Homosexuality in males may be caused in part by genes that can increase fertility in females, according to a new study.

The findings may help solve the puzzle of why, if homosexuality is hereditary, it hasn't already disappeared from the gene pool, since gay people are less likely to reproduce than heterosexuals.

A team of researchers found that some female relatives of gay men tend to have more children than average.

The scientists used a computer model to explain how two genes passed on through the maternal line could produce this effect.

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In 2004 the researchers studied about 200 Italian families and found that the mothers, maternal aunts and maternal grandmothers of gay men are more fecund, or fruitful, than average.

Recently, they 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," said Andrea Camperio-Ciani, an evolutionary psychologist at the University of Padova in Italy who led the study. "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."

The researchers detail their findings in the June 18 issue of the journal PLoS ONE.

• Click here to read the full paper at PLoS ONE.

If the same genes create both homosexuality in men and increased fertility in women, then any losses 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," Camperio-Ciani told LiveScience. "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."

A possible scenario

The question of whether homosexuality is genetically inherited has been perplexing scientists for years. While many researchers now agree homosexuality is probably caused by a mixture of nature and nurture, they are still hard pressed to explain the particulars.

Even if this sexually antagonistic genetic system is at work, it can only account for a portion of the overall causes of homosexuality in men, Camperio-Ciani said. Other factors, both genetic and social, likely also play a part.

"I think it's almost beyond a doubt that genes have some influence," said Ray Blanchard, a researcher at the Center for Addiction and Mental Health in Toronto, who studies the effect of birth order in predicting whether a male will be born homosexual. "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."

Blanchard found that with each older brother in a family, the odds increase by about a third that a boy born later will be gay. This effect is not thought to be caused by genetics, but rather by antibodies produced by the mother's immune system during pregnancy.

Eric Vilain, a professor of human genetics at the University of California, Los Angeles, has studied possible biological factors influencing homosexuality. He said the system studied by the Italian team seems plausible, but that it's too soon to be convinced.

"I would like to see the first observation reproduced in a different population and possibly with a larger sample to make sure that this holds up," he said. "If it is replicated, that's a very interesting finding. It's a possible scenario."

Research by Paul Vasey, a psychologist at the University of Lethbridge in Canada, and his graduate student, Doug VanderLaan, provides preliminary support for the Italian team's results.

The scientists studied homosexual men in Samoa, known locally as fa'afafine ("in the manner of a woman"). They found that the mothers of fa'afafine produce more offspring than the mothers of heterosexual men in that society.

"[Camperio-Ciani’s] results are consistent with a growing number of studies that suggest that the female relatives of male homosexuals are more fecund than those of male heterosexuals," Vasey said.

Loving men

Camperio-Ciani and his team hypothesize that the genes they modeled may cause people of both sexes to be extremely attracted to men, which would lead men with the genes to pursue relationships with other men, while causing women with the genes to have more sexual partners, and become pregnant slightly more often than an average woman.

This system does not address causes of homosexuality in women, he said.

"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 research may shed light on a complicated and controversial topic: 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," Camperio-Ciani said. "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."

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