I thought that this was interesting as well.
“Having one or more older brothers boosts the likelihood of a boy growing up to be gay–an effect due not to social factors, but biological events that occur in their mother’s womb, according to a study published Tuesday.
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.
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 about 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.”