Sibling sex ratio and male homosexuality.

As an explanation for the increased sex ratio observed in the sibships of male homosexuals, Slater has hypothesized that the increase, itself, may be a causative factor in predisposing a male to homosexual behavior. As a test of this hypothesis, an additive and a multiplicative risk model relating male homosexuality to the sexual configuration of the homosexual's sibship were formulated and tested against published data. Both models were found to predict a curvilinear decrease in sibling sex ratio as sibship size increased, and both models were found to fit the observed sibship size sex ratio data closely. The analysis suggests that approximately 10% of the variance in male homosexual behavior can be accounted for by the sexual configuration of the homosexual's sibship.