The brains of homosexual men are structurally different from those of heterosexual men in a region thought to influence male sexual behavior, according to a study published today.
The discovery, if confirmed, would be the first detection of a distinct pattern in the brain that could help explain sexual preference among men.
"The main result of this research is to show that it is possible to study sexual orientation at the biological level," said Dr. Simon LeVay, a neurobiologist at the Salk Institute in La Jolla, Calif. "It's not just the province of the psychologists and the psychoanalysts anymore."
In the new work, which appears in today's issue of the journal Science, Dr. LeVay reports that in homosexual men one segment of the hypothalamus, an important structure in the forebrain, is only one-quarter to one-half the size of the same region in heterosexual men.
"It's quite a striking observation, and as far as I know, it's unprecedented," said Dr. Thomas R. Insel, a behavioral neuroscientist at the National Institute of Mental Health in Rockville, Md. "Simon LeVay is a top-notch, world-class neuroanatomist, and this is a very provocative paper."
But other researchers warned that the findings are preliminary and that they involve only a small number of brain tissue samples. They said that the results by no means prove that homosexuality is caused by a particular variation in the brain.
"This just points in a possible research direction," said Dr. Richard Nakamura, chief of the cognitive and behavioral neuroscience research branch at the National Institute of Mental Health. "Biology is clearly not destiny, and this shouldn't be taken to mean that you're automatically homosexual if you have a structure of one size vs. a structure of another size."
The size of the region studied by Dr. LeVay is roughly equal in gay men to that in heterosexual women, and in each case the area is often so tiny as to be essentially non-existent.
In the new research, Dr. LeVay examined thin slices of autopsied brain tissue from 19 homosexual men, 16 presumed heterosexual men and six women also thought to have been heterosexual. The average age at the time of death for all three groups was about 40, and thus the brains had not yet undergone the profound changes known to be associated with age.
He focused on a particular segment of the hypothalamus known as the third interstitial nucleus of the anterior hypothalamus, which previous studies had shown to differ significantly between men and women. Measuring the volume of cells in the region, he found that in the heterosexual men it averaged about the size of a large grain of sand but that in the women and the gay men was almost undetectable.
Dr. LeVay proposes that the hypothalamic segment could be responsible for inspiring males to seek females and that its absence in men would be a possible element predisposing them toward homosexuality.
But Dr. LeVay and many other researchers emphasize that the results remain to be confirmed in follow-up studies.
Dr. LeVay also said it was not yet known if the difference arises during the development of the brain, or whether a man's homosexual preferences could somehow influence the contours of certain neural pathways later in life. If that were the case, he said, his finding would be a mere consequence of homosexuality, rather than a possible cause of it.
"It's axiomatic that sexual orientation is going to be represented in the brain somewhere," said Dr. John Money, a professor emeritus at the Johns Hopkins School of Medicine who has long studied the origins of sexual behavior. "The really interesting questions are when did it happen and how did it get there?"
Other caveats of the new report abound. The gay men whose brain samples were examined all died of AIDS, a disease that infiltrates the central nervous system. But Dr. LeVay said that factor was unlikely to account for the discrepancy observed in the hypothalamus because six of the men presumed to have been heterosexual also died of AIDS, contracted as a result of intravenous drug use. Nevertheless, these men had hypothalamic structures several times the size of those in gay men, he said.
Although nobody can predict the political and sociological implications of the discovery if it is confirmed, Dr. LeVay and others said that if homosexuality is viewed as something innate, rather than, say, a perverted reaction to a bad upbringing, then gays might be more easily accepted.
"The gay community has worked very hard and long to persuade psychiatrists that this shouldn't fall into the category of a mental illness," said Dr. Ingeborg L. Ward, professor of psychology at Villanova University, near Philadelphia. "This would provide possible scientific basis for that viewpoint."
Dr. LeVay, who is gay himself, said that the gay men he has discussed his work with are in general pleased by the idea that sexual orientation may be at least partly inborn.
"They say their sexual personality is something very deeply ingrained in them," he said, "so they're not surprised to be told there are structural differences in the brain."