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Gay or straight? A saliva test can predict the answer, and get it right 67 per cent of the time – for male identical twins at least.

The test, which uses clues from tiny modifications to a person’s genome, is the first that claims to detect sexual orientation. Many scientists have expressed caution over the results, while concerns over potential misuse of the test have led the study’s lead researcher to quit the project entirely.

“The scientific benefit to understanding [why people vary in sexual orientation] is obvious to anyone with an iota of curiosity,” says Michael Bailey at Northwestern University in Evanston, Illinois. “The predictive test needs replication on larger samples in order to know how good it is, but in theory it’s quite interesting.”

Over the last two decades, several studies have suggested that sexual orientation is, in part, down to our genes. Perhaps the biggest splash was made in 1993 by Dean Hamer’s team at the National Cancer Institute in Bethesda, Maryland, when they found that gay brothers tended to share a sequence of five genetic markers...