Homosexuality linked to older siblings

Until recent years there has been no real idea why 5% or so of the population are attracted to their own sex, rather than the opposite sex. It clearly isn’t a choice, but what was unknown is whether it has a genetic link, or an environmental link or both – and how much.

I have blogged previously on how some studies have shown that there is a link to having older brothers. A new study of nine million people in the Netherlands has been of much higher quality than previous studies, and confirms them. They note:

The “fraternal birth order effect” is one of the most well-documented patterns supporting a biological origin of human sexual orientation. This longstanding hypothesis proposes men’s propensity for homosexuality increases with the number of older biological brothers they have.

This effect has been attributed to a mother’s immune reaction to proteins produced by a male foetus. The proteins enter the mother’s bloodstream and trigger the production of antibodies that influence the sexual development of subsequent children. 

These maternal antibodies accumulate over successive pregnancies with male foetuses, which means men with more older brothers are more likely to experience same-sex sexual attraction.

Their findings are:

So this is pretty conclusive that the hypothesis is correct. Now it isn’t the only factor, but definitely a significant one.

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