and White, a special issue of National Geographic (04/2018) devoted to the biological and sociocultural aspects of race.  The two girls on the cover are Marcia and Millie Biggs, fraternal twin daughters of Michael Biggs, a native of Jamaica, and Amanda Wanklin, an Englishwoman (you've seen them before, in my discussion of behavior genetics).  One of the articles in the special issue, "A Color Wheel of Humanity" by Nina Strochlic,  illustrates the amazing variety of human skin tones by selections from Humanae, a project by portrait photographer Angelica Dass, who matched the skin tones her subjects to the standard color palette maintained by Pantone (discussed in the lectures on Sensation).  

The important thing to remember is that despite superficial physical differences (and even more superficial differences in biochemistry), all "races" of H. sapiens sapiens, Black, White, Mongoloid, or whatever, reflect differences in the same species of animal. Moreover, there are major individual differences among members of any single "race", and there are people with superficially similar features -- Africans and Australian Aborigines, for example, who are not members of the same "race". The bottom line is that while "race" has some degree of biological reality, it is largely a mythical concept that is better discarded. All of us have a single common ancestor. In fact, there is a theory, based on analyses of mitochondrial DNA (which is passed only from female to female) that the entire race of modern humans -- black, white, and yellow -- are the descendants of a single female who lived in East Africa 200 thousand ya. This theory has been called into question, but the essential point remains: we are all very closely related to each other, so we might as well treat each other with respect and affection.

The best guess is that any two randomly selected individuals are more than 99% identical in their genes -- if not 99.9% identical, then maybe 99.5% identical. And remember, there is about 98% similarity between the human genome and that of chimpanzees, our closest primate relatives. So, there may be some minor genetic differences between people of different continental ancestries (African, Eurasian, and East Asian), which is a nice way of saying different "races", but "race" is still largely a social construct -- a way of classifying people that has no serious biological justification.

The Beginning (and Perhaps the End) of History

Since the Ice Age ended, about 10 thousand ya, H. sapiens sapiens has continued to thrive. During the New (Neolithic) Stone Age, hunters and foragers became farmers and herders who cultivated plants and domesticated animals.

  • Rye was first grown as a crop cereal in about 11,000 BCE, and wheat about 8700 BCE, both in the Near East;
  • Rice was cultivated in China around 7000 BCE;
  • Cattle were domesticated in Africa by 5900 BCE;
  • Maize (corn) was cultivated in Central America about 3500 BCE; and
  • Pearl millet was cultivated in sub-Saharan Africa about 2000 BCE.

Neolithic people built villages and towns that gave rise to cities, the development of economies that were not devoted solely to the production of food, and the rise of hierarchical social structures. Ceramics were developed to store food. Reliance on stone tools gave way to bronze and copper -- first by hammering metal, later by forging and casting it. The earliest wooden plows date from 6 thousand ya; the first wheels for transportation, 5.5 thousand ya; the first sailing ships, and the first writing, 5 thousand ya.

The Origins of Writing

Here's the story, according to "Visible Language: Inventions of Writing in the Ancient Middle East and Beyond", an exhibit at the Oriental Institute at the University of Chicago, 2010-2011 (from "Hunting for the Dawn of Writing, When Prehistory Became History" by Geraldine Fabrikant,New York Times, 10/20/2010).

  • A clay tablet written in "proto-cuneiform", a language of ancient Sumer in Mesopotamia, has been dated to 3200 BCE. The earliest Sumerian writing appears to have been confined to a sort of shipping receipt for trade goods. Narrative writing appears about 700 years later, around 2500 BCE, in the earliest copies of the Gilgamesh epic.
  • It was once thought that Egyptian writing, in the form of hieroglyphs, was influenced by Sumerian cuneiform, but it is now believed that Egyptian writing systems emerged independently. An Egyptian alphabetic script has been dated to 1800 BCE.
  • Examples of writing have been found in Chinese archeological sites, dating from 1200 BCE.
  • Mayan culture in Mesoamerica had a written script before 500 CE. In the New World, systems for writing are generally thought to have emerged first in Zapotec culture near present-day Oaxaca, Mexico, about 300 BC, and in Mayan culture in southern Mexico and Central America about 200 AD. However, new archeological findings, reported in 2002 by Mary E.D. Pohl and her colleagues, suggests that some form of symbolic writing, known as glyphs, may have been available to the Olmec civilization in what is now Tabasco, Mexico, as early as 650 BC. The issue is not settled among archeologists: what appear to be glyphs may really be pictures, and the artifacts may not be as old as their discoverers originally thought.

See also Writing: Theory and History of the Technology of Civilization (2009) by Barry B. Powell.

Not all of these developments occurred simultaneously in every geographical area; and in some areas, some developments did not occur at all.

When writing begins, where it begins, history begins too, and science and culture begin to develop and proliferate extremely rapidly. At this point, about 5000 ya, biological evolution essentially ends: in genetic terms, and speaking metaphorically, we are the same species as Adam and Eve (created, or so Bishop Usher calculated, on the night before October 23, 4004 BCE). The development of tools, clothes, medicine, and social structure mean that we are protected -- or, more correctly,we protect ourselves -- against the biological pressures that formerly killed those who were weak or stupid. The evolution of a new species requires biological isolation, and migration and inbreeding, within and between "racial" groups, effectively precludes that. It also requires a hostile environment, which insures the survival only of the fittest. So, biologically speaking, we are pretty much at the end of our line. Now the only threats to our existence come from ourselves -- overpopulation, ecological disaster, and nuclear holocaust.

To our knowledge, few other species have lasted even three million years before their inevitable extinction. But unlike nonhuman animals, we know what the threats to our existence are, we understand that they are largely of our own making, and we have the intelligence and technology to do something about them. We can save ourselves from extinction, but only if we think, and try. That's where human intelligence comes in. The ultimate gift of evolution, the human mind, has been and will remain the key to our survival as a species.

Actually, Though, It Ain't Over 'Till It's Over

It's common to think that the biological evolution of the human species has pretty much reached its end point. That's pretty much the point of view taken here: once people start changing the environment, the environment has less chance to change them through natural selection. And, more or less, that's also the point of view taken by evolutionary psychologists (see below), who argue that patterns of human thought and behavior that evolved in the late Pleistocene era have remained pretty much unchanged up to the present.

These are good arguments, but they're apparently not quite true. Obviously, there's still opportunity for natural selection to operate on the human genome. In fact, in 2006, Jonathan Pritchard and his colleagues identified a number of segments of the human genome that have been subject to change via natural selection as recently as the last 5-10,000 years -- roughly since the beginning of agriculture (for the details, see A Troublesome Inheritance [2014] by Nicholas Wade).

  • Some of these genes code for differences in skin color -- for example, between Europeans and Africans.
    • Asians apparently acquired their light skins earlier, and through a different genetic route.
  • Another gene facilitates the digestion of lactose -- a gene that was particularly useful to early European farmers who domesticated cattle and drank their milk (the mutation first occurred in what is now central Europe 7-12,000 ya.
  • Yet another genetic mutation protects against altitude sickness; it occurs in 90% of Tibetans but only 10% of Han Chinese, and may have appeared as recently as 3,000 ya.
  • A mutation in a single gene, known as EDAR, may have given rise to a host of physical traits characteristic of East Asians (Han Chinese, Japanese, Thais) and American Indians (who are descended from East Asians), including thicker hair, extra sweat glands, and distinctively shaped teeth.  This variant first appeared about 35,000 ya in central China.

