Reported in the Proceedings of the National Academy of Sciences (US):
Gay men have, on average, a greater number of older brothers than do heterosexual men, a well-known finding within sexual science. This finding has been termed the fraternal birth order effect. Strong scientific interest in sexual orientation exists because it is a fundamental human characteristic, and because its origins are often the focal point of considerable social controversy. Our study is a major advance in understanding the origins of sexual orientation in men by providing support for a theorized but previously unexamined biological mechanism—a maternal immune response to a protein important in male fetal brain development—and by beginning to explain one of the most reliable correlates of male homosexuality: older brothers.