If this is true, then the helper in the nest theory may partly explain how a genetic trait for same-sex attraction hasn't been selected away. That hypothesis has led Vasey to speculate that the gay men who identify as men and have masculine traits - that is to say, most gay men in the West - are descended from men who had a cross-gendered sexuality.

Gay people do have children

In the US, around 37% of lesbian, gay, bisexual and transsexual people have a child, about 60% of which are biological. According to the Williams Institute, gay couples that have children have an average of two.

These figures may not be high enough to sustain genetic traits specific to this group, but the evolutionary biologist Jeremy Yoder points out in a blog postthat for much of modern history gay people haven't been living openly gay lives. Compelled by society to enter marriages and have children, their reproduction rates may have been higher than they are now.

How many gay people have children also depends on how you define being "gay". Many of the "straight" men who have sex with fa'afafine in Samoa go on to get married and have children.

"The category of same-sex sexuality becomes very diffuse when you take a multicultural perspective," says Joan Roughgarden, an evolutionary biologist at the University of Hawaii. "If you go to India, you'll find that if someone says they are 'gay' or 'homosexual' then that immediately identifies them as Western. But that doesn't mean there's no homosexuality there."

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