Scientists are going to report in this week’s Proceedings of the National Academy of Sciences that men’s chances of being gay may be influenced by birth order and the gender of the preceding siblings. Apparently, the prenatal conditions in the mother’s womb after she has given birth to multiple sons, increases a younger son’s chances of being gay. So, men with older brothers (the more the number of brothers, the greater the likelihood) are more likely to have homosexual orientation than men in the general population. It is not the social influence of having older brothers either. Older half brothers born of different mothers or older adoptive brothers, do not appear to affect the sexuality of a younger brother. Neither does living away from blood brothers. The statistical finding relates specifically to the number of older brothers, born of the same mother and is therefore thought to point to a (at least one) biological basis for homosexuality.
"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.
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. .."
Accidental Blogger 
2 responses to “Oh Brother!”
This story is a real boon to those of us oldest brothers who want to make our macho younger brothers a bit nervous. But based on the excerpt posted here, I’m not sure the “social influence” factor has been fully eliminated. That one has biological or step or adopted siblings is a socially significant datum, regardless of whether or not one lives with them. On a tangent, I do not understand why it matters, in any event.
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The push to prove a biological basis for homosexuality is quite obviously partly due to the religious right’s insistence that it is a chosen behavior as opposed to an inherent trait and therefore can be changed with counseling and/or fear of damnation.
Here is the AP report of the same story which surprisingly is actually a bit more detailed than the Scientific American report. I didn’t link to it in the main post because Houston Chronicle links have a habit of “disappearing” after a few weeks.
But nature or nurture, as you said, why does it matter?
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