- The Washington Times - Monday, June 26, 2006


Men who have several older brothers have an increased chance of being homosexual, 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 — irrespective of whether they were raised together — but not in those who had adopted brothers or stepbrothers who were older.

“It’s likely to be a prenatal effect,” said Anthony F. Bogaert of Brock University in St. Catharines, Ontario, who did the research. “This and other studies suggest that there is probably a biological basis” for homosexuality.

Mr. Bogaert studied four groups of Canadian men, a total of 944 persons, analyzing the number of brothers and sisters each had, whether they lived with those siblings and whether the siblings were related by blood.

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,” Mr. Breedlove said in a telephone interview.

The only link among the brothers is the mother, and so the effect has to be through the mother, especially because stepbrothers didn’t have the effect, said Mr. Breedlove, who was not part of the research.

Tim Dailey, a senior fellow at the conservative Family Research Council’s Center for Marriage and Family Studies, disagreed.

“We don’t believe that there’s any biological basis for homosexuality,” Mr. Dailey said. “We feel the causes are complex but are deeply rooted in early childhood development.”

There have been several attempts to establish a physical basis, he said, “and in every case, the alleged findings have been severely challenged and questioned.”

Mr. Bogaert said this needs to be looked at in context of the overall rate of homosexuality in men, which he said 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.

Mr. Bogaert said he concluded that the effect was biological by comparing men with biological brothers and those with brothers to whom they were not biologically related.

The increase in the likelihood of being homosexual was seen only in those whose brothers had the same mothers, irrespective of whether they were raised together, 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 said, 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 also is female.

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