THE biological enigma of how homosexuality evolved despite its obvious drawbacks for reproduction may finally have been resolved.

The genes that make men gay also help their female relatives to have bigger families, according to new research.

Scientists have discovered that gay men’s mothers, sisters and maternal aunts tend to have significantly more children than the norm — and that many of their nephews and male cousins are also gay.

The findings suggest that the same genes that trigger homosexuality in men also promote fertility in women, and that this could explain how they survive in the population when gay men themselves are unlikely to breed. The genes are instead passed on through the female line and the enhanced fertility they confer on these women ensures that they are inherited by plenty of children.

Some of these sons will grow up to be homosexual themselves. The study also revealed that gay men are more likely than heterosexuals to have a gay male relative, though only on their mother’s side of the family.

The results, from the University of Padua, in Italy, offer strong support for the theory that homosexuality is at least partly determined by a person’s genetic make-up, and is not just about personal choice or upbringing and environment. It also suggests an elegant solution to the biggest problem with this hypothesis — the “Darwinian paradox” that any genes that favour homosexuality ought to have died out through natural selection, as those that inherited them had fewer and fewer offspring.

Andrea Camperio-Ciani, who led the research, said: “Our data resolve this paradox by showing that there might be hitherto unexpected reproductive advantages associated with male homosexuality.”

The work also points to a likely location for the genes that have this effect: they almost certainly lie on the X chromosome, the package of DNA that men always inherit from their mothers.

In the study, published today in the journal Proceedings of the Royal Society, Dr Camperio-Ciani’s team interviewed 98 homosexual and 100 heterosexual men in detail about their extended families. In total, more than 4,600 individuals were thus indirectly involved. They found that both the mothers and maternal aunts of the homosexuals were significantly more fertile than those of the straight men: the mothers had an average of 2.69 children compared with 2.32, and the aunts 1.98 children compared with 1.51.

Fertility rates among paternal relatives and among male relatives on the mother’s side were similar for both groups.

All this points to genes that influence both male homosexuality and female fertility being passed down along the maternal line. “The results hypothesise that genetic factors, transmitted in the maternal line, increase both the probability of being homosexual in males and fecundity in females,” Dr Camperio-Ciani said.

The study did not investigate lesbianism. The notion that homosexuality has at least some basis in biology is not now seriously disputed by scientists, though there is little consensus on what the causes might be. Some scientists think that genetics are critical, while others believe that conditions in the womb are all-important.

The question of what causes homosexuality has long divided both the gay community and social conservatives who regard same-sex partnerships as wrong. Many gay activists think that identifying biological factors that contribute to homosexuality will prove that their sexual orientation is perfectly natural and encourage tolerance. Others fear that it will lead to greater hostility, with the risk that being gay will again be seen as a disorder that might one day be “cured”.

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