Study Ties Part of Brain to Men’s Sexual Orientation : Medicine: San Diego researcher’s findings offer first evidence of a biological cause for homosexuality.


A tiny segment of the brain that governs sexual activity is smaller in gay men than in heterosexual men, a trait that offers the first specific evidence of a biological cause for homosexuality, a San Diego researcher has found.

The discovery, reported today in the journal Science, is certain to enliven scientific and social debate over the role of choice in determining whether a person is homosexual. Opinions began to form this week even before the study was publicly released.

Studying cadavers, neuroscientist Simon LeVay of the prestigious Salk Institute for Biological Studies in La Jolla found that a segment of the hypothalamus no larger than a grain of sand--called INAH 3--is only half as large in homosexual men as it is in heterosexual men.

Since that area of the brain is also smaller in women than in heterosexual men, LeVay speculates that the size of the brain segment may govern the sexual preference of both men and women, with a large INAH 3 dictating a preference for women and a small one creating a preference for men.


He cautions, however, that his findings do not establish cause and effect. It may well be, he noted, that some other fundamental biological condition that causes homosexuality may also lead to a shrinkage of INAH 3 in men.

“But (the) significance is that it indicates that we can study this aspect of human nature--sexual orientation--at a biological level and with biological tools rather than leaving it to psychiatrists,” said LeVay.

Although LeVay studied only a small number of brains, 41, his results are viewed as credible by experts because previous studies in animals have shown that INAH 3 plays a major role in governing sexual activity.

“We haven’t seen this in humans before, but it fits in with the pattern of research in this area that has been going on for a couple of decades,” said psychiatrist Judd Marmor of UCLA.


The discovery, said neurologist Dennis Landis of Case Western Reserve University, “would begin to suggest why male homosexuality is present in most human populations, despite cultural constraints. It suggests it’s a biological phenomenon.”

If verified by future studies, the discovery could have far-reaching legal and cultural implications.

Anti-discrimination laws around the country, for example, provide special protection “for minority groups which have a stigmatizing trait that is essentially immutable or unchangeable, such as race,” said UCLA psychiatrist and lawyer Richard Green.

“If we can demonstrate that sexual orientation is due to an inborn brain distinction . . . it would help raise the level of protection for homosexuals,” Green said.


Furthermore, Green added, religious opposition to homosexuality is based on the view that it reflects “a sinful choosing of a pattern of sexual behavior. One would hope that those voices would be somewhat quieted if, in fact, homosexuality turned out to be a brain variation that is not much different from left-handedness.”

Some gay leaders, however, have expressed concern that LeVay’s findings would be used as ammunition against homosexuals, saying such discoveries could lead to “fixing defects” or developing in utero screening of fetuses to discriminate against gays.

“There are people who will have the notion that society could cure or repair gays if they realign this chromosome or tweak that cell,” said Robert Bray, spokesman for the Gay and Lesbian Task Force in Washington, who had discussed his concerns with LeVay earlier this week.

Others applauded LeVay’s work, saying it supports what many gays have long believed, that they are gay by nature.


“I have felt all along that there is a biological basis, and this opens up a door which has long been closed in terms of real scientific research into the biology,” said Rochelle Diamond of the National Organization of Gay and Lesbian Scientists and Technical Professionals.

In interviews on Thursday, the self-effacing LeVay--who said he is himself gay--clearly seemed surprised and overwhelmed by the intense media attention his discovery had begun to attract, as well as the reaction of activists such as Bray.

“He (Bray) expressed concern that this could fuel prejudice and discrimination,” LeVay said. “I was surprised to hear him express such negative views. I would take a much more positive view that eventually work like this could lead to more acceptance.”

The new study is LeVay’s first venture into the controversial world of sexual orientation. His previous work has been on how the visual system develops and how changes in brain structure result from visual experiences in early life. He has a reputation as a careful and thorough investigator.


“LeVay is an excellent scientist,” said neurologist Richard K. Nakamura of the National Institute of Mental Health.

The stimulus for this study was work by UCLA neurologist Roger Gorski, who has studied the same region of the brain, the interstitial nuclei of the anterior hypothalamus or INAH. Several previous studies in animals had shown that INAH seems to control much sexual activity.

When Gorski looked at human brains, he found that one of the INAH regions, INAH 3, is twice as large in men as it is in women. LeVay speculated that such a difference in size could also control sexual preference.

To test his hypothesis, LeVay obtained brain tissue from 41 people who died in seven hospitals in Southern California and New York. The study was forced to rely on cadavers because the INAH 3 region is too small to be seen in living humans even with the most sophisticated imaging equipment.


“What’s really made this study possible is the AIDS epidemic,” he said. “For the first time, we have young men dying with a known sexual orientation. I had an opportunity to take advantage of this tragedy to find out something of value to science.”

Nineteen of the brains were from homosexual men who died of complications of AIDS. Sixteen brains were from presumed heterosexual men, six of whom had also died of AIDS. And six were from presumed heterosexual women. He was unable to obtain brains from lesbians because few contract and die from AIDS.

LeVay reported that he examined the brains without knowing the sex or the sexual orientations of the donors. When he later matched his observations with the personal information, LeVay observed that INAH 3 in heterosexual men was about twice as large as in women. But he also found that the same region in the gay men was much smaller, about the same size as in women.

LeVay said he found no difference in the size of INAH 3 among heterosexuals who died of AIDS and those who died of other causes, eliminating the disease as a likely source of the size difference.


But some researchers criticized LeVay’s work, speculating that the brains from AIDS patients might reflect the havoc wreaked by the virus, which is known to affect the brain and central nervous system.

“Certainly, AIDS affects the central nervous system. It would be nice to get some confirmation of this without the confounding variable of the disease,” said Dr. Gloria Hoffman, associate professor of physiology at the University of Pittsburgh School of Medicine.

Hoffman and others agreed that LeVay’s showed the direction for further study.

“This is a very interesting initial result,” said NIMH’s Nakamura, “but it will require a much larger effort to be convinced that there is a link between this structure and homosexuality.”