WASHINGTON - Men who have several older brothers have an increased chance of being gay -- whether they were raised together or not -- a finding researchers say adds weight to the idea that sexual orientation is based in biology.
The increase was seen in men with older brothers from the same mother, but not those who had stepbrothers or adopted brothers 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.
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