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Researchers have known for years that a man's likelihood of being gay rises with the number of older biological brothers. But the new study found that the so-called "fraternal birth order effect" persists even if gay men were raised away from their biological families.
Study author Anthony F. Bogaert said the findings add to a growing body of evidence that links homosexuality to nature, not nurture.
"The research suggests that the development of sexual orientation is influenced before birth," said Bogaert, a professor of community health sciences at Brock University in St. Catharines, Ontario, Canada.