Washington - Having several older brothers increases the likelihood of a man being homosexual, a finding that researchers say adds weight to the idea of a biological basis for sexual orientation.
"It's likely to be a prenatal effect," said Anthony F Bogaert of Brock University in St Catharines, Canada. "This and other studies suggest that there is probably a biological basis for homosexuality."
S Marc Breedlove of Michigan State University said the finding confirms a physical basis absolutely.
"Anybody's first guess would have been that the older brothers were having an effect socially, but this data doesn't support that," Breedlove said in a telephone interview.
The only link between the brothers is the mother, and so the effect has to be through the mother, especially since stepbrothers didn't have the effect, said Breedlove, who was not part of the research.
Bogaert studied four groups of Canadian men, 944 people in all, analysing the number of brothers and sisters each had, whether they lived with those siblings and whether the siblings were related by blood or adopted.
He reports in a paper appearing in Tuesday's issue of Proceedings of the National Academy of Sciences that having several biological older brothers increased the chance that a man would be gay.
It's an effect that can be detected with one older brother and becomes stronger with three or four or more, Bogaert said in a telephone interview.
But, he added, this needs to be looked at in context of the overall rate of homosexuality in men, which he suggested is about three percent.
With several older brothers the rate may increase from three percent to five percent, he said, but that still means 95 percent of men with several older brothers are heterosexual.
The effect of birth order on male homosexuality has been reported previously, but Bogaert's work is the first designed to rule out social or environmental effects.
Bogaert said he concluded the effect was biological by comparing men with biological brothers to those with brothers to whom they were not biologically related.
The increase in the likelihood of being gay was seen only in those whose brothers had the same mothers, regardless of whether they were reared together, he said.
Men reared with several older step- or adopted brothers do not have an increased chance of being gay.
"So what that means is that the environment a person is raised in really makes not much difference," he said.
What makes a difference, he said, is having older brothers who shared the same womb and gestational experience, suggesting the difference is because of "some sort of prenatal factor."
One possibility, he suggests, is a maternal immune response to succeeding male foetuses. The mother may react to a male foetus as foreign but not to a female foetus, because the mother also is female.
It might be like the maternal immune response that can occur when a mother has Rh-negative blood but her foetus has Rh-positive blood. Without treatment, the mother can develop antibodies that may attack foetuses during future pregnancies.
Whether that's what happens remains to be seen, but it is a provocative hypothesis, said a commentary by Breedlove, David Puts and Cynthia Jordan, all of Michigan State.
The research was financed by the Social Sciences and Humanities Research Council of Canada. - Sapa-AP
http://www.iol.co.za/index.php?set_id=14&click_id=117&art_id=qw1151335802796B222
-------------------- Republican Values: 1) You can't get married to your spouse who is the same sex as you. 2) You can't have an abortion no matter how much you don't want a child. 3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer. 4) We need a smaller, less-intrusive government.
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