A study of gay men found evidence that male sexual orientation is influenced by genes, but not entirely, U.S. researchers say.
Ian Sample writes at The Guardian:
Michael Bailey, a psychologist at Northwestern University in Illinois, set out the findings at a discussion event held in conjunction with the annual meeting of the American Association for the Advancement of Science in Chicago on Thursday. “The study shows that there are genes involved in male sexual orientation,” he said. The work has yet to be published, but confirms the findings of a smaller study that sparked widespread controversy in 1993, when Dean Hamer, a scientist at the US National Cancer Institute, investigated the family histories of more than 100 gay men and found homosexuality tended to be inherited. More than 10% of brothers of gay men were gay themselves, compared to around 3% of the general population. Uncles and male cousins on the mother’s side had a greater than average chance of being gay, too.
… The flawed thinking behind a genetic test for sexual orientation is clear from studies of twins, which show that the identical twin of a gay man, who carries an exact replica of his brother’s DNA, is more likely to be straight than gay. That means even a perfect genetic test that picked up every gene linked to sexual orientation would still be less effective than flipping a coin.
While genes do contribute to sexual orientation, other multiple factors play a greater role, perhaps including the levels of hormones a baby is exposed to in the womb. “Sexual orientation has nothing to do with choice,” said Bailey. “We found evidence for two sets [of genes] that affect whether a man is gay or straight. But it is not completely determinative; there are certainly other environmental factors involved.”