This chapter explores possible causes of sexual orientation. In recent decades there has been a profusion of research into both proximate and ultimate causes underlying variation in sexual orientation. The latter line of inquiry is built upon evidence suggesting that there are genes predisposing individuals to homosexuality, and it seeks to explain how such genes might be maintained despite fitness costs associated with homosexuality in terms of reproductive success. Multiple hypotheses have been proposed, with modest supporting evidence. Research into proximate causation has identified...

This chapter explores possible causes of sexual orientation. In recent decades there has been a profusion of research into both proximate and ultimate causes underlying variation in sexual orientation. The latter line of inquiry is built upon evidence suggesting that there are genes predisposing individuals to homosexuality, and it seeks to explain how such genes might be maintained despite fitness costs associated with homosexuality in terms of reproductive success. Multiple hypotheses have been proposed, with modest supporting evidence. Research into proximate causation has identified neuroanatomical differences between gay and heterosexual individuals, effects of prenatal hormonal signaling on sexual orientation, and associations between the number of older brothers men have and their sexual orientation. The chapter seeks to illuminate both proximate factors influencing the spectrum of sexual orientations and ultimate causes maintaining this variation during the evolutionary history of our species.