Geneticists have found that the diversity of the DNA in the mitochondria of different people (which men and women inherit from their mothers) is far greater than the diversity of the DNA in Y chromosomes (which men inherit from their fathers). This suggests that for tens of millennia men had greater variation in their reproductive success than women. (…) These are precisely the conditions that cause sexual seletion, in which males compete for opportunities to mate and females choose the best-quality mates.

Here are a dozen kinds of evidence that suggest that the [biological] difference between men and women is more than genitalia-deep:

– Sex differences are not an arbitrary feature of Western culture (…) In all human cultures, men and woman are seen as having different natures. (…)- Many of the psychological differences between the sexes are exactly what an evolutionary biologist who knew only their physical differences would predict. Throughout the animal kingdom, when the female has to invest more calories and risk in each offspring (in the case of mammals, through pregnancy and nursing), she also invests more in nurturing the offspring after birth, since it is more costly for a female to replace a child than for a male to replace one. (…)

– Many of the sex differences are found widely in other primates, indeed, throughout the mammalian class. The males tend to compete more aggressively and to be more polygamous; the females tend to invest more in parenting. (…)

– The human body contains a mechanism that causes the brains of boys and the brains of girls to diverge during development. The Y chromosome triggers the growth of tests in a male fetus, which secrete androgens, the characteristically male hormones (including testosterone). Androgens have lasting effects on the brain during fetal development, in the months after birth, and during puberty, and they have transient effects at other times. Estrogens, the characteristically female sex hormones, also affect the brain throughout life. Receptors for the sex hormones are found in the hypothalamus, the hippocampus, and the amygdala in the limbic system of the brain, as well as in the cerebral cortex.

– Variation in the level of testosterone among different men, and in the same man in different seasons or at different times of day, correlates with libido, self-confidence, and the drive for dominance. (…) There is a causal effect (…) When women preparing for a sex-change operation are given androgens, they improve on tests of mental rotation and get worse on tests of verbal fluency. (…) Higher-testosterone women smile less often and have more extramarital affairs, a stronger social presence, and even a stronger handshake.

– Women’s cognitive strengths and weaknesses vary with the phase of their menstrual cycle. When estrogen levels are high, women get even better at tasks on which they typically do better than men, such as verbal fluency. When the levels are low, women get better at tasks on which men typically do better, such as mental rotation. A variety of sexual motives, including their taste in men, vary with the menstrual cycle as well. (…)

– The ultimate fantasy experiment to separate biology from socialization would be to take a baby boy, give him a sex-change operation, and have his parents raise him as a girl and other people treat him as one. If gender is socially constructed, the child should have the mind of a normal girl; if it depends on prenatal hormones, the child should feel like a boy trapped in a girl’s body. Remarkarbly, the experiment has been done in real life – not out of scientific curiosity, of course, but as a result of disease and accidents. One study looked at twenty-five boys who were born without a penis (a birth defect known as cloacal exstrophy) and who were then castrated and raised as girls. ALL of them showed male patterns of rough-and-tumble play and had typically male attitudes and interests. More than half of them spontaneously declared they were boys, one when he was just five years old. (…)

Things are not looking good for the theory that boys and girls are born identical except for their genitalia, with all other differences coming from the way society treats them. (…)

Of course, just because many sex differences are rooted in biology does not mean that one sex is superior, that the differences will emerge for all people in all circumstances, that discrimination against a person based on sex is justified, or that people should be coerced into doing things typical of their sex. But neither are the differences without consequences.