Brain feature linked to sexual orientation.

A comparison of 41 autopsied brains has revealed a distinct difference between homosexual and heterosexual men in the brain region that controls sexual behavior. The finding supports a theory that biological factors underlie sexual orientation, although it remains unclear whether the anatomical variation represents a cause or result of homosexuality, says neurobiologist Simon LeVay, who describes the study in the Aug. 30 SCIENCE.

LeVay, of the Salk Institute for Biological Studies in San Diego, found that a particular cluster of cells in the forefront of the hypothalamus was, on average, less than half as large in the brains of homosexual men as in their heterosexual counterparts. Although scientists have yet to identify the precise function of the clump, called the interstitial nuclei of the anterior hypothalamus 3 (INAH 3), the hypothalamus is known as the seat of the emotions and sexual drives.

LeVay obtained brain tissue from autopsies performed at seven hospitals in New York and California. His study included 19 homosexual men, 16 men presumed heterosexual, and six women presumed heterosexual. All of the homosexual men died of AIDS, as did six of the heterosexual men and one of the heterosexual women.

As a group, the heterosexual men had larger INAH 3 regions than either the homosexual men or the heterosexual women, LeVay reports. The size difference remained statistically significant whether or not the subjects died of AIDS, ruling out the possibility that it resulted from the disease, he says.

"This proves that you can study sexual orientation at the biological level," LeVay asserts. "There are differences in the brains of adult gay and straight men." However, he warns, "my data don't say how that difference arose."

Previous investigations have turned up other contrasts. In 1984, scientists at the State University of New York at Stony Brook confirmed a German study showing that male homosexuals differ from heterosexual males or females in their response to injections of the sex hormone estrogen (SN: 9/29/84, p.198). And last year, researchers at the Netherlands Institute for Brain Research reported that homosexual men had a larger suprachiasmatic nucleus than heterosexual men. The suprachiasmatic nucleus -- which plays a role in day-night rhythms -- also ] resides in the hypothalamus but has no known part in sexual behavior.

Psychologist Sandra F. Witelson at McMaster University in Hamilton, Ontario, reported last year that lesbians show a higher incidence of left-handedness than the general population. Witelson, who studies handedness as a measure of brain organization (SN: 8/17/85, p.102), told SCIENCE NEWS she has now found a similar incidence in homosexual men.

Together, the studies conducted to date "really show that there's something different in the [brain] anatomies of homosexuals and heterosexuals," she says.

Witelson and LeVay speculate that atypical levels of sex hormones may shape the brains of homosexuals in the womb or during childhood. This explanation does nto rule out environmental influences, Witelson notes. "A certain brain structure could be a predisposition to homosexual behavior that requires a certain environment to be expressed," she says.