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Chapter 2 (continued)
Biologic Theories (cont.)
Gene Studies
In 1993 Dean Hamer and others published results of a study investigating the pedigrees and DNA of 114 homosexual men and their families. They found “increased rates of same-sex orientation . . . in the maternal uncles and male cousins of these subjects, but not in their fathers or paternal relatives.”11(p321) This finding suggests that there may be a sex-linked genetic transmission of some factor that increases the likelihood of same-sex orientation. The researchers also selected 40 families with two homosexual brothers and studied their DNA, along with the DNA of their mothers and siblings, where available. They found a small section (or “marker”) on the X chromosome that was substantially different in a percentage of the known homosexual individuals. They labeled this marker Xq28. In 33 of the 40 pairs of gay brothers both men had this distinct chromosomal marker. The researchers conclude that, “it appears that Xq28 contains a gene that contributes to homosexual orientation in males.”11(p325)
Because of the complexity of the Xq28 region of the X chromosome and the small number of people tested, the researchers were not able to identify a specific gene. To determine the exact influence of any gene on human behavior requires that the exact DNA sequences be mapped out in order to distinguish the role of inheritance from the influences of environment, experience, and culture. The researchers admit that they do not yet know what percent of all homosexuals have this genetic marker, that little is known of the actual role of this marker in the families studied, and that no information about the presence or absence of this marker in heterosexuals is yet available.11(p325) Furthermore, the research does not fully explain the seven pairs of gay brothers who did not both inherit the genetic marker.
Anne Fausto-Sterling and Evan Balaban point out several criticisms of this study. First, if this marker is truly linked to homosexuality, then the heterosexual brothers of the homosexuals studied should not have the marker. But Hamer’s study provides no data on non-gay brothers. Second, the figures used by the researchers to demonstrate a significant rate of homosexuality among relatives of those studied are controversial. Hamer used 2% as the base rate of homosexuality in the general population. Using this figure, his study showed an unusual rate of homosexuality among test subjects’ uncles and male cousins, supporting the idea that homosexuality is heritable. However, the base rate of homosexuality is difficult to determine and even a slightly higher base rate would make Hamer’s findings meaningless. Third, Fausto-Sterling and Balaban point out that, “Correlation does not necessarily indicate causation. A gene affecting sexual orientation . . . might do so very indirectly.” They suggest, as an example, that having any gene in common might increase the psychological identification between brothers and thereby “influence their similarity in such matters as sexual orientation.”12
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