New research suggests that the more biological older brothers a man has, the more likely he is to be gay.

For the study, researchers collected data from 1,000 men including those who were adopted. The results showed that having older stepbrothers or older adopted brothers raised in the same household did not seem to have the same affect on sexuality as having older biological brothers, even if they weren't raised in the same household.

The theory stems from the immune response that occurs in women's bodies when they're pregnant with a male child. This produces antibodies that affect the part of the fetus' brain linked to sexual orientation.  This immune response increases with each pregnancy (although no link was found between a girl having more older sisters and being a lesbian.)

Researchers at Brock University in Ontario believe that the findings will add more weight to the whole debate on whether gays truly are "born this way" or simply being influenced by the media.

The Telegraph