Female relatives of gay men have bigger than average families, a controversial study into the biological basis of homosexuality reveals.

Female relatives of gay men have bigger than average families, a controversial study into the biological basis of homosexuality reveals.

However, this was only the case when the women were related to the man through his mother, the study found.

The findings are important because they provide an explanation for an apparent contradiction of the "gay gene" theory, which implies that if homosexuality is genetic then its determining genes would die out as gay men tend to have fewer children than heterosexual men.

However, the problem is resolved if the genetic factors that lead to a predisposition to homosexuality and a corresponding lower fecundity in men cause a higher fecundity in the men's female relatives. Such a link means that genetic factors that predispose boys to becoming homosexual will never die out in a population because their sisters, mothers and maternal aunts will continue to spread the genes by having more than the average number of children.

The findings have emerged from a study of the extended families of 98 homosexual men and 100 heterosexual men, to see if there are significant differences in fertility linked to a family member being gay.

Andrea Camperio-Ciani, of the University of Padova in Italy, and his colleagues said that by analysing family histories and the number of children in each family it was possible to study the so-called "Darwinian paradox" of homosexuality.

"The paradox is this. If male homosexuality has a genetic component, and homosexuals reproduce less than heterosexuals, then why is this trait maintained in the population?" Dr Camperio-Ciani said.

"Our data resolves this paradox by showing that there might be hitherto unsuspected reproductive advantages associated with male homosexuality," he said. The study, in The Proceedings of the Royal Society of London B, does not assume that homosexual men never have children, only that they are, on average, less likely to have as many children as heterosexual men.

Dr Camperio-Ciani said the study found that about 20 per cent of the predisposition towards being gay is caused by unknown genetic factors - the rest being the result of upbringing or personal experience. "I'm not a genetic extremist. I'm just saying that gay men might be born with a predisposition to being homosexual which is influenced by personal experience," he said. But any suggestion that male homosexuality had a genetic component had to deal with the Darwinian paradox, as the study had done, he said.

The genetic factors for homosexuality - whatever they were - consisted of several genes that tended to be passed on through the maternal line because gay men had more maternal than paternal gay relatives, the study suggested.

One possible explanation for the finding that gay men have bigger extended families is that the bigger the family size, the more likely it is that some of the male offspring will be gay. However, this did not explain why the study found that the maternal aunts of gay men had significantly larger families than their paternal aunts, Dr Camperio-Ciani said.

Other research has indicated that at least some of the genetic predisposition to being gay was carried on the X chromosome, which men inherit through their mothers, but Dr Camperio-Ciani said that genes on other chromosomes were almost certainly involved in determining sexuality.

Scientists have also demonstrated repeatedly that the chance of a man being a homosexual rises by about a third for each older brother he has. They found that a man with three older brothers was about twice as likely to be gay as a man with none. This suggests that there may be biological factors operating within the womb of a woman who has already given birth to a number of sons that increase the predisposition towards her next son being gay.