In 2011, a study of parish records from the Canadian Isle aux Coudres, which date back to 1799, revealed a steep decline in the age at which women had their first child. Other data confirms a decline in age at first reproduction, and an increased age at menopause -- both of which have been plausibly attributed to natural selection (although, frankly, it seems to me that both changes could just as easily have occurred as a result of improvements in nutrition and other aspects of health). If the evolutionary biologists are right, evolutionary change in these traits has been both very recent and very rapid.

So, evolution continues at the genetic level, at the level of body morphology, mind, and behavior, if not the mind as well. The fact that human evolution continued even after we moved out of the EEA, and into other environments that made other demands on survival, has led some evolutionary psychologists to espouse a concept of "fast evolution" -- essentially, that evolution can proceed at a much faster pace than previously believed -- possibly in response to much more recent environmental changes. And to focus on the Holocene epoch rather than the Pleistocene -- i.e., the most recent 12,000 years, since the beginnings of agriculture.

OK, point well taken, but let's not go overboard.

  • In the first place, the humans who moved out of Africa weren't just selected by their environment.  Early protohumans originally came out of the trees in the African rain forest; when the rain forest began to disappear, they moved into the African savanna; and when the savanna proved inhospitable, some of them moved out of Africa entirely.  So early humans also selected their environment, and they were able to do so precisely because they had a general problem-solving capacity, not just a set of modules that evolved in the Pleistocene EEA lasting roughly 2 million years.
  • Even "fast" evolution isn't fast enough to help individual species members, or even their immediate offspring, adapt to environmental changes. We still need a general capacity for learning and problem-solving.
  • We're not at all dependent on these evolutionary-genetic changes. Han Chinese can live in Tibet (ask any Tibetan about this) -- they just have to get there slowly, and they may need occasional doses of oxygen. Japanese can live in milk-drinking Europe and America - -they just can't drink much milk or eat much cheese.
  • For all the claims about fast evolution (which isn't very fast), the changes it's produced are pretty trivial compared to the big achievements of evolution -- language, consciousness, and the like.
  • Moreover, cultural evolution can affect biological evolution. Achieving control of fire, which is the first big step toward civilization, enabled early humans to move out of the Tropical Zone in the first place (you can live naked year-round at sea in the Tropics, but once you get outside that zone, or find yourself in a higher elevation, you had better have some way to keep warm at night and in the winter). It also allowed them to cook their food. And cooking promoted the evolution of smaller mouths and teeth, and shorter intestines. The increase in caloric intake may even have promoted an increase in brain size.

How Did We Get to Be Human?

Evolution did not draw a straight line from early hominins to modern humans. At one point, we shared the planet with a number of near-relatives.

Note: On 11/19/2018, the New York Times commemorated 40 years of its "Science Times" section by looking at "11 Things We'd Really Like to Know -- And A Few We'd Rather Not Discuss".  One of the essays, by Carl Zimmer, traces progress in understanding human origins -- and our deepening understanding of our complex evolutionary history.

I spoke recently to a scientist who was writing up a summary of what we know about human evolution. He should have had a head start, having written a similar article five ya.

But when he looked at what he had written then, he realized that little of it was relevant. “I can’t use much of any of it,” he told me.

As a journalist, I can sympathize.

In recent years, scientists have offered a flood of insights into how we became human. Fairly often, the new evidence doesn’t square with what we thought we knew.

Instead, many of these findings demand that researchers ask new questions about the human past, and envision a more complex prehistory.

When Science Times debuted 40 ya, scientists knew far less about how our ancestors branched off from other apes and evolved into new species, known as hominins.

Back then, the oldest known hominin fossil was a diminutive, small-brained female unearthed in Ethiopia named Lucy. Her species, now known as Australopithecus afarensis, existed from about 3.85 million ya to about 2.95 million ya.

Lucy and her kin still had apelike features, like long arms and curved hands. They could walk on the ground, but inefficiently. Running was out of the question.

Hominin evolution appeared to have taken a relatively direct path from her to modern humans. The earliest known members of our genus, Homo, were taller and had long legs for walking and running, as well as much larger brains. Eventually, early Homo gave rise to our own exceptional species, Homo sapiens.

Now, it’s clear that Lucy’s species wasn’t the beginning of our evolution; it was a branch that sprouted midway along the trunk of our family tree. Researchers have found fossils of hominins dating back over six million years. Those vestiges — a leg bone here, a crushed skull there — hint at even more apelike ancestors.

But even the earliest known hominins were like us in one important regard. They appear to have been able to walk on the ground, at least for short distances.

Paleoanthropologists have uncovered a wealth of new fossils from all points on the spectrum of hominin evolution. Some clearly belonged to known species, such as Australopithecus afarensis. Some were so distinct that they deserved a new designation.

But others have fallen somewhere in between. Often they look like mosaics of other species, carrying remarkable combinations of traits. Some of these mosaics may have been the result of interbreeding between species.

But it may be, too, that hominins independently evolved many traits many times, along separate lines of evolution.

All this mixing and experimentation produced as many as 30 different sorts of hominins — that we know of. And one kind did not simply succeed another through history: For millions of years, several sorts of hominins coexisted.

Indeed, our own species shared this planet with near-relatives until just recently.

In 2017, researchers found the oldest known fossils of our species in Morocco, bones dating back about 300,000 years. At that time, Neanderthals also existed. They continued to live across Europe and Asia until 40,000 ya.

At that time, too, Homo erectus, one of the oldest members of our genus, still clung to existence in what is now Indonesia. The species did not go extinct until at least 143,000 ya.

Homo erectus and Neanderthals are hardly new to paleoanthropologists. Neanderthals came to light in 1851, and Homo erectus fossils were discovered in the 1890s. But still other hominins, recent research has shown, shared the planet with our own species.

In 2015, researchers unearthed 250,000-year-old fossils in a South African cave. Known as Homo naledi, this new species had a Lucy-sized brain, but it was also a complex structure in ways that resembled our own.

The wrist and other hand bones of Homo naledi were human-like, while its long, curved fingers seemed more like an ape’s.

While Homo naledi thrived in Africa, another mysterious species could be found on an island now called Flores, in Indonesia. Known as Homo floresiensis, these hominins stood only three feet high and had brains even smaller than that of Homo naledi.

The species may have arrived on Flores as early as 700,000 ya, and these hominins endured until at least 60,000 ya. Homo floresiensis appears to have made stone tools, perhaps to hunt and butcher the dwarf elephants that once lived on the island.

Paleoanthropologists today are no longer limited to just examining the size and shape of fossils. Over the past 20 years, geneticists have learned how to extract DNA from bones dating back tens of thousands of years.

In one remarkable discovery in Siberia, researchers examining a nondescript pinkie bone discovered the genome of a separate line of hominins, now known as Denisovans.

As it turns out, we have had the planet to ourselves only in the past 40,000 years — a small fraction of Homo sapiens’ existence. Perhaps we out-competed other species. Maybe they just had bad luck in evolution’s lottery.

But in one way, we are still living with them. Both Neanderthals and Denisovans interbred with our ancestors some 60,000 ya, and billions of people today carry their DNA. Still mosaics, after all this time.

Evolutionary Psychology

Evolution doesn't just leave its mark on body morphology, giving fish scales and birds feathers, and humans opposable thumbs. It also leaves its mark on behavior, as seen in the "instincts", or fixed action patterns, discussed in the lectures on learning. In the 1970s, the evolutionary biologist E.O. Wilson coined the term sociobiology to represent the idea that patterns of social behavior evolved under the pressure of natural selection, just as physical traits did. In other words, a number of human social behaviors are instinctive, part of our innate behavioral endowment. We've seen instincts before, in the context of innate stimulus-response connections (remember reflex, taxis, instinct?). In the last chapter of his book, Wilson argued that instinctual social behavior might not be restricted to nonhuman animals like ants (the species Wilson studied) or the species studies by ethologists like Tinbergen, Lorenz, and von Frisch, but might extend to humans as well.

