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Evolutionary Theories for Homosexuality

As with all evolutionary theories, there are at least two questions that must be answered to explain homosexuality. First, we must justify how homosexual orientations could survive and/or reproduce in light of selection pressures. This is the question of adaptation. Second, any evolutionary explanation must also justify how a given trait could have arisen. No matter how adaptive a trait may be, it could not have arisen out of nothing, but must have a past. This is the question of structure. I will examine separately each of these questions.

Biological Adaptation and Male Homosexuality

There are at least three different levels at which homosexuality may be seen as adaptive, and each of these levels has its own implications with regard to how homosexuality manages to continue among humans. At the most concrete level, we might posit genetic differences between homosexuals and heterosexuals. I'll call this the "genetic" level. Some biological studies seem to support this view. At a more "abstract" level, we might assume that homosexuals and heterosexuals have the same genes, but that during ontogeny universal genetically determined "programs" get "switched on" or "switched off" depending on environmental influences. Thus, people may have the same genotypes, but these genotypes may produce different phenotypes in different situations. This might be called the "epigenetic" level. Many theories (including Freudian ideas) about the psychological dynamics behind homosexuality are of this type. At the most abstract level, homosexuality might be unrelated to genetic differences or to universal genetic programs that determine ontogeny. Instead, homosexuality may be culturally determined in a manner far removed from direct genetic influences. Following Dawkins' (1976), we might call this the "memic" level.

Evolutionary arguments have been offered for all of these levels. For example, Symons (1979) argues that homosexuality results from a meme that takes advantage of the inborn male propensity to find sexual variety interesting. Normally this propensity would be adaptive because it would encourage males to attempt sexual relations with many women, which would result in more offspring. But a meme could parasitize this propensity, redirecting sexual interest to non-adaptive objects, as in fetishism or homosexuality.

Epigenetic arguments have been inspired by work on pre-natal hormones. DÓrner and his colleagues (cited in LeVay 1994) link stress during pregnancy to hormonal effects on the fetus that would lead to homosexuality. This could be part of a mother's adaptive reproductive strategy. That is, in times of stress, when it is difficult to raise children, it may be adaptive to have some homosexual children who could help their siblings raise offspring instead of having offspring of their own.

Memic and epigenetic arguments may explain some aspects of homosexuality, such as the cultural variations in practices or identities, or specific behaviors of homosexuals (femininity for example). However, the biological studies of homosexuality, together with the extraordinary cross-cultural similarities of exclusive homosexuals (Whitam 1983) suggest that some aspects of homosexuality (e.g. orientation) are at least partly determined by genes. This part requires explanation.

The basic problem for genetic explanations of homosexuality is to account for how a gene that does not lead to offspring could survive the pressures of natural selection. One possibility is that a maladaptive gene for homosexuality might re-occur repeatedly in a population if it results from the frequent mutation of a gene that is normally adaptive. However, people with maladaptive genes rarely exceed 1% of the population, while male homosexuality apparently occurs much more frequently (Whitam and Mathy 1986; Gadpaille 1980; Diamond 1993). Another possibility is that a recessive gene for homosexuality might have advantages that allow it to pass on to future generations. For example, homosexuals could directly help their relatives (who share the recessive gene) raise more children. This "kin selection" hypothesis predicts more homosexuality in those societies where homosexuals could most help their relatives (for example, in endogamous societies where homosexuals would live nearby). Nevertheless, cross-cultural research shows that this is not the case (Werner 1979). Another possibility is that a homosexual gene itself, when combined with non-homosexual genes, may provide reproductive advantages. This latter scenario is sometimes known as the "heterozygous" hypothesis, although it is really about any type of gene combination (LeVay 1994; Sommer 1990).

The main question for the heterozygous argument is to imagine a possible advantage of homosexual genes when combined with heterosexual genes. Kirsch and Rodman (cited in Sommer 1990) suggest that this advantage may have something to do with dominance hierarchies. The maintainence of these hierarchies presumably helps animals, including humans, live peacefully together, and this peaceful living provides advantages to the group, and to the individuals in the group. The key to the argument is the disadvantage of an animal that had only dominance genes. While an animal that "fights and runs away, lives to fight another day," an animal that never gives in often dies young. In this light, consider Chagnon's (1988) discovery that Yanomamo men who had killed more enemies had more offspring than milder men. Chagnon used this correlation to argue that aggressivity really does enhance reproductive success. But Chagnon's study suffers from a sampling problem, because it included only living males. I'd wager that the more aggressive males also had a greater probability of dying before ever being able to reproduce at all! Thus, on average, milder men may have as many or more offspring than aggressive men.

The heterozygous argument suggests that homosexuality results from genes for submissive behavior. An animal possessing only submissive genes would fail to reproduce for lack of trying. But an animal possessing only genes for dominance would also fail to reproduce because it would get killed for taking too many risks. It is the "heterozygous" males, who would most likely pass on their genes. By the genetic laws of probability, this would leave every generation with a certain percentage of individuals at the extremes. For example, if each of a pair of chromosomes had only one "homosexual" locus with only two possible alleles (one for dominance and one for submissiveness) this would leave a heterozygous couple with 25% of their offspring homozygous for submissiveness, 50% heterozygous, and 25% homozygous for dominance. If, however, homosexuality were the result of the interaction of five gene loci, possibly spread across different chromosomes, then only 3.125% (1/22222) of births would be homozygous for submissiveness. As Wilson (1994) points out, some genetic diversity is valuable to individuals in that it permits them to adapt to different niches within the group, so different degrees of submissiveness may permit occupation of different niches.

The heterozygous argument still needs to be tested. It predicts, for example, that the relatives of exclusive homosexuals should have less dominant personalities than the general population, in addition to predicting that homosexuals themselves would be more likely than heterosexuals to avoid fights (at least physical ones), for which there is already abundant evidence (Whitam and Mathey 1986).

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