Edge 161 — May 20, 2005
(8,500 words)


A Talk with Simon Baron-Cohen

Baron-Cohen responds to Marc D. Hauser, Steven Pinker, Armand Leroi, Carole Hooven, Elizabeth Spelke, Alison Gopnik, David C. Geary, Helena Cronin, Linda S. Gottfredson


Diane F. Halpern, Alison Gopnik, David Haig, Nora S. Newcombe on "Pinker vs. Spelke"

A Talk with Simon Baron-Cohen

Baron-Cohen responds to Marc D. Hauser, Steven Pinker, Armand Leroi, Carole Hooven, Elizabeth Spelke, Alison Gopnik, David C. Geary, Helena Cronin, Linda S. Gottfredson


Marc Hauser raises some deep questions about how one can disentangle a genetic (particularly imprinted genetic) theory from an endocrine (particularly fetal androgen) theory of psychological sex differences. Of course, it needn't be the case that these two theories are in competition with each other. There may be both fetal androgenic effects (after the surge in testosterone produced by the testes in the male fetus at about 12-16 weeks gestation) and genetic effects that predate the production of androgens. Certainly in the animal literature there is evidence of both kinds of effects on sexual dimorphism in the brain and behaviour. It remains a challenge to conduct the right sorts of experiments that go to to the heart of which biological mechanisms underlie the observed sex differences in behaviour from birth.

It is refreshing that Hauser reacts with an open mind. He effectively says: Suppose it were the case that the two sexes differ in the mind from birth: how can we take such an idea to the next level of depth and complexity? How can we isolate the possible biological and environmental causes of such sex differences?

Steven Pinker picks up the amusing confusion that has occurred in language in using the word 'gender' and 'sex' interchangeably when they mean quite different things. He says that we have 3 different meanings (intercourse, dimorphism, and gender roles) to talk about, but only 2 words (sex and gender) available to us.

Pinker is on to something here, in that we are just emerging from decades of linguistic ambiguity and linguistic inadequacy. I would go one step further and argue that we probably have at least 7 different meanings, and only 2 words available to us. So, when we think about someone's sex, we can think about their chromosomal sex (how many X or Y chromosomes they have); their gonadal sex (do they have testes or ovaries); the sex of their brain (do they have the brain structure and function that is typical for people with their chromosomal sex); their sex-typical cognition (do they have the profile of strengths and weaknesses that is typical for people with their chromosomal sex); their sex-typical behaviour (do they act in ways that are typical for people with their chromosomal sex); their gender identity (do they identify with others who share their chromosomal sex); and finally, their sexual orientation.

The scientific studies that have been conducted are now beginning to show that whilst some of these 7 ways to think about sex may be linked. For example, if you have a Y chromosome then you are likely to develop testes, which in turn produce fetal testosterone which affected brain structure and function, cognition and behaviour. But some of these 7 ways to think about sex may be independent. For example, a girl with the genetic condition of congenital adrenal hyperplasia (CAH), which leads to over-production of fetal testosterone, is more likely to resemble a typical boy in her behaviour. Clearly we are going to need finer distinctions in our language to talk about someone's sex at all these different levels (and more), but it is interesting that even given the poverty of our vocabulary, this is not holding the science back. Key experiments (such as those with girls with CAH), are teaching us how to think about sex.

Armand Leroi wonders if children with autism are necessarily hyper-males. In particular, he asks, if this hypermasculinization is correct, why we don't see characteristics such as excessive aggression? I think Armand is right that some predictions flow from the theory. For example, do women with autism have delays in menarche, or have more male-typical interests? Experiments to look at these outcomes are underway in our lab. We should be careful though not to assume that all male characteristics are under the control of fetal rather than post-natal testosterone. Aggression for example may not be, whilst language development may be. We will need to differentiate the 'organizational' effects that testosterone may have during brain development from the 'activational' effects it may have at later points in development.

And we will need to keep in mind that if autism involves elevated levels of fetal testosterone, these may not be at the extreme levels as to cause clear changes to the sex organs, as in the female spotted hyena. Rather, these may simply be subtly higher levels that may affect brain and cognitive development. The effects may be subtle too, such as slowing language or social development. But these are empirical questions that I for one regard as important laboratory work to be undertaken.

Carol Hooven's comments are exactly on target. She raises the question: why do boys with CAH not have superior abilities in spatial tasks, if it is simply a function of how much fetal testosterone they produce? But the work of Grimshaw in Canada suggests that the relationship between fetal testosterone and spatial ability such as performance on the Mental Rotation Task is not linear; it may be an inverse U shape. Optimal performance may be in the low average male range, with either too much or too little fetal testosterone being correlated with worse performance. To me, this makes the mechanism all the more intriguing. I agree with her that we need more research into these delicate dose-dependent effects. Whether mating effort (the behaviour she has studied) has anything to do with fetal rather than current testosterone is an empirical question.