Taking a leaf from Wilson's book, some psychologists -- led by Leda Cosmides, John Tooby, and David Buss, among others -- have argued that mental traits evolved in the same way: that human beings, no less than other animals, have evolved specific patterns of thought, feeling, and desire through natural selection (in fact, Buss's first book was entitled The Evolution of Desire). The reason some of our behaviors and thought processes seem maladaptive, or at least inappropriate, in today's world is that they evolved to foster adaptation to a particular environment, known as the environment of evolutionary adaptedness (also known as the "environment of early adaptation", in either case abbreviated EEA) -- roughly the African savanna of the Pleistocene epoch (modern Ethiopia, Kenya, and Tanzania), where homo sapiens first emerged about 300,000 ya -- and have changed little since then.

Although these assertions are debatable, to say the least, the literature on instincts makes it clear that evolution shapes behavior as well as body morphology. Many species possess innate behavior patterns that were shaped by evolution, permitting them to adapt to a particular environmental niche. Given the basic principle of the continuity of species, it is a mistake to think that humans are entirely immune from such influences -- although humans have other characteristics that largely free us from evolutionary constraints.Since the emergence of humans, the cultural environment has changed a great deal, but there has not been enough time for biological evolution to produce new, more adaptive traits.

Certainly there are good reasons for believing that the uniquely human capacity for language is a product of evolution. So, arguably, are the kinds of mechanisms envisioned by Gibson's idea of direct perception. But there are reasons for thinking that the theory of biological evolution is not the answer to psychology's problems. This is because there are at lest four characteristics of mind that appear to distinguish humans from all other animals.  As outlined by Marc Hauser (himself a distinguished evolutionary psychologist) in "Origin of the Mind" (Scientific American, 09/2009), these are:

  1. Generative computation: through recursive and combinatorial thinking, humans are able to "create a virtually limitless variety of words, concepts, and things".  Hauser, Noam Chomsky, and W. Tecumseh Fitch have argued that recursion is the key to human linguistic ability (Science, 2002).  See also "The Uniqueness of Human Recursive Thinking", by Michael C. Corballis, American Scientist, 05-06/2007).
  2. Promiscuous Combination of Ideas: intermingling knowledge across different domains "thereby generating new laws, social relationships and technologies".
  3. Mental Symbols: representing both real and imagined experiences, which can be expressed to others through language.
  4. Abstract Thought: the ability to deal with objects and events that we cannot physically sense.

Evolutionary psychologists who study mating frequently place a great deal of stress on a particular dimorphism in which older males prefer younger females. This pattern makes evolutionary sense, given that young females (at least, young women, as opposed to young girls) have greater childbearing capacity than older ones (at least, women who have reached menopause). In turn, evolution is evoked to explain a wide variety of mating phenomena, from President Bill Clinton's entanglement with Monica Lewinsky to the common practice of rich men abandoning their first wives (or their second, or their third...) to take younger, ostensibly more attractive "trophy wives" (a term coined by Fortune magazine in the 1980s).


  • It should be remembered that Clinton and Lewinsky did not actually engage in sexual intercourse, but rather stuck with oral sex (and if you believe the stories, Clinton also preferred oral sex with his other extramarital partners). Therefore, whatever the causes of Clinton's behavior, it appears to have had nothing to do with any desire on his part to propagate his genes by impregnating multiple, youthful, partners.
  • While it's certainly a trend for older, successful men to divorce and marry trophy wives (someone once referred to it as "changing a 40 for two 20s"), it is not at all clear that they go on to have children by these women -- unless, perhaps, the women make it a condition of hooking up with them in the first place. In fact, an article in the New York Times notes the increasing trend to write a prohibition on children into prenuptial agreements -- although it is not clear that such provisos will hold up if challenged in court ("A Promise to Love, Honor, and Bear No Children" by Jill Brooke, 10/13/02). Some young women may make children a condition for marrying an older man, or may desire children of their own to inflate their claims for support in the event of divorce -- or just because they want to bear children; but that doesn't mean that the older man is motivated, in taking a new, younger wife, by the desire on his part to further propagate his genes.

Similar problems attached to other evolutionary explanations of mating behavior, which are commonly explained by some variant of Robert Trivers's (1972) parental investment theory.  According to the theory, men evolved to produce lots of offspring, but women evolved to be more selective about whom they'll mate with, because they have to invest more in caring for their offspring than men do.  Therefore:

  • Men are more distressed by actual infidelity; women are more distressed by "emotional" infidelity (e.g., Buss et al., 1992).
  • Men are less selective than women about whom they'll mate with. 
  • Men like causal (i.e., non-reproductive or at least non-committed) sex more than women do.
  • Over a lifetime, men have more sexual partners than women.

It all sounds good, especially when you consider the conditions of child-rearing in hunter-gatherer societies.  On the other hand, there is dispute about the facts to be explained.

  • For example, the claimed gender difference in sexual jealousy is almost wholly an artifact of method.  The original studies forced subjects to choose which would cause them more distress.  But DeSteno et al. (2002) offered an alternative explanation in terms of their double-shot hypothesis.  In their view, men assume that their unfaithful mates are also in love with their adulterous partners, while women assume that their unfaithful mates were just in the affair for sex.  So, men are wounded twice, women only once.  Further, Harris (2003) showed that when men and women are asked to rate their distress on continuous scales, the difference is trivial.  So, even the "double shot" doesn't make much of a difference in jealousy (for a review of the voluminous literature on this topic, see Carpenter, Psychology of Women Quarterly, 2012, and a response by Bendixen et al., Personality & Individual Differences, 2015).
  • Many studies studies ask subjects how many sexual partners they'd like to have, and they generally find that men want more partners than women do.  But Alexander and Fisher (2003) asked subjects how many sexual partners they had actually had.  When they just asked the question, men reported more partners than women.  But when they used a fake lie-detector (what's known in social psychology as a "bogus pipeline" technique, women actually reported having had more partners than men (though the difference was not statistically significant.
  • Studies of "speed dating" show that men don't discriminate very much among the women they meet -- they're attracted to all (or, at least, most) of them, while the women are more choosy.  But in the conventional speed-dating situation, men rotate from table to table, while women sit.  Finkel and Eastwick (2009) reversed the procedure, and found that the behavior reversed as well. 
  • In an amazing study, Clark and Hatfield (1989) had confederates approach male and female college students (previously strangers to them) on campus, and ask them one of three questions: Would you go out with me tonight?  Would you come over to my apartment tonight?  Would you go to bed with me tonight?  Men and women agreed to the date in about equal numbers, but much less likely to assent to either of the other propositions (in case you're wondering, in the first study of this type, 70% of the men said "yes" to the third question, while none of the women did).  But Conley (2011) criticized the methodology of this paper: When was the last time a complete stranger walked up to you on campus and offered sex?  In her study, she asked people to imagine this scenario, and found that when you added context, the gender difference virtually disappeared.  So, there's nothing innately automatic about either masculine promiscuity or feminine chastity.
On the distaff side, a prominent claim of evolutionary psychology is that women prefer more "masculine" faces when they are ovulating, and thus more likely to become pregnant as a result of sexual intercourse (Penton-Voak et al., Nature, 1999).  Again, this makes evolutionary sense.  According to the ovulatory shift hypothesis, women are hard-wired by evolution prefer to become pregnant by "masculine" men, but prefer to share child-rearing responsibilities with men who are more "feminine", more cooperative, more care-giving.  Put another way: they want to conceive children with more "masculine" men who presumably have better genes, but they want to marry more "feminine" men who make for better long-term partners.  And again, the question is whether it's actually true.  A fairly large literature has seemed to confirm this finding, but there are enough methodological problems in these studies to cast in in doubt (Harris, Sex Roles, 2012).  A fairly definitive study by Jones et al. (Psychological Science, 2018) suggests it's not.  These investigators used computer morphing software to emphasize or de-emphasize the "masculine" features of men by sharpening or rounding features features in their facial photographs.  Then they tested women's preference for the more- or less-masculine faces at different phases of their menstrual cycle, as confirmed by hormonal assays (as opposed to self-reports, which are, amazingly unreliable -- and one of the reasons that the "rhythm method" is not a reliable method of birth control).  In a within-subject design, in which the same 584 heterosexual women were tested at different points in their cycles, Jones et al. found no relationship -- not even when they took into account the potential effects of their subjects' use of hormonal contraceptives.

