You’re more likely to be gay if you have a younger brother or sister, according to a new survey of 200,000 Britons.
Researchers found that having both older siblings was linked to sexuality.
Although previous studies have shown the link between being gay and having older brothers, this is the first time the same link has been found for older sisters.
The scientists discovered that the link becomes stronger the more older brothers and sisters a man has.
However, the University of Melbourne researchers found that the same link did not hold for lesbians and older siblings.
The study found that the more brothers you had, the more likely you were to be homosexual
Interestingly, the study found that the association between number of siblings was only related to being homosexual.
The number of siblings a person had made no difference to the likelihood that he or she was bisexual.
However, it did relate to whether or not someone became asexual later in life; those without siblings were significantly more likely to experience no sexual attraction.
The researchers analyzed data from the UK Biobank – a long-term study of the health and lifestyle of hundreds of thousands of Britons throughout their lives.
As part of this, participants were asked four questions about their sexuality.
The researchers found that having at least one older brother was related a 10 percent increased risk of homosexuality, while this figure was five percent for men with older sisters.
The findings echo a study by the same group of researchers in the Netherlands that was published earlier this year.
This study found that the strongest relationship was found among men who have three older brothers; these participants were 41 percent more likely to be homosexual.
They found that if you have five older brothers, your chance of being homosexual is about eight percent – which is four times the chance for an average man.
“The vast majority of people who have much older brothers will be attracted to the opposite sex,” says Dr. Scott Semenyna, professor of psychology at Stetson University.
Scientists are still baffled as to why this phenomenon exists.
One theory is the ‘maternal immune hypothesis’.
Proponents say that after the birth of a boy, proteins enter the mother’s bloodstream and trigger the production of antibodies that affect the sexual development of subsequent children.
These maternal antibodies accumulate during successive pregnancies with male fetuses.
However, experts are still unsure how exactly 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 at all,” said Dr. Scott Semenyna , a 2017 study. Assistant professor at MacEwan University Department of Psychology in Canada.
However, this theory was developed before the relationship between older sisters and homosexuality was discovered.
‘There could of course also be other explanations. It is up to prospective research to make this clearer,” Kabátek said.