A Canadian research study has found that having older brothers increases a man's odds of being gay.

The study, which suggests that if you want heterosexual children you need to make sure you only have one boy in the family, lends 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.

In the study reported this week in the Proceedings of the National Academy of Sciences, by Anthony Bogaert of Brock University, it was found that 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.

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.

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 foetus, possibly affecting its development.