Having Older Brothers Increases a Man's Odds of Being Gay

The overall rate of homosexuality in men is about 3%. If men are with several older brothers the rate may increase from 3% to 5%. Is it the social or rearing factors that influence a man's sexual orientation? or rather prenatal mechanisms that begin in the womb?

A recent study by Anthony F. Bogaert of Brock University in St. Catharines, Canadan, has confirmed that this is the prenatal effect, not social. 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.