Elizabeth Spelke talks of 'systematizers' where I prefer the economy of syllables ('systemizers')! More seriously, she assumes I am discussing binary categories: either you're an empathizer or a systemizer. Nothing could be further from the claims in my theory. For me, these are continuous dimensions, not categories. For this reason, we developed metrics like the Empathy Quotient (EQ), or the Systemizing Quotient (SQ). It turns out that there are statistically significant sex differences on both of these measures. In my theory, I argue that it is the difference score that defines a typical male or female profile. It is not that female = empathizer (E) or male = systemizer (S). The claim is rather subtler than that. Rather it is that among females we find more individuals with a difference score such that E > S, and that among males we find more individuals with a difference score such that S > E.

Spelke also seems to have a concern that a single concept like systemizing can capture why individuals end up in such diverse occupations as cathedral building, ballroom dancing, or seal hunting. Interestingly, all 3 may be very good examples of systemizing, because they all involve understanding lawful or rule-based systems.

Spelke considers the newborn baby study that is currently a key piece of evidence for a stronger social interest in females and a stronger interest in objects among males. She worries that the experiment betrays something of my politics, but of course she knows nothing about my politics. If she assumes that because I tested sex differences at birth I must have conservative political leanings, or be against the equality of the sexes, she will have to take it on trust that the opposite is true. But more importantly, it is irrelevant. Hence my plea to keep politics and science clearly distinct, so that we can explore if and how and why the sexes are different, even if we are feminists or democrats.

I respect her for focusing on the experimental method of the newborn baby study. We want to know which methodological variables may have produced these results. She is right that all we know from this experiment is that more boys than girls looked for longer at the mechanical mobile, and more girls than boys looked for longer at the human face. But does this mean girls have a stronger interest in people than things, and boys have the opposite pattern of interests?

Possibly, but not necessarily. The infants' attention was being lured by two very moving objects. It could be that on average, more girls were attracted by the type of motion that faces have and, on average, more boys were attracted by the type of motion that mobiles show. Of course, re-describing the results in terms of motion-types may still be very important. For example, faces have 'biological' motion (they are self-propelled, with animacy) whilst mobiles have 'mechanical' or physical-causal motion. Are male and female brains on average tuned to react to these two types of motions differently? Further experiments would be needed to isolate if motion-type is the critical variable. In a first study we could not run additional controls to explore such possibilities, as babies will not tolerate a long experiment where such parameters can be varied systematically. But such additional experiments do need to be tried.

She worries if the experimenter was blind to the hypothesis being tested, or if the results could simply reflect experimenter-bias. I am glad I can reassure her on this point. The experimenters (there were two of them) asked mothers not to reveal the sex of their baby until after the experiment, precisely to avoid such experimenter bias. In 101 babies, there were I recall only 3 cases where the experimenter may not have been fully blind. The results were analysed with and without these 3 cases, and this did not change the pattern of results. More importantly, all that was filmed of each infant was the eye region of their face, so that later an independent panel of judges could look at each video film to measure the infants' looking time to each object. So the opportunity for experimenter-bias at the stage of coding the videos was zero. The experimenters were not the judges, and it was impossible for the judges to tell the sex of the baby from the eye-region of the face alone.

Since for some people, results of such experiments will be controversial, it is of paramount importance to control for such biases, which we took all possible human steps to do. Readers interested in the methodology can find the original article published in the journal Infant Behaviour and Development, to see how we attempted to put in place as many such controls as possible.

Spelke calls this a 'single' experiment, tested with a 'single' person and a 'single' object. If the implication is that following this novel experiment there is not yet a series of independent replication studies, then I agree that we must wait for these to be conducted. It is not that there is a set of failures to replicate. There are simply no attempts yet to replicate. I should point out that in terms of data collection, one is looking at perhaps 3-6 months of testing on a daily basis to complete a sample of about 100 babies, and I hope that other labs undertake this kind of research effort.

If the implication is whether the same results would be obtained with a different person, or a different object, these are good empirical questions. We ascertained that even testing newborns with one object compared to one person was an achievement, given a neonate's attention span. But I look forward to other scientists taking up the challenge of conducting different kinds of experimental manipulations with this age-group, so that we can start to examine issues of reliability and specificity of the results.

Spelke suggests that in 3 decades of infancy research, sex differences have not been observed. One has to remember that subtle sex differences if they exist are not going to emerge from small samples. A typical research project includes 20 babies, 10 of each sex, and researchers wisely do not test for sex differences when the power to detect such differences is not sufficient in such small samples. To demonstrate if a 3 month old infant understands a principle of folk physics, one does not need to test a sample of 100 babies. 20 may be enough. But to test for subtle sex differences, one just might need 100 (50 males and 50 females) in a single study.

