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.
In an analysis of 905 men and their siblings, Canadian psychologist Anthony Bogaert found no evidence that social interactions among family members played a role in determining whether a man was 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%. Assuming the base rate of homosexuality among men is 2%, it would take 11 older brothers to give the next son about a 50-50 chance of being gay.
But at a time when, according to one survey, 42% of Americans consider homosexuality to be a lifestyle choice, the study provides more evidence of biology's role in determining sexuality.
"People are coming to realize that biology -- in a broad sense of the word -- does play an important role," said neurobiologist Simon LeVay, who has documented anatomical differences in the brains of gay and straight men. He is not connected with the study.
A 2003 survey found that 30% of Americans believed sexual orientation was innate and 14% said it was determined by upbringing, besides the 42% who considered it a lifestyle choice. That survey was conducted by the Pew Research Center for the People & the Press and the Pew Forum on Religion & Public Life.
Polls show that people who believe sexual orientation is governed by biology tend to support gay rights, whereas those who consider it a choice don't, said Dr. Jack Drescher, who chaired the American Psychiatric Assn.'s Committee on Gay, Lesbian and Bisexual Issues for six years.
"The question of whether it's biological is playing a large role in the culture wars," said Drescher, who was not involved in the study. "Decisions about civil rights and marriage are all argued around this issue."
In a previous study, Bogaert and his colleagues estimated that about one in seven gay men in North America -- roughly 1 million people -- could attribute their sexual orientation to fraternal birth order.
Bogaert, a professor of community health sciences and psychology at Brock University in Ontario, said he didn't know what biological mechanism was behind the fraternal effect, which he and a colleague first identified 10 years ago.
