Are you a gay man? You may have your older brothers to thank.
A new study from the journal PNAS sheds new light on the "fraternal birth order effect" — a previously observed trend that gay men, on average, have a greater number of older brothers than straight men.
The research looked into an "immunlogoical explanation" to explain this trend. It found that some mothers, while pregnant, created antibodies in their bloodstream in response to a protein linked to the Y chromosome of an unborn son.
If a mother with a buildup of these antibodies becomes pregnant with another boy, these antibodies may pass through to the fetus and affect its brain development.
"That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction," said author Anthony Bogaert, a psychologist from Brock University in Canada, told CNN.
Resarchers conducted tests for this antibody, NLGN4Y, in blood samples from 142 women and 12 men. They discovered the highest concentration of NLGN4Y in women who had gay sons with older brothers.
A 2006 study found that a boy is likelier to be gay the more older biological brothers he has. This new research may help explain why. The study offers no explaination for same-sex attraction in women, bisexuality, or gay only sons.
Earlier this month, a separate study found two genes more common in gay men. One of its authors, J. Michael Bailey, called the new study "significant."
"Our studies only show that there may be genes that matter in sexual orientation," Bailey told CNN. "It is not like this study, that shows there is a potential specific mechanism by which sexual orientation may have changed prenatally. This is important work and fascinating if it proves to be true."