You are more likely to be gay if you are a younger brother, according to a new study of 200,000 Britons.
The researchers found that having 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.
Scientists discovered that the bond strengthens the more older brothers a man has.
However, researchers at the University of Melbourne found that the same link does not apply to lesbians and older siblings.
A study found that the more siblings you have, the greater your chances of being gay.
Interestingly, the study found that the association between the number of siblings was only related to being gay.
The number of siblings a person had did not influence the likelihood that they were bisexual.
However, it did relate to whether or not a person became asexual later in life: those who did not have siblings were significantly more likely not to experience 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 sibling was associated with 10 percent more likely to be homosexual, while the figure was five percent for men with older sisters.
The findings echoed a study by the same group of researchers in the Netherlands published earlier this year.
This research found that the strongest relationship was found with men who have three older brothers: these participants were considered 41 percent more likely to be gay.
They found that if you have five older brothers, the chance of being gay is about eight percent, which is four times the chance of the average man.
“The vast majority of people who have many older siblings will be attracted to the opposite sex,” said Dr. Scott Semenyna, a psychology professor at Stetson University.
Scientists are still baffled as to why this phenomenon exists.
One theory is the “maternal immune hypothesis.”
Proponents say that after giving birth to a child, proteins enter the mother’s bloodstream and trigger the production of antibodies that influence 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 that “mothers of gay sons have more of these antibodies targeting male-specific proteins, compared to mothers of non-gay sons or mothers who do not have sons,” according to the Dr. Scott Semenyna, an Assistant Professor in the Department of Psychology at MacEwan University in Canada.
However, this theory was developed before the connection between older sisters and homosexuality was discovered.
‘Of course, there may be other explanations. It is up to prospective research to clarify this,” Kabátek said.