And besides, it is not true that infancy studies have not reported significant sex differences. The MacArthur infant vocabulary scales report different norms for boys and girls at age 12 months precisely because girls vocabularies are bigger than boys from that age, and remain bigger over the next 24 months. Many independent studies show that girls on average also make more eye contact, and play with different kinds of toys to boys, from as early as 12 months old. What makes the neonatal study of sex differences new is the age at which they were tested.

Finally, Spelke concludes that "infants don't choose whether to systemize or empathize; they do both, and so do we". That's correct. We all do both. It is only a straw man who might be thought to argue that infants choose one or the other. Recall that we are interested in subtle discrepancies in the amount of a person's attention to the physical and social environment. Whilst in this experiment we may have categorized infants according to whether they looked longer at the face or longer at the mobile, we were still measuring differences in looking times at the two types of stimuli. No one is going to suppose for a moment that an infant is solely interested in one of these objects and not at all interested in the other. The world is not that black and white.

Alison Gopnik asks why on the false belief test, a test of 'theory of mind', no obvious sex differences have emerged. My answer to her excellent question is that 'theory of mind', and the false belief test in particular, is designed as a pass-fail kind of test. To pick up subtle individual differences you need a test with a fine-grained scale. To test for differences in height between the sexes you need a ruler with a resolution of centimeters. You can't just have a test such as 'can both sexes see over the table?' To test for a difference in head circumference between the sexes you need a ruler with a resolution of millimeters. Not just a test like 'can both sexes get the hat on their head?'

But there is another reason why theory of mind might not be the right place to look for sex differences, and that is because theory of mind is not the same as empathy. To empathize, you may need a theory of mind (to attribute or compute a person's mental states) but to empathize you have to do a lot more than simply attribute or compute a person's mental states. Psychopaths can attribute mental states but they don't have great empathy. And on tests of empathy, many studies find strongly significant sex differences.

David Geary proposes that sex differences in folk physics and folk biology may not equate with sex differences in systemizing. Why not? When we engage in folk physics or folk biology, surely we are trying to predict the behaviour of a system (a mechanical system, or a taxonomic system, or a digestive system, etc) in terms of its rules? That is systemizing. The concept of systemizing grew out of the older concept of folk physics, but I found the latter too restrictive because there are systems that are abstract (such as mathematics and music) that are as lawful as tools and machines, but are not well encompassed by the concept of folk physics.

Helena Cronin is of course right that we need to understand sexual dimorphism in a Darwinian framework, and she has done more than most academics to explore this framework for understanding human behaviour. The tough part of course is to test predictions from a Darwinian framework in a contemporary human population, when the relevant selection pressures may be ancient. But there are impressive examples of how this can be done.

Linda Gottfredson is the commentator who takes seriously the new idea about assortative mating of two strong systemizers as a cause of autism. Whilst this idea remains speculative, it is testable, and I am glad she pointed out that mating patterns in one generation can change rates of different characteristics in offspring in the next generation(s) with remarkable speed, and that mating patterns effectively create different environments that can affect child development.

I look forward to seeing the tests of the assortative mating theory of autism in the coming years. And I thank those who provided commentaries for their stimulating debate.

Psychologist; Director of the Berger Institute for Work, Family, and Children, Claremont McKenna College; Past-president (2005), the American Psychological Association

Response to Drs. Pinker and Spelke

Many years ago I responded to an article about sex differences in cognitive abilities with the same response that I have to the debate between Drs. Pinker and Spelke, "What you see depends on where you look." If the question being debated is the underrepresentation of women in the sciences, then we need to remember that science, like love, is "a many-splendoured thing." Approximately 50% of medical school graduates and 75% of veterinary school graduates are women. Forty-four percent of all PhDs in biology and life sciences are being awarded to women, so women obviously have the innate ability (the term used by Lawrence Summers) to succeed in science. Women are underrepresented in the number of PhDs awarded in mathematics (29%), engineering (17%), and computer/ information science (22%), and overrepresented in the percentage of PhDs in psychology (68%), and health sciences (63%) to give a few other examples. Yet, no one has asked if men have the innate ability to succeed in those academic disciplines where they are underrepresented.

There are differences in percentages of men and women who choose to study and then succeed in different fields of science. Of course, there are many other differences between men and women, aside from those directly related to reproduction. There are many psychiatric diagnoses that are primarily applied to either females or males. For example, 90% of eating disorders such as anorexia occur in females. Panic disorders (with agoraphobia) are three times more common in women than men. In contrast, the ratio of males to females with attention deficit disorder with hyperactivity ranges between 4:1 and 9:1; only 2% to 4% of all people in treatment for pathological gambling are women; and antisocial personality disorder are much more common in males. Approximately 95% of all prisoners in the United States are male. Males account for 91% of all arrests for murder and (nonnegligent) manslaughter, aggravated assault, and robbery. It seems women are unlikely kill, but when they do, they kill men. The only major crime categories in which the percentage of females exceeds that of males is prostitution and, for juveniles, run-aways.

