Men With Older Brothers More Likely to Be Gay
By RANDOLPH E. SCHMID , 06.26.2006, 02:53 PM
Men who have several older brothers have an increased chance of being
gay, researchers say, a finding that adds weight to the idea that
sexual orientation has a physical basis.
The increase was seen in men with older brothers from the same mother
- whether they were raised together or not - but not those who had
adopted or stepbrothers who were older.
"It's likely to be a prenatal effect," said Anthony F. Bogaert of
Brock University in St. Catharines, Canada, who did the research.
"This and other studies suggest that there is probably a biological
basis" for homosexuality.
Bogaert studied four groups of Canadian men, a total of 944 people,
analyzing the number of brothers and sisters each had, whether or not
they lived with those siblings and whether the siblings were related
by blood or adopted.
His findings are reported in a paper appearing in Tuesday's issue of
Proceedings of the National Academy of Sciences.
S. Marc Breedlove, a professor in the neuroscience and psychology
department of Michigan State University, said the finding "absolutely"
confirms a physical basis.
"Anybody's first guess would have been that the older brothers were
having an effect socially, but this data doesn't support that,"
Breedlove said in a telephone interview.
The only link between the brothers is the mother and so the effect has
to be through the mother, especially since stepbrothers didn't have
the effect, said Breedlove, who was not part of the research.
Tim Dailey, a senior fellow at the conservative Center for Marriage
and Family Studies disagreed.
"We don't believe that there's any biological basis for
homosexuality," Dailey said. "We feel the causes are complex but are
deeply rooted in early childhood development."
There have been a number of attempts to establish a physical basis
"and in every case the alleged findings have been severely challenged
and questioned," he said.
"If it is indeed genetically based it is difficult to see how it could
have survived in the gene pool over a period of time," Dailey added.
Bogaert said the increase can be detected with one older brother and
becomes stronger with three or four or more.
But, he added, this needs to be looked at in context of the overall
rate of homosexuality in men, which he suggested is about 3 percent.
With several older brothers the rate may increase from 3 percent to 5
percent, he said, but that still means 95 percent of men with several
older brothers are heterosexual.
The effect of birth order on male homosexuality has been reported
previously but Bogaert's work is the first designed to rule out social
or environmental effects.
Bogaert said he concluded the effect was biological by comparing men
with biological brothers to those with brothers to whom they were not
The increase in the likelihood of being gay was seen only in those
whose brothers had the same mothers, whether they were raised together
or not, he said.
Men raised with several older step- or adopted brothers do not have an
increased chance of being gay.
"So what that means is that the environment a person is raised in
really makes not much difference," he said.
What makes a difference, he said, is having older brothers who shared
the same womb and gestational experience, suggesting the difference is
because of "some sort of prenatal factor."
One possibility, he suggests, is a maternal immune response to
succeeding male fetuses. The mother may react to a male fetus as
foreign, but not to a female fetus because the mother is also female.
It might be like the maternal immune response that can occur when a
mother has Rh-negative blood but her fetus has Rh-positive blood.
Without treatment, the mother can develop antibodies that may attack
the fetus during future pregnancies.
Whether that's what is happening remains to be seen, but it is a
provocative hypothesis, said a commentary by Breedlove, David A. Puts
and Cynthia L. Jordan, all of Michigan State.
The research was funded by the Social Sciences and Humanities Research
Council of Canada.