Men With Older Brothers More Likely To Be Gay? Study Expanding To Biological Origin

Surprising Factor Could Increase Chances A Man Is Gay

Seven years ago a study made headlines for finding that men with older brothers are more likely to be gay. Today the study has resurfaced as researchers extend upon the original findings by looking deeper into the possible biological basis of homosexuality.

Published in New Scientist magazine, the 2006 study was conducted by sexologist Anthony Bogaert, a Community Health Sciences chair and professor at Ontario's Brock University. His research led him to conclude that having more older brothers makes it more likely a man will be gay. Each older brother raises the odds of homosexuality by a third, potentially going from a 3 percent chance with the first son to a 6 percent chance with the fourth.

Bogaert studied 944 gay and straight men, including some who were raised with non-biological male siblings, to pit prenatal against postnatal factors. His research, which reappeared in a Sept. 4 New Scientist article, ultimately determined that fraternal birth order seems to have a connection to sexual orientation. "That means for each male gestation that occurs, something changes in a woman’s body that makes her more likely to give birth to a gay son," as Brock University explains Bogaert's findings.

An abstract from the study, published by the Proceedings of the National Academy of Sciences, summarizes the research:

Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birth-order effect.

Today, Bogaert is looking to determine that prenatal origin.

"[The 2006 study] was an important study and it does suggest there is probably a biological basis to the older brother effect," he told The Huffington Post during a phone conversation from Canada Thursday afternoon. "It's an important study in the context of sex orientation development. We have additional research going on right now testing specifically the underlying biological effect of the older brother. ... There are no results yet. We are collecting samples of mothers of gay men and comparing them to mothers of heterosexual men and looking to see if there is evidence of a biological factor that differs between the two groups."

Bogaert explained that the 2006 study never determined a specific "mechanism" -- a biological factor or process -- behind the older brother effect. He suggested this mechanism could be a maternal immune response, a hormone change in the womb, a gestational factor during pregnancy or even genes.

"The 2006 study really strongly suggests that there is biological mechanism but it never really tested the specific mechanism itself in terms of determining what factor is in fact influencing a change in a mother's development during pregnancy," he explained to HuffPost. "What we're doing now is we're looking at the specific mechanism and seeing if we can find evidence for that. We have a study looking at ... mothers of gay and heterosexual men and seeing if there is a maternal immune response. So, that's something very unique and new."

Bogaert says that isolating the exact biological mechanism or process will support the "nature" side of the Nature v. Nurture debate. If successful, his new research could provide a biological backing for homosexuality.

"I think there's strong evidence that people who believe that there's a biological basis to sexual orientation tend to be more tolerant to sexual minorities, and that's one of the more positive [possible] social outcomes," he said. "And I am in that camp. I don't believe that homosexuality is a disorder or immoral."

The overarching purpose of the new study is to delve deeper into the origins of sexuality. Bogaert says he is trying to understand sexual variability, human sexuality and sexual orientation as a piece of the larger puzzle.

"Unerstanding the biological mechanisms may have some social ramifications, and for me that's great if it ends up being positive," he added. "But a larger motivation is just trying to understand sexuality, and understanding sexuality has its own benefits from a scientific perspective."

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