Love & Sex

The Science of Sex: The “Gay Brain” Revisited

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The "Gay Brain" Revisited

By Simon LeVay

In August of 1991, while I was on the faculty of the Salk Institute in San Diego, I published a short paper in Science in which I reported on a difference in brain structure between straight and gay men. The difference was in a group of nerve cells called "INAH3." This cell group is located in a brain region known as the hypothalamus, which among other tasks helps generate our sexual behavior. Other researchers had previously reported that INAH3 was larger (on average) in men than in women. What I reported, based on a study of about forty brains obtained at autopsy, was that INAH3 was also larger in straight men than in gay men.

Being a science nerd whose previous papers had been ignored by everyone except my mother, I didn't expect anyone to pay attention to this one either. So I was quite taken aback when the "Gay Brain," as it was quickly dubbed, provoked a tremendous media splash. On the front page of the San Diego Union-Tribune it even took precedence over the collapse of Communism, which happened on the same day. People's reactions to my report were very mixed: there were people who loved it and there were people who loathed it, but almost everyone had some opinion about it. One widely expressed opinion (with which I agreed) was that the study needed to be replicated before its findings could be considered part of accepted scientific knowledge.

One of the most outspoken critics of my paper was William Byne, a neuroscientist and psychiatrist who is now on the faculty at Mt. Sinai Medical Center in New York. Initially, Byne suggested that INAH3 might not even exist. If it did exist, he said, it was probably the same size in men and women. And any difference that I had found between gay and straight men was most likely due to differences in the men's cause of death, rather than their sexual orientation. (All the gay men in my study had died of complications of AIDS.)

Unlike most of my critics, who tended to be humanities professors or right-wing clerics, Byne had the appropriate training to do a replication study, and he quickly embarked on one. With the help of several colleagues, he set about collecting and analyzing a new set of brain samples. Before too long, Byne had verified that INAH3 does exist, that it is generally larger in men than women and that the cause of death (AIDS versus other diseases) does not affect its size. However, years went by without any announcement concerning the key question — is there a size difference related to sexual orientation?

Finally, on August 6 of this year, Byne presented his data at an international conference in Madrid. According to an account of the meeting in the Spanish newspaper La Raza, Byne reported that INAH3 was indeed larger in the straight men than in the gay men in his sample, in line with my 1991 Science report. Byne tells me that the difference was not as large as I had found, however. He declined to discuss the details of his statistical analysis prior to publication in a scientific journal.

La Raza did mention an interesting new detail. In spite of the size difference, Byne found that INAH3 contained the same number of nerve cells in the gay and straight men. If confirmed, this finding would suggest that there is no difference between gay and straight men in the earliest phase of brain development, when nerve cells are being generated and assemble into functional groups. Rather, the difference may arise at some later time, when the nerve cells in INAH3 are growing and forming connections.

What could cause such a difference in growth? There is a wide range of possibilities, ranging from genetic differences between individuals, differences in the levels of hormones (especially testosterone) that regulate cell growth in the hypothalamus and even differences in pre- or postnatal environment that could impact the growth of INAH3 through a variety of means.

Thus the findings on INAH3 to date do not prove a particular theory of sexual orientation as much as they point to ways in which such theories could be tested in the future. For example, if the technology becomes available to image INAH3 in living people, one could hope to establish the age at which the development of gay and straight men's brains diverges. Obviously, the factors causing the divergence must operate at or before that age. My prediction, based on animal experiments, is that the divergence happens before birth, but we don't yet know that for a fact.

Even without my research, we knew that there has to be some structural or chemical difference between the brains of gay and straight people. The alternative — that the difference resides only in patterns of brain activity — has been ruled out, because sexual orientation remains unchanged after all brain activity has been temporarily halted (by brain cooling or deep anesthesia, for example). What's surprising about the gay/straight difference in INAH3, then, is simply that it is so localized and obvious, rather than being diffusely spread through the synaptic architecture of the entire brain. This offers the hope that we will eventually be able to understand the origins of sexual orientation at a cellular level.