Second, many of the major sex differences — certainly some of them, maybe all of them, are universal. The idea that there are cultures out there somewhere in which everything is the reverse of here turns out to be an academic legend. In his survey of the anthropological literature called Human Universals, the anthropologist Donald Brown points out that in all cultures men and women are seen as having different natures; that there is a greater involvement of women in direct child care; more competitiveness in various measures for men than for women; and a greater spatial range traveled by men compared to by women.
In personality, we have a cross-national survey (if not a true cross-cultural one) in Feingold’s meta-analysis, which noted that gender differences in personality are consistent across ages, years of data collection, educational levels, and nations. When it comes to spatial manipulation and mathematical reasoning, we have fewer relevant data, and we honestly don’t have true cross-cultural surveys, but we do have cross-national surveys. David Geary and Catherine Desoto found the expected sex difference in mental rotation in ten European countries and in Ghana, Turkey, and China. Similarly, Diane Halpern, analyzing results from ten countries, said that “the majority of the findings show amazing cross-cultural consistency when comparing males and females on cognitive tests.”
Third, stability over time. Surveys of life interests and personality have shown little or no change in the two generations that have come of age since the second wave of feminism. There is also, famously, resistance to change in communities that, for various ideological reasons, were dedicated to stamping out sex differences, and found they were unable to do so. These include the Israeli kibbutz, various American Utopian communes a century ago, and contemporary androgynous academic couples.
In tests of mental rotation, the meta-analysis by Voyer et al found no change over time. In mathematical reasoning there has been a decline in the size of the difference, although it has certainly not disappeared.
Fourth, many sex differences can be seen in other mammals. It would be an amazing coincidence if these differences just happened to be replicated in the arbitrary choices made by human cultures at the dawn of time. There are large differences between males and females in many mammals in aggression, in investment in offspring, in play aggression play versus play parenting, and in the range size, which predicts a species’ sex differences in spatial ability (such as in solving mazes), at least in polygynous species, which is how the human species is classified. Many primate species even show a sex difference in their interest in physical objects versus conspecifics, a difference seen their patterns of juvenile play. Among baby vervet monkeys, the males even prefer to play with trucks and the females with other kinds of toys!
Fifth, many of these differences emerge in early childhood. It is said that there is a technical term for people who believe that little boys and little girls are born indistinguishable and are molded into their natures by parental socialization. The term is “childless.”
Some sex differences seem to emerge even in the first week of life. Girls respond more to sounds of distress, and girls make more eye contact than boys. And in a study that I know Liz disputes and that I hope we’ll talk about, newborn boys were shown to be more interested in looking at a physical object than a face, whereas newborn girls were shown to be more interested in looking at a face than a physical object.
A bit later in development there are vast and robust differences between boys and girls, seen all over the world. Boys far more often than girls engage in rough-and-tumble play, which involves aggression, physical activity, and competition. Girls spend a lot more often in cooperative play. Girls engage much more often in play parenting. And yes, boys the world over turn anything into a vehicle or a weapon, and girls turn anything into a doll. There are sex differences in intuitive psychology, that is, how well children can read one another’s minds. For instance, several large studies show that girls are better than boys in solving the “false belief task,” and in interpreting the mental states of characters in stories.
Sixth, genetic boys brought up as girls. In a famous 1970s incident called the John/Joan case, one member of a pair of identical twin boys lost his penis in a botched circumcision (I was relieved to learn that this was not done by a moyl, but by a bumbling surgeon). Following advice from the leading gender expert of the time, the parents agreed to have the boy castrated, given female-specific hormones, and brought up as a girl. All this was hidden from him throughout his childhood.
When I was an undergraduate the case was taught to me as proof of how gender roles are socially acquired. But it turned out that the facts had been suppressed. When “Joan” and her family were interviewed years later, it turned out that from the youngest ages he exhibited boy-typical patterns of aggression and rough-and-tumble play, rejected girl-typical activities, and showed a greater interest in things than in people. At age 14, suffering from depression, his father finally told him the truth. He underwent further surgery, married a woman, adopted two children, and got a job in a slaughterhouse.
This is not just a unique instance. In a condition called cloacal exstrophy, genetic boys are sometimes born without normal male genitalia. When they are castrated and brought up as girls, in 25 out of 25 documented instances they have felt that they were boys trapped in girls’ bodies, and showed male-specific patterns of behavior such as rough-and-tumble play.
