From Scientific American:
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.
More here.