Older-sibling and younger-sibling sex ratios in Frisch and Hviid's (2006) national cohort study of two million Danes.

Frisch and Hviid (2006) recently reported a study of variables that predicted heterosexual and homosexual marriage in a national cohort of Danish men and women. They found no evidence that older brothers increase the probability that a man will legally marry another man. They concluded that their data raise questions about the universality of the widely confirmed finding that older brothers increase the probability that a man will be sexually oriented towards other men (the fraternal birth order effect). In the present article, Frisch and Hviid's data were reanalyzed using one of the procedures that have been used in prior studies of fraternal birth order. The results showed that the sex ratio of older brothers to older sisters was significantly higher than the expected value of 106 in all four of their study groups (heterosexually married men, homosexually married men, heterosexually married women, and homosexually married women). In contrast, the sex ratio of younger brothers to younger sisters approximated 106 in all four groups. According to this analysis, the only group whose data resembled data from previous studies was the homosexually married males. The writer concluded that one cannot interpret findings about the correlates of heterosexual and homosexual marriage as if they were findings about the correlates of heterosexual and homosexual orientation, and that this is underscored by comparing the markedly different older-sibling sex ratios obtained from heterosexually married persons (in the Danish study) and those obtained from heterosexually oriented persons (in previous studies). It is unclear what implications, if any, Frisch and Hviid's findings have for the study of sexual orientation in general.