Is it more common to have brotherly babies?

Biology and sexuality : Does the mother's immune reaction trigger homosexuality in sons?

According to statistics, homosexual men are more likely to have older brothers than heterosexual men. According to this “fraternal birth sequence effect”, the likelihood of being homosexual increases with the number of older brothers a man has, according to Ray Blanchard of the University of Toronto. It is possible that during the first pregnancies with sons, “male” biomolecules get into the mother's blood and activate the body's defenses. In later pregnancies with sons, this triggers an immune reaction and influences the development in the brain of the embryo as well as its sexual orientation - according to the theory.

Antibodies to a protein that is important for brain development

Now, Blanchard's research team has actually discovered a difference in the blood of 54 women who have homosexual sons compared to the blood of 72 mothers of heterosexual sons: They had large amounts of an antibody in their blood that is directed against a protein that is important for brain development: NLGN4Y. This protein is only produced by sons because it comes from the Y chromosome. The more older sons the women had, the more antibodies against NLGN4Y the women had in their blood.

"The results suggest a connection between the immune response of the mother and the sexual orientation of the male offspring," writes Blanchard in the journal "PNAS". The researchers also found the antibodies in some women whose homosexual sons did not have older brothers - presumably triggered by biomolecules from male embryos from undetected miscarriages or by sperm.

Sexual orientation is a complex phenomenon

The researchers emphasize that this does not rule out other influences on sexual imprinting, such as hormonal or genetic ones. It is also "clear that only part of the variation in male sexual orientation can be explained by such effects." It is estimated that only one in seven gay men is homosexual due to the effect of fraternal birth order. Sexual orientation is "obviously a complex phenomenon".

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