For comprehensive analyses of gender differences in mating and other behavior, see:

  • Peterson, J.L.,  Hyde, J.S., "A meta-analytic review of research on gender differences in sexuality, 1993-2007" (2010), which concludes that "most gender differences in sexual attitudes and behaviors are small", especially in "nations and ethnic groups with greater gender equity", they also note that "Gender differences decreased with age".
  • Conley, T.D., et al. "Women, Men, and the Bedroom: Methodological and Conceptual Insights that Narrow, Reframe, and Eliminate Gender Differences in Sexuality" (2011), whose subtitle says pretty much all there is to say.
  • Carpenter, C.J., "Meta-Analyses of Sex Differences in Responses to Sexual Versus Emotional Infidelity:Men and Women Are More Similar than Different" (2012).
  • Guildersleeve,, "Do women’s mate preferences change across the ovulatory cycle? A meta-analytic review" (Psychological Bulletin, 2014).
  • Wood et al., "Meta-analysis of menstrual cycle effects on women’s mate preferences" (Emotion Review, 2014).

So far, evolutionary psychology has gone through three stages:

  • The first found evolutionary roots, and thus evolutionary explanations ( in terms of adaptiveness), for all sorts of nasty human behaviors, like war, polygamy, rape, and child murder. It's exemplified by books such as Desmond Morris's The Naked Ape and A Natural History of Rape by Randy Thornhill and Craig T. Palmer.
  • The more recent stage has found evolutionary roots, and thus evolutionary explanations, for all sorts of good things that people do, such as religion and our sense of morality. It's exemplified by books such as Marc Hauser's Moral Minds: How Nature Designed Our Universal Sense of Right and Wrong, Dacher Keltner's Born to be Good, and Frans de Waal's The Age of Empathy: Nature's Lessons for a Kinder Society.
  • There are also claims that certain aspects of cognition, like the alleged confirmatory bias in hypothesis-testing, also represent cognitive styles that evolved because they served us well in the EEA.

Evolutionary psychology tends toward the reductive -- that is, it seeks an explanation for everything about mind and behavior in terms of its role in enhancing reproductive fitness. You can see this especially in the debates over homosexuality and altruism.

  • Why is it adaptive to have homosexuals, people who won't reproduce? Because they serve as caretakers for other people's children, particularly those of other family members, thus enhancing the reproductive fitness of their relatives -- and thus, indirectly, passing on their genes.
  • Why is it adaptive to have altruists, people who risk their lives to save others? Because they do this mostly for family members -- thus, indirectly passing on their genes.

These folks have even offered an evolutionary explanation for grandmothers. Why should humans have females who, almost alone in the animal kingdom, continue to live beyond their childbearing years? Because they remain available to take care of their children's children -- thus, indirectly, enhancing the reproductive fitness of their children and grandchildren, and, indirectly, passing on their genes. Honestly, I swear this is true, you can't make this stuff up.

I've argued that biological evolution is outpaced by cultural evolution, mediated by social learning, but some evolutionary psychologists have gone so far as to claim that learning itself is a biological adaptation -- that learning itself is a product of natural selection.  In this way, they try to have it all.  But they can't, for the simple reason that, by claiming learning for biological evolution, they've distorted the meaning of both evolution and learning. Yes, there's a sense in which learning occurs via something that looks like natural selection: according to Skinner, for example, behaviors that are reinforced are maintained, while those that are not reinforced disappear.  But this is nothing more than an analogy.  Darwinian theory, and the evolutionary psychology that is based on it, assumes genetic variation, which is operated on by the environment.  But learning doesn't depend on genetic variation.  It depends on cognitive variation -- that is, variation in the thoughts, and behaviors, in the individual organism.  This is not within the scope of the Darwinian paradigm.  Learning is, in fact, more Lamarckian, as it involves the inheritance of acquired characteristics -- but not through genetic inheritance.  The inheritance that occurs in learning is cognitive and cultural inheritance, passed through social learning, not through genetic mechanisms.

Steven Pinker has written (2008): "To understand human nature, first understand the conditions that prevailed during most of human evolution, before the appearance of agriculture, cities, and government". Perhaps. Then again, it was by virtue of human nature that we invented agriculture, cities, and government in the first place. Human nature is not restricted to biological givens: it also extends to sociocultural constructions.

Top 10 Questions to Ask

Your Local Evolutionary Psychologist

(with Apologies to David Letterman)

I know this is more than 10, but you get the idea:
  1. Do you eat meat? If so, do you eat it raw? Do you confine your diet to uncooked foods? If not, why not?
  2. Does your wife have a 5:7 waist-to-hip ratio? How big are her breasts? And are they symmetrical? If not, why are you still with her?
  3. Has she reached menopause? If yes, why haven't you divorced her?
  4. If your wife has not yet reached menopause, do you practice any form of birth control? If so, why?
  5. Other than your wife, how many attractive younger women of childbearing age are you currently having sex with? Does she mind that you're sleeping with all these other young women?
  6. When having sex with these individuals, do you practice birth control? If yes, why?
  7. If you have divorced and remarried, have you killed your new wife's children from her earlier marriage -- or at least kicked them out of the house with no financial support? Has your ex-wife's new husband done the same to the children you had by her? If not, why not?
  8. Do you ever have sex when your partner isn't ovulating? If yes, why?
  9. Do you engage in any form of sexual activity other than vaginal intercourse? If yes, why?
  10. Do you always mount your sexual partners from behind, while they're standing up? If not, why not?

