Born This Way: Why Having an Older Brother Makes Men More Likely to Be Gay

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Equality ambassadors and volunteers from the Equality Campaign celebrate as they gather in front of Parliament House in Canberra on December 7. SEAN DAVEY/AFP/Getty Images

Scientists believe they may now know why gay men tend to have more older brothers. A new paper in the Proceedings of the National Academy of Sciences by Anthony Bogaert—the culmination of more than 20 years of research—shows that antibodies to one particular protein may be responsible.

Bogaert, a human sexuality researcher at Brock University in Canada, first began looking at what is now known as the "older brother effect" in 1996. Basically, what Bogaert and his collaborators found is that having older brothers increases the chances a man will be gay. In 2006, they published a study showing that each additional brother increased the chances a man would be gay by about 30 percent; Popular Science reported on those findings.

"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."

That mechanism has been confirmed in this latest paper, though he noted that other scientists will still need to conduct their own experiments to further confirm the results. He 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. Turns out that of the more than 100 mothers involved in Bogaert's latest study, those who have had more than one son have particularly high levels of these antibodies in their blood—especially if one of those sons is gay.

Australia marriage equality
Equality ambassadors and volunteers from the Equality Campaign celebrate as they gather in front of Parliament House in Canberra on December 7. SEAN DAVEY/AFP/Getty Images

Part of the reason that 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 said. After that, though, it's almost like a woman has gotten a vaccine for this particular protein; if she gets pregnant and is carrying another male child, her immune system is ready to recognize the protein and jumps into action, creating antibodies in her blood that may pass through her placenta and affect her child.

Bogaert emphasized that other labs would need to confirm the results by doing their own experiments. And there are other explanations floating around to explain the older brother effect. For example, some believe that firstborn men might be less likely to come out as gay, even if they are. However, Bogaert said, "our data really don't seem to support that."

Finding a biological basis for sexual orientation is not an invitation for anyone to start suggesting that being gay is a disease or is somehow "treatable," Bogaert emphasized. "I think it would be a misuse of this work."

In fact, Bogaert said understanding that there is a biological basis to homosexuality may be beneficial to the gay rights movement. That's because the findings suggest "that sexual orientation is not easily changed or is not a choice," he said. A Pew Research Center survey found that as of 2015, less than half of Americans believed that sexual orientation is something set from birth. If that number increases based on research like this, Bogaert said, that could mean society might become more tolerant and accepting in the future.

Uncommon Knowledge

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Newsweek is committed to challenging conventional wisdom and finding connections in the search for common ground.

About the writer


Kate Sheridan is a science writer. She's previously written for STAT, Hakai Magazine, the Montreal Gazette, and other digital and ... Read more

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