Having an older brother makes you more likely to be gay, study suggests – and nobody knows why

Having an older brother makes you more likely to be gay, study suggests

Men with an older brother are more likely to be gay, a new study has suggested, although scientists aren’t clear why this is.

The study was led by the University of Toronto and published today in the journal Proceedings of the Royal Society B.

Scientists looked at almost 5,400 men across ten scientific studies and found that those with an older brother were 38 per cent more likely to be gay – although the reason for this is unknown.

According to Dr Ray Blanchard, the study’s lead author, the effect is not seen with sisters.

“Much prior research has shown that females do not influence the sexual orientation of their younger siblings, and females’ sexual orientation is not affected by their numbers of older siblings,” he told the Daily Mail.

The study, which did not include bisexuals, compared the chances of the youngest brother in a two-son family being gay against the odds for the oldest son.

It references a controversial theory called the “maternal immune hypothesis”, which suggests that a mother who has an older son becomes exposed to “male-specific substances” from male cells during pregnancy or birth.

The theory is that she then produces antibodies to those cells, which affect the brain cells of a second male child and make his brain work differently.

The study’s results do not conclusively prove this theory, although it was able to find a correlation between fraternal birth order and likelihood of homosexuality.

Professor David Spiegelhalter, a leading statistician from the University of Cambridge, who wasn’t involved with the research, said: ‘The fascinating study estimates that having an older brother increases the odds of being gay by 38 per cent, supporting the idea that a mother’s immune response to having a male child influences subsequent boys.

“People have endlessly argued about the possible roles of genetics and upbringing, but this clear result fits in neither category.”