In 1996, a particularly devastating nail was driven into the coffin of “homosexuality is a choice” rhetoric. A team of researchers found that the more older brothers a boy has, the greater the likelihood that he will grow up to be gay. Since the research was first published in the American Journal of Psychiatry, many other scientists have replicated the fraternal birth-order (FBO) effect on male sexual orientation. One promising hypothesis to explain this phenomenon is “maternal immunization”; that is, that after delivering several sons, a woman’s body begins to produce antibodies that affect male-specific proteins. This reasoning is consistent with two important features of the FBO effect: first, the effect is not observed in daughters with older siblings of either sex. Second, it is only observed in sons with the same mother, but it seems to make no difference whether or not the male children have different fathers. Even when brothers of the same birth mother are raised in different homes, the effect is observable, suggesting that uterine environment, not home environment, is the important factor.

All things considered, about half of male homosexuality is a result of the FBO effect. Other biological factors–genetic, epigenetic, and hormonal–appear to affect sexual orientation in most of the other half of cases. Lesbianism, in general, seems a tougher nut to crack, and some researchers hypothesize that female sexuality may be simply more “fluid.” That said, fetal testosterone exposure is one promising line of investigation that explains at least some instances of female homosexuality.