You’re More Likely To Be Gay If You Have An Older Brother

It turns out that if you are significantly more likely to be gay if you have an older brother. The science behind this new finding is quite surprising.

The news comes from a new study published in the journalĀ Proceedings of the National Academy of Sciences of the United States of America. The researchers arrived at a theory that may help explain how a person’s sexual orientation could be determined before birth.

It can’t be boiled down to just one thing. However, the scientists did observe “a maternal immune response to a protein important in male fetal brain development.”

When a woman first becomes pregnant with her first male baby, the protein in question spikes in her bloodstream. The protein is a male-specific one, and the mother’s body produces antibodies to cope with it as if it were an invading pathogen. Those antibodies remain in her body after the first boy is born. If she becomes pregnant with another male child, those antibodies can infiltrate his brain during his development.

Anthony Bogaert, a Canadian psychology and community health sciences professor at Brock University, said “That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction.”


Bogaert and the other researchers involved in the study looked at 142 women and 12 men between 18 and 80 years old. The antibody related to the protein was found in greater abundance in women with a straight older son and a gay younger son. Tellingly, families with adopted sons did not fall within this pattern.

“Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings,” wrote Bogaert.

The PNAS study is the first study to theorize a specific, causal link between genes and sexual orientation. Past research has suggested a strong correlation between genetics and orientation, but we lacked specifics.

“Our studies only show that there may be genes that matter in sexual orientation. It is not like this study, that shows there is a potential specific mechanism by which sexual orientation may have changed prenatally. This is important work and fascinating if it proves to be true,” says J. Michael Bailey, a psychology professor at Northwestern University.

The finding will surely be controversial in a number of different ways to a number of different groups of people.

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