Birth order, sibling sex ratio, and maternal miscarriages in homosexual and heterosexual men and women

Several studies have shown that older brothers increase the probability of homosexuality in later-born males. The maternal immune hypothesis proposes that this phenomenon reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H–Y antigens) by each succeeding male fetus, and the concomitantly increasing effects of H–Y antibodies on the sexual differentiation of the brain in each succeeding male fetus. This hypothesis predicts that older brothers will not increase the probability of homosexuality in later-born females, because female fetuses do not produce H–Y antigen and they would not be targets of H–Y antibodies. The present study tested this prediction. The mothers of 4043 homosexual and heterosexual men and women provided information on each of their pregnancies, whether these ended with miscarriage or abortion or with a live birth. As predicted, male pregnancies increased the odds of homosexuality in later-born males but not in later-born females. The mothers of the homosexual females reported more miscarriages than the other groups of mothers; however, this result should not be accepted as meaningful until it has been replicated with mothers matched on variables that might affect reporting bias. All groups of mothers reported higher rates of miscarriage for male than for female fetuses.