“It seems as though the mother’s body is remembering the sex of previous pregnancies,” says Tuck Ngun at the University of California Los Angeles. The overall chance is still low, however, rising from around 2 per cent to just 6 per cent for a third son.
For every male pregnancy a woman has, a subsequent son has a 33 per cent higher chance of being homosexual, although no one knows why. Other observations also suggest a genetic basis for sexual orientation, such as the mysterious fraternal birth order effect.
The same region has been implicated in other studies of sexual orientation since, although researchers haven’t been able to single out “gay 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 in a region of the X chromosome. Over the last two decades, several studies have suggested that sexual orientation is, in part, down to our genes. “The predictive test needs replication on larger samples in order to know how good it is, but in theory it’s quite interesting.” “The scientific benefit to understanding is obvious to anyone with an iota of curiosity,” says Michael Bailey at Northwestern University in Evanston, Illinois.