Even college students spend their time in sex-typed activities. College men spend much more time exercising, partying, watching television, and playing video games than college women (37.2% spend 1 or more hour per week on video games compared to 6.8% of the women). Women in college report that they spend much more time on household and child care, reading for pleasure, studying, and volunteer work. On average, women and men college students live systematically different lives despite the fact that much of the time they attend class together and do the same homework. But, there is a more serious side to sex differences‹one that is clearly based on prejudice against girls, and one that makes it easy to understand the passion that underlies many of the arguments about difference between women and men. According to the U. S. Committee for UNICEF, worldwide, the apartheid of gender is responsible for the deaths of millions of females.

The selective abortion of female fetuses and infanticide of female infants has resulted in 100 males for every 92 females in India and 100 males for every 28 females in rural China, with disparate sex ratios favoring males in many other countries in the world. In discussing the missing females from the world population, Morrison (1995) commented, "Cultures that consider a double-X chromosome a deformity may be committing gender genocide" (p. 5). According to estimates from UNICEF: "More than a million children die each year because they are female."

UNICEF reports also estimate the literacy rate for females at 2/3 that of males because world-wide 20 million more girls than boys are denied access to school. Today in the US, there are still few women make it to the "O" Level‹CEO (chief executive officer), COO (chief operating officer) and CFO (chief financial officer) and CTO (chief technical officer). According to a Catalyst study in 1999, women held only 5.1% (114 out of 2,248) of the highest officer positions. This measly value is an increase over previous years. Thus, when asking about the underrepresentation of women in academic science, we need to consider the question in a much wider context‹is it something about women and science per se or is the underrepresentation part of a larger picture in which men and women differ in many categories unrelated to science?

The debate between Drs. Pinker and Spelke took place because of President Larry Summers controversial explanation for the underrepresentation of women in academic science ‹most notably tenured full professors at Research I institutions. Before considering the reasons why there are so few women in full profession positions, President Summers should have considered the base rate. In fact, there are relatively few women in full professor positions in any discipline, especially at research institutions. The Chronicle of Higher Education recently reported that women comprise approximately 22% of full professors in all academic areas combined in the US, including those universities where research is not as highly valued as Research I institutions. This is a surprisingly low number given the fact that women have received more college degrees than men every year since 1982, with the gap widening every year, so the low percentage of women at the full professor level in all disciplines is not a pipeline problem and it is not restricted to the sciences.

Science is a complex discipline where success depends upon many different cognitive abilities, such as the ability to translate complex prose to a diagram, explain a phenomenon to a laboratory partner, use manual dexterity (depending on the subfield), think creatively, use numbers effectively, and more. There are many cognitive tasks that usually show an advantage for women and others that usually show an advantage for men. These tasks are listed in the table above. Some of these differences are large; others are small. Other cognitive tasks show little or no difference and are not listed.

A great deal has been made by both Drs. Pinker and Spelke about the finding that males tend to be more variable in cognitive performance and hence overrepresented on the high (and low) end of abilities distributions, which can be seen on tests such as the SAT-M. Given the need for high mathematical ability in some areas of science and the overrepresentation of males in those sciences that are particularly math-intensive, the leap to see cause in this correlation has caused some scientific name-calling on both sides of the nature-nurture fence.

But, success in any science requires exceptionally high verbal skills as well as high math skills. Very large effects favoring females are found on tests of writing, such as the international writing assessments, which show consistent large effect sizes across countries. (Unfortunately, the new writing section of the SAT will probably not show these very large effects because it uses a 6-point grading scale, although it is only in its first administration and I have no data on which to base this prediction.) Girls and women also get higher grades in school, so the relationship among school success, standardized assessment, and career success (and life success for that matter) is far too complex to link a portion of a tail of any single distribution to career outcomes.

Nature and nurture are not two ends of a single pole. Each individual is predisposed by his or her biology to learn some skills more readily than others, and everyone selects experiences in ways that are biased by prior learning histories, opportunities afforded in their environments, and beliefs about appropriate behaviors for females and males. Similarly, many stereotypes about male and female differences reflect group differences; by learning and endorsing them, individuals may also be selecting environments that increase or decrease these differences. Experiences change neural structures, which in turn alter how individuals respond and so on. Learning, for example, is a biological, social, and environmental event. Brain structures reflect learning and experience and change into very old age, thus blurring and nature-nurture distinctions beyond usefulness for most purposes.

Even simple distinctions like dividing variables into biological and environmental categories are impossible. Consider for example, the fact that there are differences in female and male brains. The differences in brain structures could have been caused, enhanced, or decreased by environmental stimuli. Brain size and structures remain plastic throughout life. Brain imaging techniques can show changes in cortical representations that occurred after specific experiences. What we learn influences structures like dendritic branching and cell size; brain architectures in turn, support certain skills and abilities, which may lead us to select additional experiences. The interface between experience and biology is seamless. Biology and environment are as inseparable as conjoined twins who share a common heart. A psychobiosocial framework provides a more integrated way of think about a holistic process.

