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A new study suggests that having older brothers increases a man’s chance of being gay.

While scientists believe there are many factors that could affect a person’s sexual orientation, a new report published in the Proceedings of the National Academy of Sciences could offer an explanation for at least one biological reason.

According to researchers at Brock University in Canada, when a woman becomes pregnant with her first boy, it’s possible that a protein (called NLGN4Y) linked to the Y chromosome gets into her bloodstream.

As women don’t have a Y chromosome, the mother’s blood detects the Y-linked protein as a foreign substance, and her immune system reacts by creating an anti-body known as anti-NLGN4Y.

If she becomes pregnant with another boy, these antibodies can cross the placental barrier and enter the brain of the fetus.

“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 author Anthony Bogaert, a Canadian scientist and professor at Brock University.

The report also suggests that this effect becomes increasingly likely with each male born, meaning that men with more older brothers have a higher chance of being gay – a phenomenon known as the ‘fraternal birth order effect’.

Bogaert and his co-authors tested a group of 142 women and found a higher concentration of the NLGN4Y anti-body in women who had gay sons with older brothers compared to women with no sons and women with only heterosexual sons.

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

It’s certainly not the first time scientists have noticed the pattern of gay men having older brothers, however this is the first time that a potential biological explanation has been discovered.

Of course, the findings don’t mean that all men with older brothers will be gay, and the researchers admit that further studies are required, but it’s intriguing nonetheless.

Earlier this month, researchers from North Shore University, Illinois reported that they may have identified ‘gay genes’ in men for the first time ever.