The study of homosexuality has been a controversial subject in psychology. Before 1973 homosexuality was considered abnormal behaviour – a mental illness. Homosexuality was believed to be rooted in family problems (an overbearing mother, an absent father) or “arrested development.” Homosexuality was also labelled as “criminal behaviour;” the behaviour was illegal in many states of the US as well as several European countries. To this day, homosexuality is punishable in many countries around the world.

The removal of homosexuality from the DSM as an “abnormal behaviour” by the American Psychological Association in 1973 encouraged psychologists to examine the potential roots of sexuality. The prevailing thought at the time was that sexuality was the result of various environmental factors.

It was difficult to study gay men and lesbian women in the 1970’s and 80’s because of the stigma of being homosexual. “Coming out” in order to be studied was risky to family relationships, career, community membership, and in many cases, freedom from arrest. It wasn’t until the late 1980’s, with the spread of HIV infection and the disproportionate number of deaths from AIDS among gay men, that gay men were willing to be open about their sexuality. This allowed early research which appears to indicate a biological root for sexuality.

One of the first studies of this kind was done by Simon LeVay (1991).

The aim of LeVay’s study was to examine the nuclei of the anterior hypothalamus of the brain. This part of the brain plays a role in the regulation of male-typical sexual behaviour. Up until this study, it was believed that the INAH 3 nucleus of the hypothalamus was sexually dimorphic – that is, that males had larger INAH 3 nuclei than women – and this was due to sexual differences. LeVay hypothesized that the INAH3 would be larger in individuals sexually oriented toward women (heterosexual men and homosexual women) and smaller in individuals sexually oriented toward men (heterosexual women and homosexual men). In other words, he believed that it might not be one’s sex, but one’s sexual orientation that affected the size of the nucleus.

Brain tissue was obtained from 41 subjects at routine autopsies of people who died at seven metropolitan hospitals in New York and California. 19 were homosexual men who had died of complications resulting from AIDS. 16 were presumed heterosexual males – six of them died of AIDS and ten of other causes. Six subjects were presumed heterosexual women – one had died of AIDS, the five others of other causes. No lesbian women were used in the study as this was a “hidden population” (see background information above). The mean age of each sample was 38.2, 42.8, and 41.2 respectively.

All brain tissue was stained in order to measure the size of the nuclei using an electron microscope. The tissue samples were coded and subsequent analysis was done without the knowledge of the subject group to which each sample belonged.

Consistent with the hypothesis, the volume of the INAH3 nuclei was more than twice as large in the heterosexual men (0.12 mm³) as in homosexual men (0.051 mm³). The difference was still significant when the homosexual men were compared with only the six heterosexual men who died of complications from AIDS. There was a similar difference between the heterosexual men and the women (0.056 mm³). There was no significant difference in the volume of INAH3 between the heterosexual men who died of AIDS and those who died of other causes.

The data support the hypothesis that INAH3 is dimorphic not with sex but with sexual orientation, at least in men. However, LeVay noted in his report that the study, though opening up the possibility that sexuality may have biological roots, does not allow us to determine whether the size of the INAH 3 in an individual is the cause or consequence of that individual’s sexual orientation – or if the size of the INAH 3 is actually influenced by a third variable.

There have been several criticisms of LeVay's study. Many of these criticisms he discusses in his original report of the study. First, one cannot infer cause and effect. Such data show only correlation, not causation. If one interprets the data as indicating that the INAH3 of male homosexuals is smaller than that of male heterosexuals, one still does not know whether that is a cause of homosexuality or a result of it.

LeVay also showed concern from the role of AIDS on the brain. Although his research attempts to rule out the role of HIV infection by comparing the heterosexual and homosexual men that died of AIDS, the sample size is very small, so it may not be possible to do this effectively. In addition, AIDS manifests different symptoms. How the individuals died – as well as the treatments that they received – may have played a role in the results.

Another criticism is his choice of descriptive statistics. In using the mean, an average density for each group was determined. However, this does not allow us to understand the individual differences. In fact, one of the INAH3 from a homosexual male was larger than all but one of those from the 16 heterosexual males in the study.

Finally, there is the question of construct validity. Homosexuality has many forms and is probably not a single phenotype. LeVay also uses the words “presumed heterosexual” in his report to show that he cannot guarantee that HIV was not contracted through gay sex. In addition, human sexuality is a very complex behaviour; to argue that it is determined by a singular nucleus in the brain is considered by many to be overly reductionist.

The greatest strength of LeVay’s study is the way in which it opened up biological research on the question of the origins of sexuality. It played a significant role in changing attitudes about “lifestyle choice,” and started a discussion about whether sexual orientation may not be a choice – a position taken today by the APA and BPS.

LeVay, Simon. A Difference in Hypothalamic Structure between Heterosexual and Homosexual Men. Science, New Series, Vol. 253, No. 5023 (Aug. 30, 1991), pp. 1034-1037 Published by American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/2878803