In understanding how sex-differentiated cognitive patterns are developed and maintained, it is important to remember that we all develop in a social context and there have been many studies about the ubiquity of sex role socialization practices.

In thinking about an explanation, every scientist needs to consider alternative hypotheses.

Any job can be disproportionately male or female for different reasons. For example, it is likely that there are few female piano movers in New York City because women, on average, have less upper body strength than males. Other highly sex-segregated jobs are more readily explained by the fact that women do the bulk of the caretaking responsibilities in society (childcare, sick care, and elder care), and therefore often work "around" these responsibilities. Consider for example how this alternative hypothesis can be used to explain the underrepresentation of women in academic science. Summers noted that in addition to the greater variability among males in science and mathematics abilities, women might also be missing from academic science because they would be dissuaded by the fact that science is an 80-hour a week job.

He could have added the following: Academia is one of the few places where young talent has to prove itself at a young age in order to keep its job. If graduate school is followed by a post doc (as many in the sciences will do) and then six years at the assistant professor level, the young academic will be approximately 36 years old before applying for tenure, (assuming everything has gone smoothly) and if denied tenure, she or he will be fired ("not renewed") and have to explain why tenure was denied to hiring committees at some less prestigious college.

For women, tenure clocks and biological clocks run on the same time zone, and although maternal and paternal leaves are available at most universities, there are also subtle and not-so-subtle pressures not to take advantage of these leaves. The conditions of academic life that are particularly difficult for any woman who has caregiving responsibilities such as child care, which is a more likely reason for the underrepresentation of women in academic science, with its additional requirements for laboratory hours, than the fewer number of women at the highest tails of math and science standardized tests. The fewer number of men with high scores on writing tests or with high grades in school were ignored and the tenure system with its requirements to show excellence at a young age and insistence on full-time employment were not questioned as gender-fair workplaces.

We are both social and biological animals. There is much we can do to make the academic environment more welcoming to women scientists, such as part-time tenured positions during parenting years for men and women (and at other times when needed such as the need to care for aging parents). It is possible that more women scientists would broaden the research questions being asked, or the balance of methods being used, and bring new students with them because of role modeling and other subtle influences. These debates are about the future of science and how we use social science to explain. The questions we ask today and explanations we accept may seem hopelessly old fashioned to the next generation of scientists who wonder why we wondered about the innate the abilities of half the human race to do the work of the future.

Psychologist, UC-Berkeley; Coauthor, The Scientist in the Crib

The Pinker/Spelke debate brings a refreshing level of scientific seriousness to the arguments over Summers' remarks. But both Pinker and Spelke, as well as many others who have written about the issue, have a fundamentally over-simplified theoretical account of development.

Both Pinker and Spelke take a strongly nativist view — they assume that many, indeed perhaps most, complex adult traits are determined genetically, at birth, and remain essentially unchanged over time. Their debate is over whether one such trait, mathematical ability, is or is not different in men and women. Their debate, and related debates about whether there are "genes for" everything from homosexuality to depression to criminality, reflect a pervasive misunderstanding of human development and the relationship between nature and nurture. This misunderstanding is reflected not only in nativist views but also in the apparently opposing view that emphasizes social and cultural factors.

Most ordinary people, and even not so ordinary ones like Summers, Spelke and Pinker, seem to think that we can divide up the contributions of genes and the environment — we can take some trait like mathematical ability or scientific success or sexual preference and determine that it is, say, 30% due to genes and 70% due to the environment. But most developmental psychologists, and even most developmental biologists, would say that this way of thinking about the problem is profoundly misconceived.

Even in simple biological cases it is a truism that genes only lead to traits via a particular environment. Incontrovertibly genetic traits like having male genitals are actually environment–dependent. The fetus' male genes trigger the production of male hormones by the mother and those hormones bathing the fetus are what lead to male genitals. If a genetically female fetus is exposed to the same hormones she will also develop male genitals. Even biological traits are the result of this causal cascade of gene-environment interactions — the genes modify the environment which modifies gene expression and so on.

In the case of biological traits, of course, we can specify what a "normal" environment will be like, or at least specify the range of likely possible environments. But we can't do this for human psychological traits. The reason is that the most distinctive genetic trait of homo sapiens — the source of their greatest evolutionary advantage — is their ability to intentionally and radically change their environments. Even very young children already can picture the world around them, imagine alternative ways the world could be, and intentionally act to change the world accordingly. In particular, even very young children have causal theories of the world — they know how one thing makes another happen. One of the primary advantages of such theories is that they let us imagine alternatives to our current environment, and let us act to change the environment. We are starting to understand something about the computational, and perhaps even neurological bases of this kind of causal knowledge, and the associated capacities for hypothetical and counterfactual thought, planning and intervention. Eventually we will understand their genetic bases as well.

