Birth weight, sexual orientation and the sex of preceding siblings

This study's first objective was to compare the mean birth weights of homos exual and heterosexual men and women. Its second objective was to investiga te whether prior male and female fetuses have different effects on the birt h weight of subsequent fetuses. The subjects were 3229 adult men and women (the probands), who weighed at least 2500 g at birth, and whose mothers kne w the sex of the child (or fetus) for each pregnancy prior to the proband. Information on birth weight, maternal gravidity and other demographic varia bles was reported on questionnaires completed by the probands' mothers. The results confirmed earlier reports that boys with older brothers weigh less at birth than boys with older sisters, but they did not confirm reports th at girls with older brothers weigh less than girls with older sisters. The results did not show across-the-board differences in the mean birth weights of homosexual versus heterosexual women or homosexual versus heterosexual men. However, the homosexual mates with older brothers weighed about 170 g less at birth than the heterosexual males with older brothers. It is sugges ted that this pattern of results may reflect a maternal immune response to Y-linked minor histocompatibility antigens (H-Y antigens). According to thi s hypothesis, when the maternal immune response is mild, it produces only a slightly reduced birth weight, but when it is stronger, it produces a mark edly reduced birth weight as well as an increased probability of homosexual ity.