A new study shows having an older brother can increase the odds among men of being gay.
German and Canadian researchers compiled data from ten different studies on sexual orientation focusing on families with two sons giving them a dataset of 5,390 gay and straight men.
Crunching the numbers they found males with an older brother had a 38 percent higher likelihood of being gay than men who don’t have older male siblings.
The study was published Wednesday in the Proceedings of the Royal Society B journal.
Additionally, the data indicated the more older brothers a man had, the greater the odds he would be gay.
The New York Post reports 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.”
In their paper, the researchers wrote, “[Mothers] of homosexual males produce more offspring than the mothers of heterosexual males.”
Professor David Spiegelhalter, of the University of Cambridge, told the Daily Mail, “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.”
The study’s authors hypothesize that immune systems of mothers who give birth to a male baby react to a specific protein that has been shown to be important for brain development in male fetuses. The antibodies developed in reaction to that protein impact the brain development of future male children.
The researchers noted that no correlation was found between the birth order for women and sexuality.