For example, once our ancestors uncovered the causal link between friction and fire, they could introduce light, warmth and cooked food into their environment in a way that radically altered their lives. Over history, this human capacity has meant that we now live in environments, for good or ill, that are nothing like the environments we evolved in. As a result we behave in ways that are nothing like the ways we behaved in the pleistocene. And since, as a social species, our conspecifics behavior is the most important part of our environment, our ability to causally represent and so alter, both our own behavior and the behavior of others plays a particularly important role.

What this means is that when we describe the genetic contribution to a human trait we have to consider the effects the genes would have on that trait in all the possible environments that human beings could create. Take a simple and vivid example. Some babies are born with phenylketonuria or PKU, a genetic condition that makes them unable to metabolize certain chemicals in food. In a "normal" environment these babies will become severely retarded and die. But since we human beings have discovered this causal fact, we can intervene to change it. We can, and do, test babies for the gene at birth and provide PKU babies with special diets. How much does the gene cause mental retardation and how much does the environment cause it? The answer, of course, is that it's a dumb question. Both genes and environments are 100% responsible. Most significantly, although for other animals a genetic defect like PKU would universally lead to retardation, our (genetically determined) human capacity for environmental alteration means that there is now almost no causal link between the genes and retardation.

Work in behavioral genetics demonstrates over and over that the effects of genes depend crucially on the range of environments in which people grow up. "Heritability" is a measure of the correlation between variance in genes and variance in some particular trait. Heritability can be measured with techniques like twin studies, and it is often taken to be an indicator of genetic effects. But heritability changes in different environments. IQ, for example, is highly heritable for rich children, but hardly heritable at all for poor children. Smoking once was much more heritable for men than women, but recently has become more heritable for women as well. In an environment of poverty, variations in the environment make a big difference, in a uniformly rich environment they make less difference. In an environment where women's smoking is taboo, genetic variability has less effect.

Because humans create and change their own environments we don't and can't, in principle, know what the range of possible environments will turn out be. And we don't know how those possible environments might interact with genetics over the course of development to cause a particular distribution of adult traits. This means that we simply don‘t and can't know how much genes contribute to complex human traits in general — the question is incoherent. In a particular case, with a particular specified environment, and a complete developmental history of the causal interactions between the organism and the environment, we might be able to give a causal account of the path from gene to trait. But there is no general answer about how gene and trait are related across all environments.

This point is directly applicable to the case of women in science. Several thousand years ago, no human being, male or female, had the innate capacity to become a professor of mathematics, because there were no professors and no mathematics. A scientific environment did not yet exist. As human beings have created and altered the environments that allow institutional science those environments have interacted with genes in different ways.

In one sense, it is obvious that genetic differences between men and women kept women from succeeding in science and mathematics. 50 years ago, the rules at the University of Pennsylvania, where my mother went to school, banned women from taking high-level mathematics courses. Her innate genetic endowment, the endowment that, as Elizabeth I said, made her cloven and not crested, was directly responsible for her inability to take those classes. In that environment, having two X chromosomes caused you to do less well in mathematics, just as PKU caused retardation in the environment rich in phenylaline. But just as in the PKU case, none in their right mind would think that this was a justification for keeping that environment the same — it was instead a reason to change it.

In the current environment, there is excellent empirical evidence that an indubitably genetic difference between men and women — their fertility curves — plays an important role in discouraging women from pursuing scientific careers. Arbitrary institutional structures like tenure demand that scientists work hardest between the ages of twenty-five and forty, and so make it much harder for women than for men to combine a career and family. The most dramatic effects on women in science come in the transition from graduate school to tenure-track jobs at research universities. Women with children in particular are much less likely to make this transition successfully. These effects might even interact with the innate "motivational" differences that Pinker hypothesizes. But in a different possible environment, an environment with family-friendly policies like parental leave and tenure-clock stoppage, this genetic difference, either the definite difference in fertility or the more dubious motivational differences, would no longer have the same effect, And indeed we already see that these environmental alterations have this effect.

The greatest genetic gift of humanity is our ability to change our environments in ways that enable our genetic inheritance to be expressed in unprecedented ways. This gift comes with an equally great responsibility. If we decide that we want a particular kind of world — a world with as much good science as we can get — or a world with as much individual justice as we can get — we need to experiment with new possible environments to try to get the outcomes we want. We need to use our innate capacities for theory formation and change to conceive of new environments, and to determine empirically what the effects of those environments will be. Oversimplified conceptions of genetics and development not only make that job harder, they lead to an impoverished view of the human nature they aim to describe.

Evolutionary geneticist/theorist; Harvard University; Author, Genomic Imprinting and Kinship

Steve Pinker discusses the various uses of sex and gender.

