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Does Having Older Brothers Increase Men’s Chances of Being Gay? Here’s What the Studies Say

WASHINGTON, DC - APRIL 28:  A man reaches out to a large rainboWASHINGTON, DC - APRIL 28:  A man reaches out to a large rainbo

Is being gay linked to genetics? It’s a controversial question which brushes up against issues of personal sexual choice and the ethics of reducing something as complex and fluid as sexuality down to a biological signal. But a major new study, conducted over 20 years, suggests that men who are gay tend to have a greater number of older brothers—and for an actual, scientific reason.

“Gay men have, on average, a greater number of older brothers than do heterosexual men, a well-known finding within sexual science,” wrote the researchers from Canada’s Brock University, the University of Toronto, and Harvard Medical School in a study published in the scientific journal PNAS.

What could explain such a phenomenon? The answer may be linked to a specific kind of antibody produced in some mothers who carry male fetuses. This Y-linked protein, NLGN4Y, may be associated with early fetal brain development. And that could have implications for a woman’s future offspring, as the protein may become more prevalent after the birth of the first male child.

“This explanation posits that some mothers develop antibodies against a Y-linked protein important in male brain development, and that this effect becomes increasingly likely with each male gestation, altering brain structures underlying sexual orientation in their later-born sons,” wrote the study authors, who also stressed that more research needs to be done on the issue to confirm their results.

Brock University’s Anthony Bogaert, one of the lead study authors, further emphasized the research is not meant to suggest that being gay is some sort of illness or genetic defect. Rather, the findings seek to explain widely observed realities in human sexuality—and suggest that some people truly are just geared toward certain kinds of attraction from birth.