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Chapter 2 (continued)
Biologic Theories (cont.)
Neuroanatomic Studies
Several studies have been conducted on the hypothalamus, which is a portion of the brain that regulates such things as body temperature and metabolism. Certain parts, or nuclei, of the hypothalamus are also believed to be involved in regulation of sexual functions. In studies on rodents, researchers have identified several nuclei that are different in size between males and females. The evidence obtained from rat studies sparked interest in searching for gender differences in the human brain. Several studies have subsequently been performed on the human anterior hypothalamus leading to the discovery of several nuclei in this region. But the studies produced inconsistent, and as yet unconfirmed, results regarding gender-related size differences in these nuclei.3(p234)
One of these studies, performed by Dr. Simon LeVay, reported a size difference in one nucleus (known as INAH3) between homosexual and heterosexual men.7(p1034) The findings of this study were well publicized as “the first evidence of a biological cause for homosexuality.”8 Many scientists were skeptical, however, such as Anne Fausto-Sterling, professor of medical science at Brown University, who said simply: “My freshman biology students know enough to sink this study.”9 Byne and Parsons point out several problems with LeVay’s research. First, LeVay cites studies on a nucleus found in the rat hypothalamus (known as SDN-POA) to support his conjecture that the INAH3 is important in the development of male sexual behavior. Evidence cited by Byne and Parsons shows LeVay’s assumptions regarding the comparison of INAH3 with SDN-POA to be either false or inapplicable.
Second, LeVay’s study had several technical flaws. The most important of these are the small sample size (only 19 homosexual men, 16 other men, and six women were sampled); the fact that all of the homosexual men and six of the other men had died of AIDS, which may affect the size of the nuclei being measured; inadequate sexual histories (the 16 men and the six women were assumed to be heterosexual although their actual sexual orientation was unknown); and inadequate medical histories of the subjects.3(p234-235)
The most important caution regarding this study comes from LeVay himself: “the results [of this study] do not allow one to decide if the size of INAH3 in an individual is the cause or consequence of that individual’s sexual orientation.”7(p1036)
Research conducted since LeVay’s study has provided strong evidence that sexual behavior may indeed cause changes in brain structure. Marc Breedlove of U.C. Berkeley carefully controlled the sexual behavior of two groups of rats, allowing one group to copulate while preventing mating behavior in the other group. When the spinal cords of these rats were compared following dissection, a significant difference was found in the size of neurons in the spinal nucleus of the bulbocavernosus (SNB), a part of the rat brain known to be active in males during copulation. Breedlove’s conclusion is that “It is possible that differences in sexual behavior cause, rather than are caused by, differences in brain structure.”10
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