The use of 'sex' in the sense of sexual intercourse is a relatively recent euphemism. The first use listed in the OED (second edition) is from D. H. Lawrence (1929). As with all euphemisms, the new term soon comes to be contaminated with the associations it was first employed to avoid. Now 'gender' is often employed instead of sex to avoid the connotation of copulation.

Sex shares a root with sect, section, dissection, etc. Gender shares a root with genre, genus and gentle (as in 'of a good family'). It is sometimes claimed that the use of gender should be restricted to grammatical categories and that its use as a synonym for sex is recent. The Oxford English Dictionary quotes uses of gender for sex from the fifteenth century, although in the first edition of the Dictionary in 1899 this usage was described as jocular. A few Victorian literary examples will show the venerable nature of this synonym:

Charles Dickens in David Copperfield:

"… both these gifts inevitably attaching, as they believed, to all unlucky infants of either gender, born towards the small hours on a Friday night."

"But with Miss Mills observing, with despondency, that it were well indeed for some hearts if this were so, I explained that I begged leave to restrict the observation to mortals of the masculine gender."

Charles Dickens in David Copperfield:

" 'The pleading of the feminine —' Mr Fledgeby began, and there stuck so long for a word to get on with, that Mrs. Lammle offered him sweetly, 'Heart?'

'No,' said Mr. Fledgeby, 'Gender — is ever what a man is bound to listen to…' "

"That worthy couple were delayed by a portentous old parishioner of the female gender, …"

George Eliot in The Mill on the Floss:

"Public opinion, in these cases, is always of the feminine gender—not the world, but the world's wife: …"

The current rise in the use of gender for sex can be traced to a paper by a 1955 paper by John Money entitled "Hermaphroditism, gender and precocity in hyperadrenocorticism: Psychological findings" [1]. He wrote:

"The term gender role is used to signify all those things that a person says or does to disclose himself or herself as having the status of boy or man, girl or woman, respectively. It includes, but is not restricted to, sexuality in the sense of eroticism."

I have detailed the initially slow, but later precipitate, increase in the use of gender in my 2004 article "The inexorable rise of gender and the decline of sex: social change in academic titles, 1945-2001" [2]

The key factor in this rise was the adoption of a sex/gender distinction by feminist scholars. Sex was biologically determined but gender was socially constructed. Now, this distinction is observed only sporadically.

1. Money, John (1955) Hermaphroditism, gender and precocity in hyperadrenocorticism: Psychological findings (Bulletin of the Johns Hopkins Hospital 96: 253-264).

2. Haig, D. (2004) The inexorable rise of gender and the decline of sex: social change in academic titles, 1945-2001. Archives of Sexual Behavior 33: 87-96.

Psychologist, Temple University; Coauthor, Making Space: The Development of Spatial Representation and Reasoning

Steven Pinker and Elizabeth Spelke ranged over a wide variety of topics in their stimulating exchange. Commenting on each of these points would be a task for a book, not a brief commentary. What I'd like to do here is three things: (1) highlight the importance of the very new questions that Spelke has raised concerning the origins of sex-related cognitive differences, (2) argue that we are far too fixated on sex-related cognitive differences, at the cost of ignoring how much everyone can improve their spatial and mathematical abilities, and (3) point out that evolutionary psychology currently provides a far-from-rigorous framework for thinking about these differences (if we must).

Developmental origins of sex-related differences

In the debate, Spelke presented data that there are no sex-related differences in infants and toddlers performing spatial and number tasks. In my lab, we have also consistently failed to find such differences on spatial tasks. (And I note that, in a prior Edge commentary on Simon Baron-Cohen's work, Alison Gopnik said that she has never seen sex-related differences on theory of mind tasks, when Baron-Cohen's arguments might lead one to expect female superiority.) What are we to make of this paucity of early differences?

One thing to make of it is nothing. In replying to Spelke, Pinker argued that these basic abilities are irrelevant, because it is sophisticated spatial and mathematical reasoning that matter for success in elite science. (He also said that lack of differences could be due to lack of statistical power, but that possibility seems to me unlikely. The failure to find differences has been so consistent across studies, in a field where journal editors typically require that possible sex differences be examined, that simple nonparametric statistics with studies as the unit of analysis would argue for a lack of a reliable difference. Someone should write that paper)

Another thing to make of early equality is a great deal—it reframes how we think about the emergence of the differences we see in adult life. Unless we suppose that sophisticated reasoning appears like Athena from the forehead of Zeus, completely unrelated to earlier developmental achievements, the pattern of data strongly suggests that something happens during childhood to create divergent abilities. We don't know what that thing is, and it might of course be either biological or social in nature. Later emergence does not exclude biological explanation, but it is crucial to a social or biosocial explanation. It is exciting to think that research in the future may focus on the transmutation of foundational cognitive abilities to perform increasingly sophisticated tasks.

