Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity

A. Camperio-Ciani, F. Corna, C. Capiluppi

Abstract

The Darwinian paradox of male homosexuality in humans is examined, i.e. if male homosexuality has a genetic component and homosexuals reproduce less than heterosexuals, then why is this trait maintained in the population? In a sample of 98 homosexual and 100 heterosexual men and their relatives (a total of over 4600 individuals), we found that female maternal relatives of homosexuals have higher fecundity than female maternal relatives of heterosexuals and that this difference is not found in female paternal relatives. The study confirms previous reports, in particular that homosexuals have more maternal than paternal male homosexual relatives, that homosexual males are more often later-born than first–born and that they have more older brothers than older sisters. We discuss the findings and their implications for current research on male homosexuality.