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 today.

Anthony Bogaert of Brock University in Ontario looked at the family structures of 944 gay and straight men, including men who grew up with multiple stepbrothers rather than biological brothers and men whose mothers had many sons before them but who did not grow up with those brothers.

The results, in short: nature, 1; nurture, 0.

It is the number of boys a boy’s mom has already had – not the number of boys he grows up with – that best predicts whether that boy will be gay.

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.