A more scholarly critique of evolutionary psychology has been provided by David J. Buller, a philosopher, who lists "Four Fallacies of Pop Evolutionary Psychology" (Scientific American, 01/2009) -- where "pop evolutionary psychology" (PopEP) "refers to a branch of theoretical psychology that employs evolutionary principles to support claims about human nature for popular consumption".  Here's his list of fallacies:

  1. "Analysis of Pleistocene Adaptive Problems Yields Clues to the Mind's Design".  PoPEP-ists generally trace the mind's design to the problem of mate selection, but Buller points out that the paleontological record "is largely silent regarding the social interactions that would have been of principal importance in human psychological evolution".
  2. "We Know, or Can Discover, Why distinctively Human Traits Evolved": The comparative method on which evolutionary biology relies involves studying species who share a common ancestor, but which developed different adaptations to deal with different environments -- as in the case of Darwin's finches.  That works well for finches, who come in great variety, but it does work for humans, who diverged from our closest relative, the chimpanzee, about 6 million ya.  the relatives that would allow the comparative method to work, like australopiths and other hominins, just aren't around for us to compare ourselves to.
  3. "Our Modern Skulls House a Stone Age Mind".  Some human traits, like our basic emotions, arose long before the Pleistocene era; and environmental change since the Pleistocene has arguably altered human thought patterns as well.
  4. "The Psychological Data Provide clear Evidence for Pop EP".  Here Buller criticizes PopEP-ists' failure to consider alternative explanations.  The fact that some human mental or behavioral characteristic fits an evolutionary explanation doesn't matter if there is an alternative explanation that fits better.  Consider, for example, the PopEP claim men are more upset by sexual infidelity, while women are more upset by emotional infidelity.  First, this isn't exactly true.  This finding emerges only from forced-choice questionnaires that require subjects to choose which kind of infidelity upsets them more.  When subjects are asked to rate how much they would be upset by each kind of infidelity, men and women come out about even.  Moreover, any such difference may have nothing to do with the environmental pressures on Stone age hunter-gatherers.  The same sex difference could reflect men's belief that sexual infidelity in women is generally accompanied by emotional infidelity, and women's belief that male sexual infidelity is generally not accompanied by emotional infidelity. 

Evolutionary psychologists have been extremely creative in conjuring up plausible accounts of how this or that characteristic of human mental life is adaptive -- or was adaptive in the EEA. But are these anything more than "Just So" stories, a la Rudyard Kipling? To quote Richard Lewontin again ("Not So Natural Selection",New York Review of Books, 05/27/2010):

"The success of evolutionary biology as an explanatory scheme for its proper subject matter has led, in more recent times, to an attempt to transfer that scheme to a variety of other intellectual fields that cry out for systematic explanatory structure....

"One answer has been to transfer the formal elements of variation and natural selection to other aspects of human activity.... We have evolutionary schemes for history, psychology, culture, economics, political structures, and languages. the result has been that the telling of a plausible evolutionary story without any possibility of critical and empirical verification has become an accepted mode of intellectual work even in natural science....

"Even biologists who have made fundamental contributions to our understanding of what the actual genetic changes are in the evolution of species cannot resist the temptation to defend evolution against its known-nothing enemies by appealing to the fact that biologists are always able to provide plausible scenarios for evolution by natural selection. But plausibility is not science. True and sufficient explanations of particular examples of evolution are extremely hard to arrive at because we do not have world enough and time. The cytogeneticist Jakov Krivshenko used to dismiss merely plausible explanations, in a strong Russian accent that lent it greater derisive force, as 'idel specoolations'.

"Even at the expense of having to say 'I don't know how it evolved', most of the time biologists should not engage in idle speculations."

 For another critical view of evolutionary psychology, see "It Ain't necessarily So" by Robert Gottlieb, New Yorker, 09/17/2012.  Here's an extract:

There are plenty of factions in this newish science of the mind.  The most influential... focuses on the challenges our ancestors faced when they were hunter-gatherers on the African savanna in the Pleistocene era..., and it has a snappy slogan: "Our modern skulls house a Stone Age mind."  This mind is regarded as a set of software modules that were written by natural selection and now constitute a universal human nature.  We are, in short, all running apps from Fred Flintstone's not-very-smartphone.  Work out what those apps are -- so the theory goes -- and you will see what the mind was designed to do.

Evolutionary Change and Cultural Change: The Case of Violence

Some inkling of the comparative speed of biological and cultural evolution, with respect to human experience, thought, and action, is afforded by an analysis of historical changes in violence by Steven Pinker -- himself a prominent proponent of evolutionary psychology. In The Better Angels of Our Nature: Why Violence Has Declined (2011), Pinker argues from archival data that the rate of violence among humans has declined radically from the Stone Age until now. For example, criminal records from the 14th century indicate that London's homicide rate was about 55 deaths per 100,000 population, compared to 2 per 100,000 today. And London is far from a unique case. Outside of the United States, for example, capital punishment has virtually vanished from the western world -- and, despite the large number of inmates on death row in states like Texas and California, actual executions are quite rare (OK, maybe not in Texas). Despite the horrors of World Wars I and II, and headline-grabbing terrorist bombings, campus shootings, domestic violence, and gang killings in our inner cities, Pinker asserts that "The decline of violence may be the most significant and least appreciated development in the history of our species".

Pinker's data has been controversial -- the two World Wars were pretty bad; there are persistently high levels of violence in Africa, Asia, and South America; and the homicide rate in American cities in 21st-century Detroit and New Orleans (though not New York City) rivals that of 14th-century London. But if he's right, this radical and rapid decline in violence cannot be accounted for by evolutionary change. After all, as noted earlier, when it comes to mind and behavior there's been no change in the human genome since the time of Adam and Eve. So it has to be a product of cultural change.

Actually, it doesn't have to be a product of cultural change alone. Pinker notes that, in addition to innate tendencies toward competition and violence -- our inner demons -- we also have countervailing innate tendencies toward cooperation and empathy -- our better angels (the phrase comes from Abraham Lincoln's first inaugural address). But Pinker is an evolutionary psychologist, and if our better angels were innately stronger than our inner demons there wouldn't ever have been high levels of violence, and there would have been no historical decline, either -- because there would have been no high level of violence to decline from. So, even from an evolutionary-psychological point of view, the key to the decline in violence has to lie in cultural change. Compared to 50,000 (or 5,000, or even 500) ya, the cultural environment has changed to favor our innate goodness rather than our innate badness. Following the sociologist Norbert Elias, Pinker calls this "the civilizing process".

So what are the elements of the civilizing process. Elias thought that it consisted of the elevation of state power over feudal loyalty, and also the development of commerce. Pinker's view is more monolithic. In addition to four innate better angels (like cooperation and empathy), and five innate inner demons (like competition and violence), he identifies six trends and five historical forces that have fostered the decline of violence. For example:

  • The emergence of some some form of centralized ruling authority, culminating in modern state power, put an end to the "war of all against all".
    • In Leviathan (1651), the British political philosopher Thomas Hobbes pointed out that in the absence of a state, life is "nasty, brutish, and short".
  • The rise of cities demanded that people adhere to stricter codes of conduct -- which, once internalized, literally changed individuals' psychology.
  • The spread of literacy expanded the "circle of empathy".
  • The rights movements of the 20th century -- the expansion of male suffrage, women's rights, civil rights, gay rights, even animal rights -- led much violent behavior -- beating your wife or your dog -- into antisocial behavior.
  • Trade transformed potential enemies into paying customers.
  • Democracy, and especially the concept of minority rights, incorporated the peaceful resolution of conflict through compromise.
  • Individual thinking generalized into collective rationality, and the recognition of others as rational agents who deserved to be treated the way we would wish ourselves to be treated (i.e., the Golden Rule).

Ever the evolutionary psychologist, Pinker asserts that this changing environment selected for our better angels over our inner demons. But he neglects the simple fact that this changing environment was, itself, the product of human cognitive activity -- the collective rationality through which we built a world that would reinforce these innate tendencies. Evolutionary psychology has to embrace a Darwinian notion of the organism as, essentially, passive in the fact of the environment -- traits evolve precisely because they were selected by an environment that changed autonomously. The dinosaurs didn't make the asteroid whose collision with Earth rendered them extinct, and gave mammals a chance. But we made the cultural environment that increased the power of our better angels over our inner demons.

This is the Doctrine of Interaction on a large scale: We are not creatures of our environments. We make the environments in which we live.

