Recent research has revealed that having several older brothers increases the chances of a man being homosexual, which is a finding that adds weight to the idea of a biological basis for sexual orientation.
As Anthony F Bogaert of Brock University in St Catharines, Canada said, "It's likely to be a prenatal effect. This and other studies suggest that there is probably a biological basis for homosexuality."
S Marc Breedlove of Michigan State University confirmed that 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.
According to Breedlove 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.
The research was conducted on four groups of Canadian men, 944 people in all, analysing the number of brothers and sisters each had, whether the siblings were related by blood or adopted and whether they lived with those siblings.
The results of the research have been reported in a paper of Proceedings of the National Academy of Sciences. Bogaert concluded that having several biological older brothers increased the chances of a man being gay. This effect that can be detected with one older brother and becomes stronger with three or four or more, Bogaert added.
He said that when this was looked at in the context of the overall rate of homosexuality in men, which is about 3 per cent, with several older brothers the rate may increase from 3 per cent to 5 per cent.