IF YOU鈥橰E male, having more older brothers makes it more likely you鈥檒l be gay. It now seems clear that it鈥檚 how many brothers you share with the same mother that matters, not how many you grew up with.
Each older brother increases your chance of being homosexual by about 30 per cent, but this statistic has been dogged by the suggestion that it鈥檚 due to social rather than biological factors. Sceptics propose that rough-and-tumble play between brothers may lead the younger boys to become gay.
Now Anthony Bogaert, at Brock University in St Catharines in Canada, has largely ruled that out. He looked at a total of 944 homosexual and heterosexual men, including one group raised with non-biological male siblings. Bogaert reasoned that if simply being brought up with a lot of older brothers produced the effect, it shouldn鈥檛 matter if they had the same mother or not.
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In fact, it did matter: only the number of biological older brothers was linked to sexual orientation. This was true even when the biological older brothers lived separately (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0511152103). 鈥淚t鈥檚 pretty strong in suggesting a prenatal origin,鈥� he says.
