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Ironically, researchers are not entirely sure how or even why humans produce two sexes in the first place. (Why not just one -- or even three -- as in some species?) What is clear is that the two sexes originate with two distinct chromosomes. Women bear a double dose of the large X chromosome, while men usually possess a single X and a short, stumpy Y chromosome. In 1990 British scientists reported they had identified a single gene on the Y chromosome that determines maleness. Like some kind of biomolecular Paul Revere, this master gene rouses a host of its compatriots to the complex task of turning a fetus into a boy. Without such a signal, all human embryos would develop into girls. "I have all the genes for being male except this one, and my husband has all the genes for being female," marvels evolutionary psychologist Leda Cosmides, of the University of California at Santa Barbara. "The only difference is which genes got turned on."
Yet even this snippet of DNA is not enough to ensure a masculine result. An elevated level of the hormone testosterone is also required during the pregnancy. Where does it come from? The fetus' own undescended testes. In those rare cases in which the tiny body does not respond to the hormone, a genetically male fetus develops sex organs that look like a clitoris and vagina rather than a penis. Such people look and act female. The majority marry and adopt children.
The influence of the sex hormones extends into the nervous system. Both males and females produce androgens, such as testosterone, and estrogens -- although in different amounts. (Men and women who make no testosterone generally lack a libido.) Researchers suspect that an excess of testosterone before birth enables the right hemisphere to dominate the brain, resulting in lefthandedness. Since testosterone levels are higher in boys than in girls, that would explain why more boys are southpaws.
Subtle sex-linked preferences have been detected as early as 52 hours after birth. In studies of 72 newborns, University of Chicago psychologist Martha McClintock and her students found that a toe-fanning reflex was stronger in the left foot for 60% of the males, while all the females favored their right. However, apart from such reflexes in the hands, legs and feet, the team could find no other differences in the babies' responses.
One obvious place to look for gender differences is in the hypothalamus, a lusty little organ perched over the brain stem that, when sufficiently provoked, consumes a person with rage, thirst, hunger or desire. In animals, a region at the front of the organ controls sexual function and is somewhat larger in males than in females. But its size need not remain constant. Studies of tropical fish by Stanford University neurobiologist Russell Fernald reveal that certain cells in this tiny region of the brain swell markedly in an individual male whenever he comes to dominate a school. Unfortunately for the piscine pasha, the cells will also shrink if he loses control of his harem to another male.