Having one or more older brothers boosts the likelihood of a boy growing up to be gay - an effect not of social factors, but of biological events that occur in their mother's womb, according to a study published today.
In an analysis of 905 men and their siblings, Canadian psychologist Anthony Bogaert found no evidence that social interactions among family members play any role in determining whether a man is gay or straight.
The only significant factor was the number of times a mother had previously given birth to boys, according to the report in the Proceedings of the National Academy of Sciences.
The so-called fraternal birth order effect is small - each older brother increases the chances by 33 percent. Bogaert said that assuming the base rate of homosexuality among men is 2 percent, it would take 11 older brothers to give the next son about a 50-50 chance of being gay.
In a previous study, Bogaert and his colleagues estimated that one in seven gay men in North America - roughly 1 million people - can attribute their sexual orientation to fraternal birth order.
Bogaert said he doesn't know what mechanism is behind the effect, which he and a colleague first identified 10 years ago.
The leading theory is that women pregnant with boys are exposed to male proteins that their bodies view as foreign, and they respond by making antibodies to fight them, said Bogaert, a professor at Brock University, Ontario.
Those antibodies could affect the developing fetus, and the more times a woman has carried boys, the stronger the antibody response would be.
This theory, dubbed the maternal immunization hypothesis, was first proposed in 1985 to explain why boys are more likely than girls to develop conditions such as attention deficit hyperactivity disorder, autism and dyslexia.
The study provides more evidence of the role biology plays in determining sexuality.