An evolutionary perspective on homosexuality: Testing the sexually antagonistic genes hypothesis through familial fertility analysis

There is robust evidence to the effect that homosexual individuals have systematically less offspring. The sexually antagonistic genes hypothesis claims that this fitness-related disadvantage of homosexual individuals is compensated by their other-sex relatives exhibiting greater fertility than the relatives of straight individuals. It would allow explain how genetic variants associated with homosexuality persist in the human population. Nevertheless, previous studies yielded conflicting results and, moreover, they focused almost solely on the fertility of gay men's relatives. We present the results of a large preregistered study based on data on fertility of both gay men's and lesbian women's relatives. We have analyzed the fertility and biological kin fertility of 693 gay men, 843 straight men, 265 lesbian women, and 331 straight women from Czechia and Slovakia. As expected, gay men and lesbian women sired significantly less offspring than straight individuals. However, we also found no evidence supporting the sexually antagonistic genes hypothesis, i.e., no difference between the fertility of other-sex relatives of homosexual and heterosexual individuals. Interestingly, though, paternal grandparents of gay men had more offspring than the paternal grandparents of straight men did. The mothers of lesbian women had a higher fertility than the mothers of straight women, but that could be attributed to intervening effects, such as the excess of older brothers in homosexual individuals. Our results suggest that mechanisms other than those predicted by the sexually antagonistic genes hypothesis are involved in the evolution of human homosexual orientation.