Birth order might have more to do with sexuality than many realize.
New research found that having an older brother increases men’s chances of being gay, according to a study published Wednesday in the journal Proceedings of the Royal Society B.
The findings from German and Canadian scholars showed that men with older brothers have a 38% higher likelihood of being homosexual than men who don’t have an older brother.
“[Older] brothers increase the probability of homosexuality in later-born males,” read the study’s abstract.
Researchers analyzed 10 separate studies on sexual orientation, involving a total of 5,400 men and information regarding their birth order. The findings also showed that the more older brothers a man had, the higher the likelihood he would be gay. Having three older brothers doubled a man’s chances of being gay, according to the research.
The study also found a correlation between mothers giving birth to homosexual males and having more children, as compared to moms of straight males having fewer children. “[Mothers] of homosexual males produce more offspring than the mothers of heterosexual males,” the researchers wrote.
“The fascinating study estimates that having an older brother increases the odds of being gay by 38 percent, supporting the idea that a mother’s immune response to having a male child influences subsequent boys,” University of Cambridge professor and statistician David Spiegelhalter told the Daily Mail. “People have endlessly argued about the possible roles of genetics and upbringing, but this clear result fits in neither category.”
Interestingly, the study authors did not find a correlation between sexuality and birth order for women. Indeed, no particular pattern of siblings, their gender or their age appeared to determine whether a female would be gay.
“Much prior research has shown that females do not influence the sexual orientation of their younger siblings, and females’ sexual orientation is not affected by their numbers of older siblings,” study author Dr. Ray Blanchard told the Daily Mail.
Although the study authors admit they are not sure why their findings are the case, they believe it may have something to do with a theory called “maternal immune hypothesis,” which posits that women who give birth to a male baby develop certain antibodies that impact the brain development of future male offspring.