The Cultural View of Development

There is actually a third view of development, the cultural point of view, which is concerned with the effect of social development on the development of the individual's mind. In general, psychology has tended to ignore sociocultural differences in mental life. Psychology is universalistic, in that it assumes that the basic principles of mental functioning are found in all normal adult humans; and it is particularistic, in that it assumes that the course of individual lives reflect individual differences in knowledge and skills. But cognitive anthropology and sociology take on the task of understanding how social and cultural processes affect what we now and how we know it. Cognitive anthropology (also known as anthropological psychology or cultural psychology) arose in the late 19th century, with an interest in characteristic patterns of thought associated with people in so-called "primitive" and "advanced cultures. In the context of late 19th-century European imperialism (and its American counterpart in the drive westward towards "Manifest Destiny"), cognitive anthropology essentially studied the differences between conquerors (e.g., British, American) and the conquered (e.g., Arabs, Africans, and Native Americans). But setting politics aside, there are other aspects of cultural development that might also affect individuals' mental processes:

  • literacy, or the proliferation of written language (not to mention, more recently, the proliferation of electronic media such as radio, television, and the Internet);
  • economic development, as in the progression of societies from hunter-gatherer through agricultural and industrial to "post-industrial" forms of social organization (this definition of development was especially pursued by psychologists in the Soviet Union, such as Lev Vygotsky, but it has since outgrown its Marxist overtones).
  • modernization, typically defined in terms of two dimensions: traditional beliefs versus secular rationalism; and a concern with survival and physical security versus a concern with self-expression (Inglehart & Baker, American Sociological Review, 2000).

The Three- (Maybe Four-) Age System

In cultural terms,H. (sapiens) sapiens are "Stone Age" humans:

  • they made needles, handles, fishhooks;
  • they hunted with harpoons;
  • they used mechanical devices to throw spears.
  • Perhaps as long as 77,000 ya, they left pots in Blombos Cave, in what is now South Africa, that display abstract, symmetrical, geometric designs that may represent the earliest known art.
  • About 40,000 ya, they left the wall-art in the caves at Altamira (Spain) and Lascaux (France).

The term "Stone Age" refers to a "three-age" system for organizing human prehistory, based on the use of tools, introduced by C. J. Thomsen, a Danish archeologist, in the early 19th century. The three canonical ages are the Stone Age, Bronze Age, and Iron Age. The Stone Age is further subdivided into early, middle, and late periods. Sometimes a fourth age, the Copper Age, is interpolated between the Stone Age and the Bronze Age (image from "Complex Behavior Arose at Dawn of Humans" by Ann Gibbons, Science 03/16/2018).

As their names imply, the prehistoric ages are determined largely by the kinds of tools in use, but in fact the ages also carry broad implications for social organization.

  • The Stone Age. These humans were hunter-gatherers, living a mobile lifestyle close by sources of water. They made tools by hand from sharpened stones, bones, reeds, branches, and other objects found in nature.
    • Paleolithic (Early Stone Age) peoples lived in small bands of up to 100 people.
    • Mesolithic (Epipaleolithic, or Middle Stone Age) peoples divided into tribes and bands as their population increased.
    • Neolithic (Late Stone Age) peoples domesticated animals and otherwise began the transition to agriculture on stable farmsteads, and a hierarchical social organization based on the tribal chief.
  • The Bronze Age.These humans fashioned tools from copper and then bronze alloys, and used a potter's wheel (as opposed to their hands) to make pottery. Bronze age agriculture involved the deliberate breeding of livestock, as well as the beginnings of trade.
  • The Iron Age. These humans used -- well, duh! -- iron. Social development included the emergence of cities and city-states. 
    • One of the earliest of these cities is Catalhoyuk, on the Konya Plain in central Turkey, near the present-day city of Antalya, established in roughly 7,000 BCE.  For an overview of this Neolithic archeological dig, see "The Origin of Home" by Annalee Newitz, Scientific American, 03/2021.  See also "Women and Men at Catalhoyuk" by Ian Hodder, Scientific American 01/2004.

The three-age system was developed to organize our understanding of European history, though a similar progression, with some glitches, can be found outside Europe as well. Although we know little about the mental lives of prehistoric people, the general thrust of evolutionary psychology is that the heuristics, errors, and biases that litter modern thought processes are, in fact, evolutionary holdovers from prehistoric times. In other words, these patterns of thought evolved precisely because they aided survival in the EEA.

Paleolithic Cave Art

The meaning of paleolithic cave art remains a mystery. The most common interpretation is that it has a spiritual or supernatural nature. Other authorities suggest that much of it, especially the paintings of genitalia and other sexual anatomy, is the work of adolescent boys with too much time on their hands.

Two recent books that review the controversy are:

  • The Cave Painters: Probing the Mysteries of the World's First Artists by Gregory Curtis (Knopf, 2006), which tends toward the conventional view;
  • The Nature of Paleolithic Art by R. Dale Guthrie (Chicago, 2006), which argues for the revisionist view.
For succinct coverage of the controversy, see "Secrets of the Cave Paintings" by William H. McNeill (New York Review of Books, October 19, 2006).

One thing is for certain: at least by 40,000 ya, something similar to the modern mind had emerged.  This is indicated not just by the "representational" cave paintings such as found at Altamira, Lascaux, and other sites, but also by works of imagination.  A prime example is "The Lion Man", uncovered in southwest Germany and now in the British Museum, in which a mammoth tusk has been carved with a lion's head and a human body.  The Altamira painters might have seen the bison, horses, and deer that they painted on the cave walls; but they never saw anything like a man with a lion's head.  They had to have imagined it.  At the time of its discovery, the "Lion-Man" was the earliest work of the imagination that has been preserved.

Meanwhile, new archeological techniques have pushed the dates for the earliest cave art somewhat backwards in time. Employing a "radium-thorium" dating technique that is, by virtue of the chemistry involved, more reliable and precise than the traditional radiocarbon technique for the dating of older antiquities, a group led by Alistair Pike has dated some cave art in El Castillo, in Spain to 40,800 ya, and the cave art at Altamira to at least 35,600 ya.  Bone flutes were discovered in a cave in Germany have been dated to as early as 43 thousand ya; and figurative sculptures discovered in Germany have been dated to as old as 40,000 ya.

The conventional wisdom is that these discoveries indicating that the first flowering of human creativity -- the emergence, if you will, of the characteristically human mind -- occurred about 40,000 ya, and took of from there (and then).

And, importantly, not just in Europe.  In 2018, a team of archeologists discovered a set of cave paintings in Borneo -- hand prints and depictions a kind of cattle, among other things --  that are more than 40,000 years old, as determined by an alternative form of dating based on the decay of uranium into thorium.  The image to the left, part of a panel that is about 5 yards wide, depicts a local buffalo being hunted by creatures who share some human and some animal features (much like the "Lion-Man" above).  The point isn't that imaginative, narrative art emerged in Borneo before it emerged in Europe (though that may have been the case, as some of the images may be as old as 52,000 years).  The point is that art emerged at roughly the same time on opposite sides of the globe (for more detail, see "The First Story" by Kate Wong, Scientific American, 03/2020).

On the other hand, recent discoveries from Africa appear to push back the date for the emergence of human creative thinking.  European cave art may go back about 41,000 years, but there is other evidence of human technological and artistic creativity dating from much further back in time -- in Africa.  Flaked-stone tools have been uncovered in Ethiopia dating back 2.6 million years, and at Turkana, in Kenya, 1.76 million ya, and in South Africa to 500 thousand ya; there is evidence that cave dwellers in South Africa controlled fire 1 million ya.  And a block of red ocher marked with completely non-functional parallel lines and cross-hatchings, dating to 75,000-65,000 MP was discovered in the Blombos Cave in South Africa in 2000.

