LeVay’s study is very preliminary. He analyzed brain tissue from only 41 cadavers (16 straight men, 6 straight women and 19 gay men–LeVay was unable to obtain brain tissue from lesbians). He found that one bundle of neurons in the hypothalamus (diagram) was nearly three times as large in heterosexual men as in homosexual men and heterosexual women. But his conclusions were loaded with caveats. As LeVay noted, some of the subjects died of AIDS, which in its late stages can affect the brain. LeVay ruled out the disease as a confounding factor: he found that homosexual men had smaller hypothalamic bundles than straight men who had died of AIDS, suggesting that it is not AIDS that shrinks the clump of neurons. Other neuroscientists weren’t as ready to dismiss an AIDS connection, however. Also, measuring brain structures is notoriously difficult and controversial. The areas LeVay scrutinized are even smaller than snowflakes, and neuroscientists cannot agree on whether the most meaningful gauge is the volume of the region (the yardstick LeVay used) or its number of neurons.
Despite the questions, the study fit the emerging theory that sexual orientation is determined more by nature than nurture. In 1978, experiments on male primates found that lesions in the hypothalamus left the monkeys’ sex drive vigorous but made them lose interest in females. Studies that followed human twins separated at birth concluded that identical pairs are more likely to be gay than are nonidentical pairs, notes Scott Mendelson of Rockefeller University, “suggesting it’s because of similarities in the brains and not simply the way they’re raised.”
But it is simplistic to conclude that homosexuality has only biological causes, for LeVay’s results leave a chicken-and-egg question. Does a small bundle of neurons in the hypothalamus cause homosexuality, or might homosexual orientation cause that portion of the brain to shrink? Scores of studies have found that neurons change in response to experience, such as learning a maze. “You could postulate that brain change occurs throughout life, as a consequence of experience,” says neurophysiologist Kenneth Klivington of Salk. Alternatively, an unknown factor might cause both homosexuality and the neuron differences that LeVay reports. The “cause” of homosexuality would then be that mystery agent–perhaps unusual exposure to testosterone in utero. (Such exposure has been shown to make boys left-handed and subject to allergies, for instance.) The next step is to try to replicate LeVay’s findings, discover what determines the size of the hypothalamus region, and when: before birth or after. The debate over the origins of sexual orientation is far from over.
