The mystery of why homosexuality is not phased out of the genetic pool has been explained by Italian scientists.
Andrea Camperio-Ciani, an evolutionary psychologist at the University of Padova, says that homosexuality in males may be caused in part by genes that can increase fertility in females.
The researcher says that he and his colleagues have observed that some female relatives of gay men tend to have more children than average.
He says that a study on about 200 Italian families conducted in 2004 showed that mothers, aunts, and maternal grandmothers of gay men were more fecund than average.
With a view to explaining how two genes passed on through the maternal line could result in such an effect, Camperio-Ciani and his colleagues used a computer model.
He says that his team tried to explain their findings with a number of genetic models, and found one that fit the bill.
"This is the first time that a model fits all our empirical data. These genes work in a sexually antagonistic way - that means that when they're represented in a female, they increase fecundity, and when they're represented in a male, they decrease fecundity. It's a trait that benefits one sex at the cost of the other," Live Science quoted Camperio-Ciani as saying.
Writing about their observation in the journal PLoS One, the researchers say that this scenario may help explain the seeming paradox of hereditary homosexuality if it turns out to be true.
Camperio-Ciani points out that, if the same genes lead to both homosexuality in men and increased fertility in women, any loss in offspring that come about from the males would be made up for by the females of the family.
"Sexually antagonistic selection is an old idea by Richard Dawkins, but this has never been proven in humans. There are a large quantity of these traits found in insects, for example, and recently in deer sexually antagonistic traits have been discovered, showing that high-ranking males produce rather unsuccessful daughters. We found that sexually antagonistic selection is operating also in our species, and we found it in a very important trait, which is homosexuality," he said.
He believes that the genes his team modelled might cause people of both sexes to be extremely attracted to men, thereby causing men to pursue relationships with other men, and women to have more sexual partners and become pregnant slightly more often than an average woman.
Camperio-Ciani, however, added that such a phenomenon could not lead to homosexuality in women.
"We're still working on lesbianism, but were not getting to the same result, and possibly we'll come out with a completely different explanation," he said.
The researchers also said that even if the sexually antagonistic genetic system was behind such effects, it could only account for a portion of overall causes of homosexuality in men, and that other genetic and social factors might also play a part.
"I think it's almost beyond a doubt that genes have some influence. My personal view is that there is probably more than one biological mechanism contributing toward homosexuality. I think it's also safe to say that there is at least one non-genetic influence," said Ray Blanchard, a researcher at the Centre for Addiction and Mental Health in Toronto, who studies the effect of birth order in predicting whether a male will be born homosexual.
Camperio-Ciani said that the research might help understand whether homosexuality is a choice, or whether it is caused by factors beyond a person's control.
"I think this is an example where the results of scientific research can have important social implications. You have all this antagonism against homosexuality because they say it's against nature because it doesn't lead to reproduction. We found out this is not true because homosexuality is just one of the consequences of strategies for making females more fecund," he said.