Evolutionary Perspectives on Male Homosexuality: A Literature Review. Yasmina Mashmoushi, Mitan Mzouri

Evolutionary Perspectives on Male Homosexuality: A Literature Review. Yasmina Mashmoushi, Mitan Mzouri. Proceedings of Manitoba's Undergraduate Science and Engineering Research, Vol 4, issue 1, 2018, http://dx.doi.org/10.5203/pmuser.201841619

Abstract: This review provides a comprehensive coverage of the leading evolutionary hypotheses to date on male homosexuality (namely the sexual antagonism model, the tipping-point model, and the kin selection hypothesis). It does so by first (1), surveying prominent findings on the nature and biological causes of male homosexuality; second (2), discussing the effects of male homosexuality on individual fitness; and third (3), outlining the contending evolutionary theories on male homosexuality and critically evaluating each against current pertinent empirical evidence. This review reveals that male homosexuality is a complex, multifaceted phenomenon influenced by an interplay of genomic and environmental factors that may have had unique evolutionary trajectories. Thus, there is likely more than one evolutionary mechanism at play responsible for the maintenance of gay alleles in the human population. Current research largely supports the notion that gay alleles bestow fitness benefits on heterosexual carriers. The tipping-point model and sexual antagonism model, but not the kin selection hypothesis, are in line with current empirical evidence. Future research into the genomic underpinnings of sexual orientation in homosexual males and its genetic equivalents in heterosexual males and females may allow for further evaluation of these hypotheses.

Keywords: human evolution, evolutionary psychology, mating preferences, sexual orientation, homosexuality

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The tipping-point model of male homosexuality, popularized byEdwardMiller, posits that the group of alleles that code for a homosexual orientation in gay men confer strong fitness benefits in heterosexual men by coding in them a certain level of psychological femininity68. According to Miller, if only a few of these alleles are inherited by males, their reproductive success is enhanced via the expression of attractive, albeit feminine traits such as kindness, empathy, andsensitivity68. However, if too many of these alleles are inherited by males, a tipping-point is reached, at which even their mate preferences become feminized68. Miller came up with a simplified version of his theory to better illustrate it. He asks the reader to imagine that there are five different genes that each help code for an individual's place along a masculine-feminine continuum. Each of these five genes have two respective alleles: one that pulls the individual to the masculine side of the continuum and one that pulls the individual to the feminine side of the continuum. According to his simplified model, if a man inherits all five of the "feminine-pulling alleles", he will be homosexual and if he inherits less than five,he will not. Homosexuality would continue to persist in the human populationif a strong reproductive advantage is conferred on individuals possessing some copies of these feminine-pulling alleles. According to Miller, a low doseo f these feminine-pulling  alleles significantly enhances a heterosexual male carrier's reproductive success. But in the less common, spontaneous occasion that a significantly large dose of these feminine-pulling alleles is inherited, the male carrier’s sexual orientation is altered and his fitness adversely affected. Nonetheless, these alleles would continue to persist in the population if they confer an overall reproductive advantage on thei rmale carriers68. Consistent with the tipping-point hypothesis, homosexual men are reported to be more sensitive, kind, and empathetic than heterosexual men,which have been characteristically deemed to be feminine attributes70. Furthermore, studies have found that a higher level of psychological femininity in straight men is associated with a greater number of female partners, suggesting that psychological femininity is attractive to women71,72. This could be because psychological femininity indicates a nurturing disposition which could help rear offspring. In another study, researchers predicted that if the tipping-point model of male homosexuality were correct, then heterosexual men with a homosexual maletwin should have more attractive feminine-pulling alleles and thus more opposite-sex partners than members of heterosexual twin pairs15. The findings of this large community-based twin study(N=4904)supported this prediction; heterosexual males with a homosexual male twin had significantly more children, significantly more opposite sex partners, and were significantly younger at their first age of intercourse than members of heterosexual male twin pairs (p<0.001)15. The results of these and similar studies have made the tipping-point model one of the leading evolutionary theories on male homosexuality to date67.

Another possibility is that the alleles responsible for male homosexuality code for psychologically or physically feminizing traits in both men and women21,67. Thes exual antagonism model suggests that an allele that is detrimental tothe fitnessof one sex could be maintained in the populationso long as it is beneficial to thefitness of the other sex21. An allele that make sits bearer attracted to men and more feminine provides an obvious reproductive advantage to women, but an obvious reproductive disadvantage to men21. This allele would code for same-sex attraction if it appears in a male's genome but would maintainanet evolutionary benefit if this occurs rarely21. There is asignificant amount of evidence for this theory. Numerous studies have found significantly greater fecundity in the female matrilineal relatives of homosexual men (i.e. their mothers, aunts and grand-mothers)as compared to heterosexual men 21,73,74,75. Some other studies have also found that the female relatives of homosexual males have significantly fewer abortions and gestational complications than the female relatives of heterosexual males12,74. Moreover, homosexual men have been found to have an excess of matrilineal but not patrilineal male homosexual relatives as compared to heterosexual men21,73. According to researchers, even a modest increase in the reproductive capacity of females carrying these gay alleles could easily account fort heir maintenance at high levels in the population21,76.