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 3 per cent. With several older brothers the rate may increase from 3 per cent to 5 per cent, he said.