Researchers have found new evidence that men with older brothers are more likely to be gay.

Previous studies suggest that for each older brother a male has, the odds his is gay goes up by about a third—even if the brothers were raised in separate households.

But a new study in Proceedings of the National Academy of Sciences (PNAS) points to a biological factor for that pattern.

Researchers at Brock University theorize that when a woman is pregnant with her first male child, NLGN4Y, a protein linked to the Y chromosome enters her bloodstream. Because women don't have Y chromosomes, it could cause her immune system to create antibodies that affect subsequent male fetuses.

"That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction," lead author Anthony Bogaert told CNN.

Bogart's team tested 142 women and 12 men ages 18 to 80 for antibodies to NLGN4Y, and found the highest levels in women who had gay male children with straight older brothers. Men in general, women who had no sons, and women who'd given birth only to heterosexual boys had lower levels of NLGN4Y antibodies.

The sample size in this study was relatively small and the research doesn't offer a biological explanation for gay oldest sons, bisexuality or asexuality among men, or lesbianism in women. Still, researchers working to find a genetic link to sexual orientation believe the new findings are significant.

"[Previous] studies only show that there may be genes that matter in sexual orientation," Psychologist J. Michael 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."

Last week researchers at lllinois’ North Shore University reported on two gene variants that are more common in gay men.