Admittedly, the evidence from before 40,000 ya is scattered and spotty, but it's there.  The revisionist interpretation is that technological and artistic creativity first emerged in Africa more than a million ya, but didn't really begin to bloom until much more recently, when human population density reached a critical point -- which happened to occur in Europe, not Africa (see "the Origins of Creativity" by Heather Pringle, Scientific American, March 2013). 

Stay tuned for further revision!

For an illustrated timeline of paleolithic art, see "First Artists" by Chip Walter, National Geographic, January 2015. 

Epochs in Human History

Just as it seems likely that human mental life changed with progress from the Stone Age to the Bronze age, it is also a reasonable hypothesis that patterns of thought continued to change with further economics, political, and social development.

Some of the milestones of historical development have already been listed -- literacy, industrial capitalism, and modernism.

Here are some other possibilities:

  • The Ancient Era (roughly up to the sack of Rome in the 4th century CE
  • The Middle Ages (4th-15th c.)
  • Early Modern Period (14th-18th c.)
  • Modern Era (18th-20th c.)
  • Post-Modern Era (since World War II)

Again, these epochs were developed with reference to Europe, but analogs can be found outside of European culture. It is entirely possible that, in significant ways, people who lived in these eras thought differently than we do.

The cultural view of development, which holds that societies and cultures develop much like species evolve and individuals grow, has long been popular in other social sciences, such as economics and political science.  To a great extent, the origins of the cultural view of development can be traced to the writings of Karl Marx.  Originally, Marx argued that all societies went through four stages of economic development.  Later, working with Friedrich Engels, he added two other stages:

  1. tribal,
  2. ancient,
  3. feudal,
  4. bourgeois [capitalist],
  5. socialism, and
  6. communism.

But long before Marx, the 18th century philosopher Giambattista Vico (1688-1744) argued that history proceeded in repeating cycles of three stages (actually, the pattern is more like a spiral, because history does not repeat itself exactly, although the general theme does):

  1. The age of gods, characterized by the emergence of a "family state" governed by a patriarch who holds absolute power (in recent times, think of the early Christian Church).
  2. The age of heroes,characterized by aristocratic commonwealths (think of the heroic warriors of medieval Christian Europe).
  3. The age of man, characterized by the rise of democratic republics, which eventually generates unrest and disorder, leading to a new barbarism that starts the cycle all over again (think of the Enlightenment in Europe).

In 1960, the American economic historian W.W. Rostow offered a non-Marxist alternative conception of "The Stages of Growth" (the title of his book):

  1. the traditional society
  2. the preconditions for take-off,
  3. the take-off,
  4. the drive to maturity; and
  5. the age of high mass-consumption.

Along the same lines, in 1965 A.F.K. Organski, a comparative political scientist, proposed four stages of political development:

  1. the politics of primitive unification,
  2. the politics of industrialization,
  3. the politics of national welfare, and
  4. the politics of abundance.

Most recently, Francis Fukuyama traced political development through a series of stages in The Origins of Political Order: From Prehuman Times to the French Revolution (2011); a second volume, Political Order and Political Decay: From the Industrial Revolution to the Present Day (2014) tracks political development since the french Revolution of 1789.  According to this view:

  1. Primitive hunter-gatherers inherited the violent tendencies of their primate forebears, which forced them to gather together into small, protective social groups.
  2. From these beginnings emerged tribes (and religion, first in the form of ancestor worship).
  3. Then organized states -- first led by warlords, later by hereditary kinds, city-states and later ration-states.
  4. The rule of law emerged when monarchies were made accountable to elected bodies (as in England's Magna Carta).
  5. Democratization, as well, proceeded along a series of stages.  These, in turn,were traced in a book by Fukuyama's mentor, Samuel P. Huntington (who famously predicted a "clash of civilizations" between Christianity and Islam).
  6. The phases of political development are, in Fukuyama's view, independent of corresponding stages of economic and cultural development.  When liberal democracy is coupled with a market-oriented economy, you get what Fukuyama described, in an earlier book, as "The End of History".  That is to say, no further development is possible, because there's nothing left to develop toward.  In this respect, Fukuyama departs from Marx, who believed that political and economic evolution would end with communism.

Stage theories of political and economic development are about as popular in social science as stage theories of cognitive or socio-emotional development have been in psychology!  Note, however, the implications of the term development, which suggests that some societies are more "developed" -- hence, in some sense better than others.  Hence, the familiar distinction between developed and undeveloped or underdeveloped societies.  This implication is somewhat unsavory, just as is the suggestion, based on a misreading of evolutionary theory, that some species (e.g., "lower animals") are less developed than others (e.g., humans).  For this reason, contemporary political and social thinkers often prefer to talk of social or cultural diversity rather than social or cultural development, thereby embracing the notion that all social and cultural arrangements are equally good.  This emphasis on diversity is also characteristic of modern social and cultural psychology.

From Cultural Development to Cultural Psychology

In addition to the new evolutionary psychology, a new cultural psychology is emerging that addresses cultural differences in thought processes without necessarily implying that one culture is more or less "developed" than another.

