Biology 202
2000 First Web Report
On Serendip

The quest to find a biological substrate for homosexuality resembles an earlier movement in research to determine the nature of I.Q. scores. Both revolve around finding a biological basis for differences in human behavior. Like the previous research on intelligence, the research on homosexuality is plagued with difficulties. One large obstacle in this type of research is the difficulty in making statements about causality from correlational studies (simple, linear relationships between two variables) (4). In other words, it is difficult to determine what comes first the chicken or the egg.

Recently, a plethora of research has been done on determining brain differences between homosexuals and heterosexuals. Simon LeVay's work on the anterior hypothalamus is the most widely discussed and debated research in the area of sexual orientation and neuroscience. This paper will examine Simon LeVay's research which declares that there is a physiological difference between the brains of heterosexual and homosexual men (5). Furthermore, it will discuss the shortcomings of his research and the need for more conclusive studies on sexuality and its link to biology.

Before delving into LeVay's study, it is important to examine pre-existing evidence for brain differences and sexual dimorphism in humans. Prenatal and perinatal hormones organize the components of the brain and create either a male or female brain and thereby produce male or female typical behavior (5). The most sexually dimorphic region of the brain is located in the hypothalamus. The sexually dimorphic nucleus of the preoptic area (SDN-POA) is the most differentiated portion of the brain between sexes. In humans, the SDN is twice as large in young adult men than it is in women. The SDN-POA also plays an essential role in the secretion of gonadotropin, maternal behavior, and sexual behavior in many mammals and therefore implies that it is critical in human sexual behavior. There are four cell groups located in this preoptic-anterior hypothalamic area. These four groups have been termed the Interstitial Nuclei of the Anterior Hypothalamus (INAH 1-4). Research done on these cell groups shows that the INAH-3 is about three times larger in the male brain than it is in the female brain (6). The INAH-2 is twice as large in the male brain than the female brain (6). The importance of these regions in sexual differentiation (between the sexes) led scientists to believe that sexual orientation could be on a continuum in this area of neurological research (7).

>From this previous work on the anterior hypothalamus, LeVay hypothesized that the INAH 2 or 3 would be larger in men who sexually prefer women than it would be for men who prefer males. In essence, LeVay proposed that male homosexuals have brains that are different from heterosexual men and similar to the brains of women. LeVay examined post-mortem tissue from 41 subjects most of whom had died from AIDS. His research found a correlation between brain structure and sexual orientation. (7) LeVay found a significant difference in the size of the INAH-3 in heterosexual men and homosexual men. The INAH-3 was two to three times larger in heterosexual men than in gay men. He found no significant difference in size between homosexual men and women. Regarding his results, LeVay said, "The discovery that a nucleus differs in size between heterosexual and homosexual men illustrates that sexual orientation in humans is amenable to study at the biological level." (2)

However, there are numerous problems with Levay's study which have clouded the reliability of his findings. It is quite possible that the correlation of INAH 3 volume and sexual orientation are due to confounding factors such as AIDS (3). This casts doubt on already ambiguous results of whether or not sexual orientation is caused by brain differences or whether the samples LeVay used were unreliable due to the degenerative effects of AIDS on the brain. The latter, however, is unlikely, since LeVay's control subjects were also men who had died of AIDS. Another problem with LeVay's study was that the INAH 2 was not dimorphic although it was predicted to be. As mentioned above, the INAH 2 was twice as large in the male brain than in the female brain and LeVay expected to see smaller INAH 2 nuclei in homosexual men (6). LeVay did not find a significant difference in the size of the INAH 2 of homosexual and heterosexual men (7). The existence of exceptions in Levay's study also challenges his conclusions. For example, he found heterosexual men with small INAH-3 nuclei and homosexual men with very large INAH-3 nuclei. Other questions have also been raised about the ambiguous nature of the sexual histories of his subjects. A common argument that disputes LeVay's research is his dualistic view of sexuality. LeVay's subjects are labeled as either homosexual or heterosexual. Although this make research simpler, it is unlikely that division works well in the real world (1) . Sexuality would best be defined with as continuum with varying degrees of behavior (1) .

Even if LeVay's work is reproduced and receives the backing of the scientific community, the ineffable nature of human sexual preference makes this research difficult. For instance, observing differences between the brains of homosexual and heterosexual men does not tell us when these differences arose. It is quite possible that the INAH-3 in homosexual men started out the same size as the INAH-3 in heterosexual men and then became smaller due to their experienced sexual behaviors (7).

In light of LeVay's work, further research on homosexual dimorphic brain regions is being done. Allen and Gorski found that the anterior commissure was larger in homosexual men than in heterosexual men as well as in women (2) . This study does not support LeVay's idea that homosexual men's brains are analogous to those of females. However, it is important in exhibiting that there are differences in the brains of homosexuals and heterosexuals. Only through more research can these differences be identified.

Recently, concerns have arisen regarding the effects of this research on the public. The research into the biology of homosexuality can have profound implications on American society. If a biological factor for homosexuality were determined, it would have both a positive and negative impact on homosexual individuals. A positive implication of a biological determinant for homosexual behavior would be a greater acceptance of homosexuals in mainstream society. This would be accompanied with less discrimination and the inclusion of sexual orientation in hate crime legislation. Instead of being viewed as a moral decision, homosexuality would be viewed as an innate characteristic (like race) that justifies special protection (1) . However, finding a biological determinant for homosexuality could also adversely affect homosexuals by leading to genocide of gays and lesbians. Possible abuses could include the "rewiring" of homosexuals or abortions of fetal homosexuals (2) . Although the potential for abuse exists in this research, many scientists agree that this possibility exists in most biomedical research and is not reason enough to discontinue investigation (2) .

Currently, biological research into sexual preference is muddled and inconclusive. Furthermore, three distinct biological fields are involved in the most recent work on sexual orientation: neuroanatomy, psychoendocrinology, and genetics (2) . Among these three fields, more and more evidence for a biological determinant of homosexuality is surfacing. It is predicted that a biological substrate for sexual orientation will be found within the next ten years (1) . Genetic studies such as one done by Bailey and Pillard have shown a 52% concordance rate of homosexuality in monozygotic twins (4). This suggests that homosexuality is highly attributable to genetics. These findings as well as those of LeVay's (although debatable) are setting the groundwork for a biological cause of homosexuality. They are the key to unlocking the mystery of sexuality and are proving more than ever that brain does equal behavior.

WWW Sources

2) Atlantic Monthly , An Atlantic Monthly article which has a thorough introduction to scientific research on sexual orientation.

3)Johns Hopkins, Criticism of LeVay's study of sexual orientation on Johns Hopkins web site.

4) University of Texas , web site outlining the biological determinism of homosexuality.

5)Rice University, Lecture notes from Rice elucidating the relationship between hormones and behavior.

6)Journal of Neuroscience, Allen, Hines, Shryne, and Gorski journal article titled "Two sexually dimorphic cell groups in the human brain."

7)Simon LeVay's Web Page, Provides details about LeVay's work from his own perspective.

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