The number of biological older brothers a boy's mother has carried--whether they live with him in the same household or not--affects his chances of being gay. The findings, reported this week in the Proceedings of the National Academy of Sciences, by Anthony Bogaert of Brock University, lend credence to the theory that it's not the social or rearing factors that influence a man's sexual orientation, but rather prenatal mechanisms that begin in the womb.

The idea that prenatal mechanisms may influence sexual orientation has been around for a couple of decades. In 1996, Bogaert along with colleague Ray Blanchard correlated sexual orientation in men with the number of older brothers, but it wasn't clear if that influence was occurring because the boys shared the same household or because they had shared the same womb.

In the new study, Bogaert pitted prenatal against postnatal by examining four samples of homosexual and heterosexual men, for a total of 944 participants. The data for three of the samples had been collected previously, and included detailed information about the men's sexual orientation, as well as their family life. Because most of the men from these three study groups came from unbroken families, Bogaert looked at a fourth group, composed of men who had been adopted or raised with half- or step-siblings. He also gathered data from this group about how long members lived with each sibling and whether they had brothers or sisters with whom they had never lived.

He reasoned that if the social or rearing factor theories were correct, he would expect to see certain things. First, it wouldn't matter whether a gay man's older brothers had been biologically related or not, the social influence would be there. Second, the amount of time the young boy lived with his older brothers, biological or not, should affect his sexual orientation. Third, if the boy did not live with older brothers, then the numbers should not impact his sexual preference.

Bogaert found the opposite to be true. First, he found that only the number of biological older brothers predicted sexual orientation in men--even when the number of non-biological older brothers was significantly higher. Second, his study showed that the amount of time reared with older brothers--either related or not--did not predict a young boy's becoming homosexual. And surprisingly, Bogaert discovered that even if a young man did not grow up in the same house as his older brothers, the fact that he had older biological brothers increased his odds of being gay.

The fact that the common denominator between the older and younger biological brothers is the mother hints at a prenatal influence on sexual orientation. What it could be is still a mystery. But one theory suggests that after delivering a boy, a woman's immune system produces antibodies to male-specific proteins. During subsequent pregnancies the mother's placenta may deliver the antibodies to the fetus, possibly affecting its development.