  • In some ways, cultural psychology has its origins in 19th century imperialism, as researchers from the countries of Europe tried to understand how the thought patterns of those they colonized, in Africa, Asia, and the Americas, might differ from their own.  Naturally, this quest for understanding the "primitive" or "savage" mind often had more than a little tinge of racism in it -- at the very least, the investigators seem to have been under the sway of cultural stereotypes of the people they studied.  One popular hypothesis, for example, was that the "primitive mind" might be more susceptible to visual illusions.
    • A famous example is the 1895 Torres Island Expedition, to a part of New Guinea then under British control, by W.H.R. Rivers, a pioneering British psychologist.  In fact, Rivers and his colleagues found only minimal striking differences in sensory acuity between the Torres Island natives and European control subjects. Of course, you wouldn't expect to find much cultural variation in mental functions like perception, which lie so close to the physiology that we all share in common.  And it turned out that while Torres Islanders were indeed more susceptible to the horizontal-vertical illusion, they were less susceptible to the Muller-Lyer illusion.
    • In 1895, psychologists really didn't have the methods or equipment to study the "higher" mental functions, where such differences might be observed.  Nor, for that matter, did they have any philosophical warrant to do so.  Recall from the Introduction that Wilhelm Wundt, the leading psychologist of the time, denied that anything other than sensation and perception were amenable to controlled experimental investigation. 
    • The first cross-cultural investigation of "higher" mental functions was undertaken by F.C. Bartlett, he of the "War of the Ghosts" and the "Reconstruction Principle" of Memory, who studied herdsmen in Swaziland, a British colony in East Africa (Bartlett had been a student of Rivers). Bartlett found that these herdsmen had extraordinarily good memory where their cattle were concerned -- an outcome he interpreted in line with his reconstructive theory of memory.  In his view, this superior memory reflected the "strong sentiments" that people develop around institutionalized, culturally valued activities.  Cattle are more important for Swazi herdsmen than for Cambridge undergraduates, and this interest led to superior memory.  But notice that culture didn't change the reconstruction principle itself -- it only determined what the subjects were interested in.
  • Cultural psychology was also stimulated by the rise in Marxism, and the view that political and economic changes in society would alter how individuals thought.  Chief among these theorists were Aleksandr Luria and Lev Vygotsky, much of whose work was published in Russian in the 1930s, and translated into English only much later.  Especially important in this respect are Vygotsky's essays on Mind and Society, translated and edited by Michael Cole in 1978. 
    • Vygotsky's general law of cultural development states that "Any function in children's development appears twice or on two planes.  First it appears on the social plane and then on the psychological plane.  First it appears between people as an interpsychological category and then within the individual child as an intrapsychological category....  Social relations or relations among people genetically underlie all higher functions and their relationships ("The Genesis of Higher Mental Functions", 1981, p. 163).
    • Vygotsky emphasized the importance of social interaction in learning.  He defined the child's zone of proximal development as "the distance between the actual developmental level as determined by independent problem solving and the level of potential development as determined through problem solving under adult guidance, or in collaboration with more capable peers (Mind in Society, p. 86).  In contrast to Piaget, who believed that children must move from one stage of development to another on their own, through their own discovery learning, Vygotsky insisted that cognitive development proceeded best when adults actively supported and promoted the child's learning. But they shouldn't push the child too far, so as to go beyond the boundaries of the child's zone of proximal development.
    • If none of this sounds particularly Marxist, you'd be right.  But if you were a Russian psychologist living in Stalin's Soviet Union, you'd give your theory of culture and cognition a Marxist twist too.
  • One of the principal hypotheses of the early cultural psychology was that there would be differences in patterns of thought between literate and non-literate cultures.  As noted in the lecture on Language, words are a powerful medium for representing knowledge, and syntax is a powerful tool for thinking.  But, as far back as Plato, philosophers had speculated that the availability of a written language might change how people thought (in particular, Plato thought that writing would wreck memory, because people would no longer have any need for it).
    • Scribner and Cole (1981) studied the Vai people of Liberia, a tribal group which had developed its own idiosyncratic written language, otherwise rare in the tribal cultures of Africa.   The majority of Via men (and almost all women) are illiterate, but some know written Vai, while others also know Arabic (e.g., from Koranic schooling, in which students are taught only to memorize the Koran), while still others learned English via formal schooling.  Literacy in Vai improved performance on a number of cognitive tasks, but English literacy, as a product of formal schooling, had even stronger effects.  Koranic or Arabic literacy had few positive effects, which means that formal schooling, not literacy per se, was really responsible for most of the apparent cognitive effects of literacy.  
      • Scribner and Cole summarize their findings as follows: "Literacy makes some difference to some skills in some contexts" (1982, p. 234).
  • And, of course, some evidence of cross-cultural differences come from studies of the Sapir-Whorf hypothesis, also discussed in the lecture on Language.  On the assumption that language is a reflection of culture, evidence for the Whorffian hypothesis becomes evidence of cross-cultural differences.

Notice however, that there's a difference between studying the effects of language on cognition and studying the effects of literacy.  While not everybody's literate, everybody's got language -- it's part of being human.  So while it's easy to characterize literacy as an aspect of cultural development, it's not so easy, or even appropriate, to imply that, for example, speakers of English are any more "developed", just by virtue of knowing English, than speakers of the Vai language.  So with the Sapir-Whorf hypothesis, interests shifts from cultural development to culture per se.

Partly as a result of increasing cultural diversity in America and Europe, and increasing appreciation of cultural differences, recent years have seen a great increase of interest in cross-cultural psychological research -- but without the implication that one culture is more "developed" than another.  Cultures, in this view, are just different, and these differences are psychological as well as behavioral, affecting how people think.  For a review of the early literature in this area, see Triandis & Brislin (1984).  This literature has focused mostly on cultural differences in social interaction, rather than in"pure" cognition or emotion.

Certainly the most popular cultural difference studied today has been characterized on a dimension of individualism vs. collectivism (e.g., Triandis et al., 1988; Triandis, 1996).

  • In individualistic cultures, such as Western Europe, North America, and Australia, people view themselves as independent individuals whose behavior is guided by their own attitudes, beliefs, and interests. 
    • They foster an "independent" sense of self (Markus & Kitayama, 1991).
    • And they promote "linear", ""analytic", categorical" modes of thinking (Nisbett, Peng, Choi, & Norenzayam, 2001).
  • In collectivist cultures, such as China, Japan, and South Asia, as well as many parts of Africa and Latin America, people view themselves as intimately connected to their communities, with their behavior highly responsive to the expectations of others and other situational demands.
    • They foster an "interdependent" sense of self.
    • And they promote "holistic" or "dialectical" thinking.

There is some evidence to support these propositions, chiefly from studies comparing Chinese or Japanese subjects with Americans.  However, these conclusions should be qualified. There is plenty of variability within cultures, especially given the opportunity for cultural contact.  It is not clear that a third-generation Japanese-American undergraduate at Berkeley, for example, thinks any differently than her Polish-American roommate. Sometimes, you get the feeling that some cultural psychologists have a stereotyped vision of the "exotic" cultures that interest them.  If early cultural psychology was sometimes motivated by racism, the later version sometimes smacks of what Edward Said called Orientalism.

Not to put too fine a point on it: the notion of "dialectical" thinking, in which the confrontation between a thesis and its antithesis is resolved by a synthesis, has its origins in European philosophy, particularly Hegel and Marx.  And perhaps the most linear, categorical thinker of all time was Mao Tse-tung, a Chinese who read both Marx and Hegel and -- again, not to put too fine a point on it -- imprisoned or killed everyone who disagreed with him. 

Cultural differences are most frequently cast in terms of developed vs. underdeveloped cultures, or Eastern vs. Western cultures, but such differences can be seen even within one of these categories.

  • Within American culture, for example, Cohen and Nisbett (1995) described a "culture of honor" characteristic of the Old South (i.e., the states of the former Confederacy) which differed in significant ways from the culture of the North.
  • Along the same lines, Hazareesingh (2015) has listed five features that distinguish French thinking from that characteristic of other countries:

    1. The use of history to structure reasoning.
    2. Importance of the nation and collective identity as "French".
    3. Intense public debate about ideas.
    4. The role of the public intellectual in society.
    5. Give-and-take between rationality and creative imagination.

From its beginning, scientific psychology has been based on empirical findings obtained from subjects of Western European heritage -- the 19th-century psychophysicists were Germans, after all, as was Ebbinghaus; and as scientific psychology grew strength in the 20th century, most of its subjects were American college students -- who, themselves, were mostly white, mostly of European heritage, and mostly relatively wealthy (at least, from the middle class or above).   In a provocative article, Joseph Henrich, Steven Heine, and Ara Norenzayan labeled these subjects WIERDos -- for Western, Educated, Industrialized, Rich, and Democratic (Behavioral & Brain Sciences, ).  They suggested that psychological theory had been distorted by excessive reliance on them, and that psychologists should expand their subject populations to include non-WIERDos -- and, perhaps, to repeat classic experiments, on which so much theory has already been established, on them as well.  This idea seems particularly relevant to personality, social, and developmental psychology (we'll discuss culture-specific syndromes of mental illness in the next lectures, on Psychopathology and Psychotherapy) -- though even such basic phenomena as the Ebbinghaus illusion, discussed in the lectures on Sensation and Perception, appear to vary across cultures.

For introductions to cultural psychology, see the following books by Prof. Michael Cole of UCSD, who -- along with Harry Triandis, at the University of Illinois -- is the doyen of cultural psychology:
  • Culture and Thought: A Psychological Introduction by Michael Cole & Sylvia Scribner (1974)
  • Cultural Psychology: A Once and Future Discipline by Michael Cole (1996).

This page last revised 08/06/2021.