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The Evolutionary Origins of Same-Sex Attraction: Assessing Weak Negative Selection and Positive Selection Arguments

Menelaos Apostolou

Abstract

About one in five individuals is not attracted exclusively to members of the opposite sex. The presence of same-sex attraction in such a high frequency constitutes a major evolutionary puzzle. One solution that has been proposed is weak negative selection pressures exercised on deviations from exclusive heterosexual attraction in ancestral human societies. The current paper employs a population genetics model in order to assess the explanatory power of this hypothesis. By investigating different scenarios, it is found that weak selection pressure falls short in accounting for the observed prevalence rates of same-sex attraction. On this basis, it is argued that positive selection is also required. By examining the shape of the distribution of same-sex attraction, it is further argued that positive selection pressure has been exercised on heterosexual orientation with same-sex attraction in women and possibly in men, and on homosexual orientation in men. In addition, it is proposed that male preferences for same-sex attraction are the main source of positive selection of female same-sex attraction. The source(s) of positive selection of male same-sex attraction is, however, obscure.

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