The interaction of fraternal birth order and body size in male sexual orientation.

A late fraternal birth order has been demonstrated numerous times in homosexual men. Body size has been less studied with regard to the development of sexual orientation and has demonstrated contradictory results. In this research, the relations among fraternal birth order, body size, and sexual orientation were examined in a Canadian sample of homosexual and heterosexual men. An interaction between fraternal birth order and height was observed, with a homosexual orientation most likely to occur in men with a high number of older brothers and shorter stature. No significant interactive effects were observed for weight. The results suggest that the mechanism underlying the fraternal birth order phenomenon has an effect on physical development that lasts and is detectable into adulthood (i.e., adult stature).