Results of the study, which builds on 20 years of research by Canada’s Brock University, the University of Toronto, and Harvard Medical School exploring the link between the number of brothers a man has and his sexual orientation, were published this week in scientific journal PNAS.

This is referred to as the ‘fraternal birth order effect’ and has been recognised by scientists since the 1950s. However, this new study offers a reason for the phenomenon.

The study revealed that when a woman gets pregnant with her first male child a protein associated with the Y chromosome only found in males called NLGN4Y enters into her bloodstream.

However, the body deems this protein a foreign substance and the immune system responds by creating antibodies which, in some cases, can enter the brain of the next male child she gets pregnant with.

After testing 142 women (along with a control group of 12 men) researchers found the highest concentration of antibodies in women with gay younger sons who had older brothers, in comparison with 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 brothers 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.”