Sizing Up The Sexes

Scientists are discovering that gender differences have as much to do with the biology of the brain as with the way we are raised

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Many researchers suspect that, in humans too, sexual preferences are controlled by the hypothalamus. Based on a study of 41 autopsied brains, Simon LeVay of the Salk Institute for Biological Studies announced last summer that he had found a region in the hypothalamus that was on average twice as large in heterosexual men as in either women or homosexual men. LeVay's findings support the idea that varying hormone levels before birth may immutably stamp the developing brain in one erotic direction or another.

These prenatal fluctuations may also steer boys toward more rambunctious behavior than girls. June Reinisch, director of the Kinsey Institute for Research in Sex, Gender and Reproduction at Indiana University, in a pioneering study of eight pairs of brothers and 17 pairs of sisters ages 6 to 18 uncovered a complex interplay between hormones and aggression. As a group, the young males gave more belligerent answers than did the females on a multiple-choice test in which they had to imagine their response to stressful situations. But siblings who had been exposed in utero to synthetic antimiscarriage hormones that mimic testosterone were the most combative of all. The affected boys proved significantly more aggressive than their unaffected brothers, and the drug-exposed girls were much more contentious than their unexposed sisters. Reinisch could not determine, however, whether this childhood aggression would translate into greater ambition or competitiveness in the adult world.

While most of the gender differences uncovered so far seem to fall under the purview of the hypothalamus, researchers have begun noting discrepancies in other parts of the brain as well. For the past nine years, neuroscientists have debated whether the corpus callosum, a thick bundle of nerves that allows the right half of the brain to communicate with the left, is larger in women than in men. If it is, and if size corresponds to function, then the greater crosstalk between the hemispheres might explain enigmatic phenomena like female intuition, which is supposed to accord women greater ability to read emotional clues.

These conjectures about the corpus callosum have been hard to prove because the structure's girth varies dramatically with both age and health. Studies of autopsied material are of little use because brain tissue undergoes such dramatic changes in the hours after death. Neuroanatomist Laura Allen and neuroendocrinologist Roger Gorski of UCLA decided to try to circumvent some of these problems by obtaining brain scans from live, apparently healthy people. In their investigation of 146 subjects, published in April, they confirmed that parts of the corpus callosum were up to 23% wider in women than in men. They also measured thicker connections between the two hemispheres in other parts of women's brains.

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