Researchers believe that increasing levels of certain antibodies in a mother could explain the occurrence of the “fraternal birth order effect”, where the more older brothers a male has, the more likely he is to be gay.

The research, published in the Proceedings of the National Academy of Sciences, looked for a biological factor in mothers that could accumulate over multiple male pregnancies and influence their offspring's sexual orientation.

The team, led by Anthony Bogaert from Brock University, found that women pregnant with boys develop antibodies that target protein NLGN4Y, which is produced by the Y chromosome. It's not uncommon for women to produce immune responses to molecules produced by the fetus, it happens if mother and baby have different blood groups, but the particular response to NLGN4Y piqued the researchers' interest. The protein plays a role in how brain networks develop.

The researchers assume that this protein influences the regions of the brain where attraction is moderated and that exposure to higher anti-NLGN4Y levels makes same-sex attraction more likely.

They analyzed the blood of 142 women and found that those with gay sons with older brothers had, indeed, higher levels of NLGN4Y antibodies than mothers of gay sons with no older brothers. Women with straight sons, no sons, and also men had progressively lower levels of these antibodies.

“I wouldn’t say we’ve solved the fraternal birth order effect puzzle, but we are getting close to finding a mechanism,” Bogaert told New Scientist.

It's important to note that the research only involved a small number of women, so it's not possible to conclude whether this is the true biological mechanism behind the fraternal birth order effect. It is also necessary to be aware that said effect is estimated to play a role in the sexual orientation of only 15 percent of gay men.

Even if levels of NLGN4Y are involved in determining sexuality, this can only be a small part of the puzzle. Many studies looking at the biological factors behind homosexuality forget about women, as well as bisexual and pansexual people. If the conundrum has to be solved, a more wide-reaching approach is probably necessary.

[H/T: New Scientist]