THE likelihood of a man being gay is related to how many older brothers he has, with each sibling increasing his chances, a new study claims.
The study builds on 20 years of research into the links between how many siblings a man has and his sexual orientation, which has been conducted by experts from Canada's Brock University, the University of Toronto, and Harvard Medical School.
The findings were published in the scientific journal PNAS on Monday.
"Gay men have, on average, a greater number of older brothers than do heterosexual men, a well-known finding within sexual science," wrote the researchers.
The phenomenon is referred to as the 'fraternal birth order effect' and has been known since 1958 when it was noted that homosexual men tend to have a greater number of older siblings than heterosexual men.
In 1996, research conducted by Ray Blanchard and Anthony Bogaert confirmed the findings only applied to homosexual men with older brothers and not older sisters.
Since then multiple studies have been conducted on the subject, finding that each older brother increases a male's odds of having a homosexual orientation by 28 to 48 per cent.
So while the presence of a pattern isn't new, researchers now believe they may have discovered a biological explanation to the occurrence.
The study found that when a woman gets pregnant with her first male child a protein associated with the Y chromosome called NLGN4Y - which isn't produced in females - enters into her bloodstream.
Her body then recognises this as a foreign substance and her immune system responds by creating antibodies and, in some cases, these antibodies can enter the brain of the next male child she gets pregnant with.
"That may alter the functions in the brain, changing the direction of how the male foetus may later develop their sense of attraction," lead researcher Anthony Bogaert and professor of psychology and community health sciences at Brock University, told CNN.
To put their findings to the test, Bogaert and his team collected samples from 142 woman, along with a control group of 12 men, and tested them for antibodies to the NLGN4Y protein.
They found the highest concentration of antibodies in women with gay younger sons who had older brothers, compared to women who had no sons or who had given birth to heterosexual boys.
"The implications of this study, especially if and when it is replicated by an independent team, are profound. Along with more deeply understanding the exact origin of the older brother effect, it helps solidify the idea that, at least in men, there's a strong biological basis to sexual orientation," Dr Bogaert told Medical Express.
"This is the culmination of more than 20 years of research where we started looking at the older brother, or fraternal birth order, effect. The current study adds to the growing scientific consensus that homosexuality is not a choice, but rather an innate predisposition."
The study emphasised that, while these findings were significant in further understanding sexual predispositions, it did not suggest that every man would have certain sexual preferences just because he had older brothers.
"Indeed, although most of our key effects were of a notable statistical magnitude, it is also clear that only a portion of variation in men's sexual orientation is accounted for by these effects," the study read.
"Sexual orientation is clearly a complex phenomenon with likely many factors influencing it."
Update your news preferences and get the latest news delivered to your inbox.