The more older brothers a boy has, the more likely he is to be gay, according to a study published in the weekly New Scientist magazine. The study, conducted by a Canadian researcher, found that boys with a statistical average of 2.5 older brothers are twice as likely to be gay as boys with no older brothers, Agence France-Presse reports. A boy with four older brothers is three times as likely to be gay. One theory behind the study is that a male fetus triggers an immune system response in the mother, which in turn affects the fetus's brain at a critical stage of its development. The more boys a mother carries, the stronger that antibody response. If the Canadian researcher is correct, "then clearly, as average family size decreases, so will the incidence of male homosexuality," the New Scientist article says. "It also follows that, historically, there have been more gay men than there are today."