Seventh, a lack of differential treatment by parents and teachers. These conclusions come as a shock to many people. One comes from Lytton and Romney’s meta-analysis of sex-specific socialization involving 172 studies and 28,000 children, in which they looked both at parents’ reports and at direct observations of how parents treat their sons and daughters — and found few or no differences among contemporary Americans. In particular, there was no difference in the categories “Encouraging Achievement” and “Encouraging Achievement in Mathematics.”
There is a widespread myth that teachers (who of course are disproportionately female) are dupes who perpetuate gender inequities by failing to call on girls in class, and who otherwise having low expectations of girls’ performance. In fact Jussim and Eccles, in a study of 100 teachers and 1,800 students, concluded that teachers seemed to be basing their perceptions of students on those students’ actual performances and motivation.
Eighth, studies of prenatal sex hormones: the mechanism that makes boys boys and girls girls in the first place. There is evidence, admittedly squishy in parts, that differences in prenatal hormones make a difference in later thought and behavior even within a given sex. In the condition called congenital adrenal hyperplasia, girls in utero are subjected to an increased dose of androgens, which is neutralized postnatally. But when they grow up they have male-typical toy preferences — trucks and guns — compared to other girls, male-typical play patterns, more competitiveness, less cooperativeness, and male-typical occupational preferences. However, research on their spatial abilities is inconclusive, and I cannot honestly say that there are replicable demonstrations that CAH women have male-typical patterns of spatial cognition.
Similarly, variations in fetal testosterone, studied in various ways, show that fetal testosterone has a nonmonotic relationship to reduced eye contact and face perception at 12 months, to reduced vocabulary at 18 months, to reduced social skills and greater narrowness of interest at 48 months, and to enhanced mental rotation abilities in the school-age years.
Ninth, circulating sex hormones. I'm going to go over this slide pretty quickly because the literature is a bit messy. Though it's possible that all claims of the effects of hormones on cognition will turn out to be bogus, I suspect something will be salvaged from this somewhat contradictory literature. There are, in any case, many studies showing that testosterone levels in the low-normal male range are associated with better abilities in spatial manipulation. And in a variety of studies in which estrogens are compared or manipulated, there is evidence, admittedly disputed, for statistical changes in the strengths and weaknesses in women's cognition during the menstrual cycle, possibly a counterpart to the changes in men's abilities during their daily and seasonal cycles of testosterone.
My last kind of evidence: imprinted X chromosomes. In the past fifteen years an entirely separate genetic system capable of implementing sex differences has been discovered. In the phenomenon called genetic imprinting, studied by David Haig and others, a chromosome such as the X chromosome can be altered depending on whether it was passed on from one's mother or from one's father. This makes a difference in the condition called Turner syndrome, in which a child has just one X chromosome, but can get it either from her mother or her father. When she inherits an X that is specific to girls, on average she has a better vocabulary and better social skills, and is better at reading emotions, at reading body language, and at reading faces.
A remark on stereotypes, and then I’ll finish.
Are these stereotypes? Yes, many of them are (although, I must add, not all of them — for example, women’s superiority in spatial memory and mathematical calculation. There seems to be a widespread assumption that if a sex difference conforms to a stereotype, the difference must have been caused by the stereotype, via differential expectations for boys and for girls. But of course the causal arrow could go in either direction: stereotypes might reflect differences rather than cause them. In fact there’s an enormous literature in cognitive psychology which says that people can be good intuitive statisticians when forming categories and that their prototypes for conceptual categories track the statistics of the natural world pretty well. For example, there is a stereotype that basketball players are taller on average than jockeys. But that does not mean that basketball players grow tall, and jockeys shrink, because we expect them to have certain heights! Likewise, Alice Eagly and Jussim and Eccles have shown that most of people’s gender stereotypes are in fact pretty accurate. Indeed the error people make is in the direction of underpredicting sex differences.
To sum up: I think there is more than "a shred of evidence" for sex differences that are relevant to statistical gender disparities in elite hard science departments. There are reliable average difference in life priorities, in an interest in people versus things, in risk-seeking, in spatial transformations, in mathematical reasoning, and in variability in these traits. And there are ten kinds of evidence that these differences are not completely explained by socialization and bias, although they surely are in part.
A concluding remark. None of this provides grounds for ignoring the biases and barriers that do keep women out of science, as long as we keep in mind the distinction between fairness on the one hand and sameness on the other. And I will give the final word to Gloria Steinem: "there are very few jobs that actually require a penis or a vagina, and all the other jobs should be open to both sexes."
