Study says men with older brothers "more likely to be gay"

There's been a tremendous amount of progress for gay rights, with countries all over the world legalizing same-sex marriage. Society's become more accepting of homosexuality, yet the factors that determine our desires remain mysterious. For decades, scientists have searched for a biological explanation for our sexual orientation.

Now scientists claim they have broken new ground, in a fascinating study published by the Proceedings of the National Academy of Sciences. It all traces back to earlier research, which revealed a unique pattern in families: "Gay men have, on average, a greater number of older brothers than do heterosexual men, a well-known finding within sexual science," write the researchers. "This finding has been termed the fraternal birth order effect."

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Therefore, the more older brothers a man has, the more likely he is to be gay. Scoff if you want, but this trend has been observed over and over again in various cultures. The pattern remained even if the man was raised separately from his biological brothers. In fact, according to 2006 study, with each additional brother, the chances that a man is gay increases by a third. In the new study, researchers hope to explain this phenomenon:

"Our study is a major advance in understanding the origins of sexual orientation in men by providing support for a theorized but previously unexamined biological mechanism – a maternal immune response to a protein important in male foetal brain development."


Basically, women have two X chromosomes. Men have an X and a Y chromosome. When a woman becomes pregnant with a boy, a protein linked to the Y chromosome - named NLGN4Y - enters her bloodstream. Her body treats NLGN4Y like a foreign substance, and her immune system reacts, creating large quantities of the antibody "anti-NLGN4Y." When the woman becomes pregnant with another boy, the anti-NLGN4Y antibodies can cross the placenta and affect mental development.

"[The antibodies] may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction," write the researchers. "This effect becomes increasingly likely with each male gestation, altering brain structures underlying sexual orientation in their later-born sons."

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Now, this doesn't mean scientists suddenly found "the gay gene," or that if you're your straight and have a lot of older brothers you are now gay. The researchers stress that is just one of many factors that govern our sexual orientation. More evidence is needed, because there are still many questions. For example, what is the biological explanation for bisexual men, and asexual men? Why don't women experience a 'fraternal birth order effect'? What about oldest sons that are gay? Or 'only children' that are gay? Or women attracted to women?

We'll have to wait and see what the next study reveals. As the researchers note, "Strong scientific interest in sexual orientation exists because it is a fundamental human characteristic, and because its origins are often the focal point of considerable social controversy." We've learned so much about genetics, but there are still so many mysteries, particularly when it comes to sexuality.