Homosexuality could be influenced by genetics as well as culture.© Punchstock

A survey of Italian men has provided evidence that homosexuality may be partly influenced by genetics. The same genes that are proposed to predispose to homosexuality may also boost reproduction in women, solving the apparent paradox of why these genes have not been removed by natural selection.

By quizzing around 200 men of different sexual orientations, researchers at the University of Padua have discovered that maternal relatives of homosexual men tend to produce more offspring than those of heterosexuals. This suggests that the mothers and maternal aunts of homosexuals have a genetic advantage - but one that reduces reproduction when passed to male offspring.

"For a long time it has been a paradox," says Andrea Camperio-Ciani, who led the study. "But we have found that there might be a set of genes that, in males, influences homosexuality but in females increases fecundity."

We don't know what these genes might be, but they are probably spread across a range of different chromosomes, says Camperio-Ciani. If the effect were down to a single gene it would probably spread easily throughout the population, and homosexuality would presumably be much more common, he explains.

But whatever the genes are, the X chromosome is almost certainly involved, he adds. Men have an X and a Y chromosome as opposed to women's pair of X's, so a man's single X chromosome is always inherited from his mother. In the survey, the difference in reproductive power was evident only in the mother's side of the family.

In the family

The researchers selected homosexual and heterosexual men from Italy's bars and clubs and asked them to fill in a questionnaire about their family trees. Besides having more fecund mothers and aunts, homosexual men had more fellow homosexuals in their maternal family, again hinting that their sexuality is influenced by their mother's genes.

What's more, homosexual men were more likely to have older brothers, which supports a separate theory that homosexuality is linked to changes in the mother's immune system during previous male pregnancies. The findings are reported in an online version of Proceedings of the Royal Society of London B¹.

Overall, about 14% of the variation between homosexual and heterosexual men is accounted for by increased maternal reproduction, and about 6% by the trend to have older brothers, Camperio-Ciani says. This means that, if homosexuality is influenced by genetics or in the womb, the effect is only one of many social and environmental factors.

"We know we've only explained 20% of the pattern," says Camperio-Ciani. The remaining 80% could perhaps be due to formative sexual and social experiences during early life or even childhood, he speculates.

Culture has a part to play in both the real and perceived abundance of homosexuality, Camperio-Ciani points out. "Homosexuality is present in almost any culture, but often it is not reported because of prejudice," he says.

But this study shows that, although there may not be a 'gay gene', it is possible that genetic factors can influence sexuality. "We strongly believe that this set of genes influences sexual expression, but they don't determine it," Camperio-Ciani says. 

University of Padua

University of Padua

• References

  1. Camperio-Ciani A., Corna F. & Capiluppi C., Proc. R. Soc. Lond. B, published online, doi:10.1098/rspb (2004).