Changes in the Womb Tied to Homosexuality In Boys With Brothers
Tuesday, June 27, 2006; A02
Many men are gay because of biological differences wrought by their mothers before the boys were born, according to a study that opens a new and contentious front in the same-sex marriage wars.
Anthony F. Bogaert of Brock University in Ontario had already documented that boys who have several older brothers are more likely than others to grow up gay -- a phenomenon known as the fraternal birth order effect.
But why? Some scientists have suspected social and environmental factors, such as the large amount of time such boys spend fraternizing with male siblings during their sexual development. Others have wondered whether, after carrying multiple male fetuses, women undergo biological changes that affect the development of brain areas related to sexual orientation in subsequent sons.
To find out, Bogaert looked at the family structures of 944 gay and straight men, including men who grew up with multiple stepbrothers rather than biological brothers and men whose mothers had many sons before them but who did not grow up with those brothers.
The results, in short: nature, 1; nurture, 0.
As described this week in the early online edition of the Proceedings of the National Academy of Sciences, it is the number of boys a boy's mom has already had -- not the number of boys he grows up with -- that best predicts whether that boy will be gay.
The mechanism behind this apparent maternal alchemy remains a mystery. But many scientists suspect that women mount a subtle immune system response against male fetuses that becomes stronger with each male pregnancy, ultimately affecting fetal brains in ways that influence sexual orientation.
No similar links have been noted for lesbianism. And even among boys, the birth order effect -- which becomes a prominent influence in boys with two or three brothers or more -- accounts for only about one in every seven gay men, Bogaert and colleagues have calculated.
-- Rick Weiss
© 2006 The Washington Post Company