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.
The leading theory is that women's bodies react to male fetuses' proteins as foreign, making antibodies to fight them, Bogaert said.
Such 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 originally proposed in 1985 to explain why boys are more likely than girls to develop conditions such as attention deficit hyperactivity disorder, autism and dyslexia.
"We thought it might be an interesting explanation for this," Bogaert said.
Scientists have not found any antibodies that may be responsible, but Michigan State University neuroscientist Marc Breedlove is trying to identify them in pregnant mice.
"We would love to identify the protein that she is targeting, or find out which brain regions are being affected," said Breedlove, who coauthored a commentary that accompanies the study. "Right now, it's the only plausible mechanism we can think of."
Scientists have found other genetic links to sexual orientation. For example, if one identical twin is gay, there is a 52% chance that the other twin -- who has the same DNA -- is gay, according to a 1991 report in the Archives of General Psychiatry. Among fraternal twins, who share about half their DNA, the figure drops to 22%, and for other brothers it is 9%, according to the study.
Bogaert first reported a link between sexual orientation and older brothers in a 1996 study conducted with Ray Blanchard, who runs the Clinical Sexology Program at the Center for Addiction and Mental Health in Toronto. That finding has been replicated since then in other data on men in the U.S., Canada and Europe, as well as in data collected by the pioneering sex researcher Alfred Kinsey in the 1940s and 1950s.
In the new study, Bogaert's aim was to figure out whether older brothers influence the sexuality of younger ones through nature or nurture.
If the influence were due to social factors as the boys were growing up, he reasoned, then older brothers would have an impact as long as they were reared together. On the other hand, if the explanation hinged on prenatal biological factors, the physical presence of older brothers during childhood would be irrelevant.
Bogaert collected biodemographic data on gay and straight men raised in families with various combinations of older and younger brothers and sisters. Some were full siblings, some shared only a mother or a father, some were step siblings, and some siblings were adopted.
"It doesn't seem to be that having an older brother around, regardless of whether that brother is a biological brother or a nonbiological brother, seems to have an effect on a man's sexual orientation," he said. "Biological older brothers, even ones they are not reared with, seem to be increasing the likelihood of male homosexuality."
Previous studies have looked at the impact of older sisters on the chances of a girl growing up to be a lesbian, but they found no correlation. That result bolsters the maternal immunization theory, because female fetuses do not produce proteins that would be unfamiliar to pregnant women and thus prompt the production of antibodies.