Scientists Say Having Older Brothers Could Make You Gay

Having lots of boys can affect a woman’s immune response // Aleksandar Nakic/Getty

New Scientist reports:

The more older brothers a boy has, the more likely he is to be gay when he grows up – an effect called the “fraternal birth order effect.” Now it seems that increasing levels of antibodies in a mother’s immune system could play a role.

Anthony Bogaert at Brock University, Canada, and his team think that some women who are pregnant with boys develop antibodies that target a protein made by the Y chromosome. Our immune systems make antibodies to recognise foreign molecules, which have the potential to be from dangerous bacteria. But pregnant women sometimes also produce antibodies against fetal molecules – for example, if their fetus has a different blood group. Bogaert’s team wondered if maternal antibodies might play a role in shaping sexual orientation.

The team collected blood from 142 women, and screened it for antibodies to a particular brain protein that is only made in males. They thought this would be a good candidate, because it plays an important role in how neurons communicate with each other, and because it is produced on the surface of brain cells, making it relatively easy for antibodies to find and detect it.

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