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If You Have an Older Brother, New Research Claims You’re More Likely to Be Gay

The good, the bad and the ugly things we learned about our bodies today

Do you have an older brother? Are you gay? Well, if your answer to both questions is “yes,” then according to a new paper in the Proceedings of the National Academy of Sciences — the culmination of more than 20 years of research by by Anthony Bogaert — that’s pretty normal.

A human sexuality researcher at Brock University in Canada, Bogaert first began looking at what is now known as the “older brother effect” in 1996. Basically, what he found is that having older brothers increases the odds that a man will be gay. In 2006, Bogaert’s team published a study showing that each additional brother increased the chances a man would be gay by about 30 percent.

“We didn’t know what the mechanism was — what was, in fact, behind it,” Bogaert told Newsweek. “But we did posit a biological mechanism in those early papers.”

In his latest research, Bogaert and other researchers began looking at factors that might influence a male child before he is even born. One of those factors was the antibodies produced in a mother’s body as a reaction to a protein made by a male fetus. This protein, NLGN4Y, is thought to be linked with the way that neurons in the brain connect with each other during development.

In Bogaert’s latest study, more than 11 mothers who have had more than one son had particularly high levels of these antibodies in their blood — especially if one of those sons is gay. In other words, a single antibody to one particular protein may be somewhat responsible for establishing someone’s sexuality.

One explanation for why these antibodies don’t usually affect firstborn sons may have to do with how women are exposed to the protein to begin with. “It’s likely that the protein may not get into the mother’s bloodstream until during or just after the birth,” Bogaert told Newsweek.

Still, Bogaert emphasized that there are other explanations floating around to explain the older brother effect — some believe that firstborn men might be less likely to come out as gay, even if they are — and that other labs would need to confirm the results by doing their own experiments. However, Bogaert said, “Our data really don’t seem to support that.”

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