According to a new study, having older brothers improves a man’s chance of being gay.

Although experts believe many factors could affect a person’s sexual orientation, a brand-new report published in the Proceedings of the National Academy of Sciences introduced at least one biological cause.

Researchers at Brock University in Canada discovered that when a woman is pregnant with her first son, it’s likely that a protein (called NLGN4Y) connected to the Y chromosome enters her bloodstream.

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Women don’t have a Y chromosome, so when it enters the mother’s blood, her body identifies the Y-linked protein as a foreign substance. Consequently, her immune system responds by producing an antibody termed anti-NLGN4Y.

Later, if the woman becomes pregnant with another boy, these antibodies can pierce the placental barrier and access the brain of the fetus.

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“That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction,” claims study writer Professor Anthony Bogaert, a Canadian scientist at Brock University.

The study also insinuates that this effect becomes more likely with each male born which implies that men with multiple older brothers have a higher likelihood of being gay. This phenomenon is known as the “fraternal birth order effect”.

Bogaert and his co-authors examined 142 women and found a higher concentration of the NLGN4Y anti-body in those who had gay sons with older brothers than women with no sons or only heterosexual sons.

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“Our study is a major advance in understanding the origins of sexual orientation in men by providing support for a theorized but previously unexamined biological mechanism — a maternal immune response to a protein important in male fetal brain development — and by beginning to explain one of the most reliable correlates of male homosexuality: older brothers.”

Of course, the learnings don’t confirm that all men with older brothers will be gay. The researchers acknowledge that additional investigations are needed.