According to a group of Italian researchers, the genes that make men gay evolved because they actually make their female relatives more fertile.

The team discovered that the mothers, aunts and sisters of gay men tend to have more children than those women related to straight men.

And that, they say, is why nature decided to preserve a trait that would normally prevent the genetic line furthering itself through children.

Their findings, published today, show that female maternal relatives of gay men have more reproductive success than their counterparts who are related to heterosexuals.

The difference is not seen in female relatives on the father's side.

The "Darwinian paradox" of gayness has puzzled scientists for decades. If male homosexuality is genetic, and homosexuals reproduced less than heterosexuals, the trait should eventually disappear.

Because this is clearly not the case, many people insist that there is nothing genetic about gayness.

Critics of the genetic explanation argue that homosexuality must be the result of nurture rather than nature.

The researchers, led by Dr Francesca Corna from the University of Padua, handed out anonymous questionnaires to 98 homosexual and 100 heterosexual men in northern Italy.

They were asked to provide information about their sexual orientation and about their siblings, first cousins, parents, aunts, uncles, and grandparents.

Analysis of the replies showed that the maternal relatives of homosexuals had higher reproductive success than those of heterosexuals, and that the difference did not appear in the paternal line.

The research is published today in the Proceedings of the Royal Society B: Biological Sciences.