WASHINGTON -- 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 they lived with those siblings, and whether the siblings were related by blood or adopted.
His findings are reported in a paper appearing in today'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."
Bogaert said the increase can be detected with one older brother and becomes stronger with three 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.
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.
One possibility, Bogaert 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.
Whether that is 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.