Having an older sibling increases the odds of being gay by a fifth, major study finds

You are more likely to be gay if you are a younger sibling, according to a new study of 200,000 Britons.

Researchers found that having both older brothers and sisters was associated with sexuality.

While previous studies have shown the link between being gay and having older brothers, this is the first time the same association has been identified for older sisters. 

The scientists found the link becomes stronger, the more older siblings a man has. 

However, the researchers, from The University of Melbourne, found the same link doesn’t apply to lesbians and older siblings. 

Interestingly, the study found that the association between the number of siblings was only related to being homosexual. 

The number of siblings a person had made no difference to the likelihood of them being bisexual.

It did, however, relate to whether or not a person became asexual in later life — those with no siblings were significantly more likely to have experience no sexual attraction.  

The researchers analysed data from the UK Biobank — a long-term study into the health and lifestyle of hundreds of thousands of Brits across their lifetimes. 

As part of this participants were asked four questions about their sexuality.

The researchers found that having at least one older brother was associated with a 10 per cent increased chance of homosexuality, while the figure was five per cent for men with older sisters. 

The findings echoed a study by the same group of researchers in Holland published earlier this year.

This research found the strongest relationship was found with men who have three older brothers — these participants were seen to be 41 per cent more likely to be gay. 

They found that if have five older brothers, your chance of being gay is about eight per cent— which is four times the probability for an average man.

‘The vast majority of people who have a lot of older brothers are going to come out opposite-sex attracted,’ Dr Scott Semenyna, a psychology professor at Stetson University.

Scientists are still baffled as to why this phenomenon exists.

One theory is ‘maternal immune hypothesis’.

 Advocates say that, after birthing a boy, proteins enter the mother’s bloodstream and trigger the production of antibodies that influence the sexual development of subsequent children.

These maternal antibodies accumulate over successive pregnancies with male foetuses.

However experts are still unsure as to exactly how these antibodies lead to personality differences.

A 2017 study supported this theory concluding, ‘mothers of gay sons have more of these antibodies that target male-specific proteins, compared to mothers of sons who are not gay or mothers who have no sons whatsoever,’ according to Dr Scott Semenyna, an Assistant Professor at the MacEwan University Department of Psychology in Canada.

However this theory was developed before the relationship between older sisters and homosexuality was discovered.

‘Of course, there can be other explanations. It’s for prospective research to make this clearer,’ Kabátek said.