Maximizing everyone's cognitive ability should be the important goal

Even though sex differences in some kinds of spatial ability are substantial, levels of spatial ability do not seem to be biologically fixed. Spatial ability increases a great deal with practice, training and schooling, for both men and women. The difference between someone who has engaged in spatial thinking and been supported educationally in doing so is, in fact, far larger than the difference between men and women. The same is true for mathematical reasoning. If we want to maximize the human capital available for occupations that draw on spatial and mathematical skill, such as engineering, architecture, physical science and computer science, we would do much better to concentrate on understanding how to educate effectively for spatial and mathematical skill, rather than focus with such fascination on the explanation of sex differences. Our fixation on this latter topic speaks volumes concerning our approach to social change.

Whether or not men and women would converge in their abilities in supportive educational environments remains to be determined. If they do not, Pinker might well argue that we would still expect a disparity in elite achievement. I am not so sure. There are multiple factors that determine elite achievement, and beyond some (high) threshold, I doubt that extra increments of the same cognitive ingredients explain much variance. As Diane Halpern mentions, thinking creatively, explaining one's data, or inspiring a research team may be pretty important as well!

Evolutionary psychology provides a far-from-compelling explanation

Evolutionary psychologists like Steven Pinker have a ready explanation for sex-related differences in spatial ability. Men are generally the hunters in hunting-gathering societies, and hunting seems to require spatial skill in several of its component activities, including tracking animals, aiming at them, and fashioning the weapons with which to take aim. The protein obtained from hunting helps to ensure the survival of a man's children, and prowess in hunting may also enhance a man's access to women (who wish to have their children provisioned by a skilled hunter). In addition, aiming may come in handy in any struggles for dominance with other males over sexual access to females.

This neat explanation requires close examination, however, not head-nodding acceptance. Man the Hunter may need spatial skills, but so does Woman the Gatherer. These skills likely far transcend the plant-spotting skills highlighted in Silverman's work, which in any case has not clearly stood the test of replication. Gathering may require quite long trips away from home base to find various kinds of edible vegetation in their respective ripening seasons. True, animals move while vegetation sits still. But consider that humans are far from the swiftest creatures around, and much hunting by our ancestors may have consisted of setting traps, waiting at waterholes and so on rather than tracking animals over meandering paths. What about the spatial skills needed for manufacturing artifacts to use in hunting? Well, note that spatial skill is required to weave or make baskets or pottery, as much as for fashioning arrows and spearheads. Thought experiments like these suggest the crying need to think probingly not reflexively about human evolution.

Another explanation for sex differences focuses not on Man the Hunter but on the Man Who Gets Around. In this story, men need spatial skill in order to navigate around the territory required to have sexual access to as large a number of fertile women as possible. There is in fact beautiful observational and experimental evidence that this kind of effect is seen in voles. But the Man Who Gets Around is not a vole. Human females live in social groups rather than occupying widely-separated home territories. The skills it takes to impregnate many human females probably do not include the ability to find one's way among a cluster of huts—charm or stealth come to mind as more plausible candidates for success.


Curious Minds: How a Child Becomes a Scientist (Pantheon)

All new essays by 27 leading Edge contributors..."Good, narrative history, combined with much fine writing...quirky, absorbing and persuasive in just the way that good stories are."—Nature "Some of the biggest brains in the world turn their lenses on their own lives...fascinating...an invigorating debate."—Washington Post "Compelling."—Discover " An engrossing treat of a book...crammed with hugely enjoyable anecdotes ...you'll have a wonderful time reading these reminiscences."New Scientist "An intriguing collection of essays detailing the childhood experiences of prominent scientists and the life events that sparked their hunger for knowledge. Full of comical and thought-provoking stories."Globe & Mail "An inspiring collection of 27 essays by leading scientists about the childhood moments that set them on their shining paths."—Psychology Today

Published in the UK as When We Were Kids: How a Child Becomes a Scientist (Jonathan Cape)

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The New Humanists: Science at the Edge (Barnes & Noble)

The best of Edge, now available in a book..."Provocative and fascinating." La Stampa "A stellar cast of thinkers tackles the really big questions facing scientists."The Guardian "A compact, if bumpy, tour through the minds of some of the world's preeminent players in science and technology." — Philadelphia Inquirer "What a show they put on!"— San Jose Mercury News "a very important contribution, sparkling and polychromatic."Corriere della Sera

Published in the UK as Science at the Edge (Weidenfeld)
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The Next Fifty Years:
Science in the First Half of the Twenty-first Century

Original essays by 25 of the world's leading scientists..."Entertaining" —New Scientist "Provocative" —Daily Telegraph "Inspired"—Wired "Mind-stretching" —Times Higher Education Supplement "Fascinating"Dallas Morning News "Dazzling" —Washington Post Book World