There is, nonetheless, a sense in which sexual differences cannot be strictly left to the genes. If a gene appeared in a Pleistocene man for, say, better sense of direction at the expense of poorer social intuition, it might have been of benefit to him: But, as well as his sons, his daughters would have inherited it from him: In them the gene might have been positively disadvantageous because it left them less socially intuitive. So its net effect, over time, would be neutral, and it would not spread."
The genes that would spread would therefore be ones that responded to signals of gender: if in a male, improve the sense of direction; if in a female, improve the social intuition. And this is precisely what we find. There is no evidence of genes for different brains, but there is ample evidence of genes for altering brains in response to male hormones. (For reasons of historical accident, the normal brain is female unless masculinized:) So the mental differences between men and women are caused by genes that respond to testosterone.
We last met the steroid hormone testosterone in fish and birds where it was rendering them more vulnerable to parasites by exaggerating their sexual ornaments. In recent years more and more evidence has been found that testosterone affects not just ornaments and bodies but also brains: Testosterone is an ancient chemical, found in much the same form throughout the vertebrates. Its concentration determines aggressiveness so exactly that in birds with reversed sex roles, such as phalaropes and in female-dominated hyena clans, it is the females that have higher levels of testosterone in the blood. Testosterone masculinizes the body; without it, the body remains female, whatever its genes. It also masculinizes the brain:
Among birds it is usually only the male that sings. A zebra finch will not sing unless there is sufficient testosterone in its blood. With testosterone, the special song-producing part of its brain grows larger and the bird begins to sing. Even a female zebra finch will sing as long as she has been exposed to testosterone early in life and as an adult: In other words, testosterone primes the young zebra finch s brain to be responsive later in life to testosterone again and so develop the tendency to sing. Insofar as a zebra finch can be said to have a mind, the hormone is a mind-altering drug.
Much the same applies to human beings. Here the evidence comes from a series of natural and unnatural experiments. Nature has left some men and women with abnormal hormonal doses, and in the 1950s doctors changed the hormonal conditions of some wombs by injecting some pregnant women with certain hormones. Women with a condition known as Turner s syndrome (they are born without ovaries) have even less testosterone in their blood than do women who have ovaries: (Ovaries produce some testosterone, though not as much as testicles do.) They are exaggeratedly feminine in their behavior, with typically a special interest in babies, clothes, housekeeping, and romantic stories: Men with less than usual testosterone in their blood as adults—eunuchs, for example—are noted for their femininity of appearance and attitude. Men exposed to less than usual testosterone as embryos—for example, the sons of diabetic women who took female hormones during pregnancy—are shy, unassertive, and effeminate: Men with too much testosterone are pugnacious: Women whose mothers were injected with progesterone in the 1950s (to avert miscarriage) later described themselves as having been tomboys when young; progesterone is not unlike testosterone in its effects. Girls who were born with an unusual condition called either adrenogenital syndrome or congenital adrenal hyperplasia are equally tomboyish: This disorder causes the adrenal gland, near the kidney, to produce a hormone that acts like testosterone instead of cortisol, its usual product."
Somewhat like in the zebra finches, there are two periods when testosterone levels rise in male children: in the womb, from about six weeks after conception, and at puberty. As Anne Moir and David Jessel put it in a recent book, Brain Sex, the first pulse of hormone exposes the photographic negative; the second develops it:This is a crucial difference from the way the hormone affects the body. The body is masculinized by testosterone from the testicles at puberty, whatever its womb experience. But not the mind. The mind is immune to testosterone unless it was exposed to a suf ficient concentration (relative to female hormones) in the womb. It would be easy to engineer a society with no sex difference in attitude between men and women: Inject all pregnant women with the right dose of hormones, and the result would be men and women with normal bodies but identical feminine brains: War, rape, boxing, car racing, pornography, and hamburgers and beer would soon be distant memories: A feminist paradise would have arrived:
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