BEIJING, Sept. 18 (Xinhuanet) -- New research suggests that older men who "rob the cradle" seeking companionship and sex with younger women are actually doing mankind a favor by helping extend our species' lifespan.

    The study, published in the Aug. 29 issue of PLoS ONE, notes that while women often lose their reproductive capacity around age 50, men can often remain productive into their 70s. From an evolutionary perspective, women who can no longer reproduce are non-players, and since "it takes two," men partnered with menopausal women are also irrelevant.

    More interesting, when old men father children, their genes seem to increase the lifespan of both sexes over evolutionary time.

    Thus, Charles Darwin would say it's advantageous for males to live longer lives providing they can find a woman capable of reproducing. Natural selection should favor longevity-boosting genes, which would get passed down from fathers to both sons and daughters. So women would benefit as well in future generations, the scientists say.

    "By increasing the survival of men you have a spillover effect on women because men pass their genes to children of both sexes," said study team member Cedric Puleston, a doctoral candidate at Stanford University.

    The result is that over time, May-December relationships would increase the lifespan for both men and women in the next generations and so on.

    To figure out whether male fertility could help explain human longevity, Shripad Tuljapurkar of Stanford University and his colleagues examined lifespan and fertility data from both men and women.

    They studied four societies thought to closely mimic lifestyles of our ancestors, including two hunter-gather groups, the Dobe !Kung of the Kalahari and the Ache of Paraguay (one of the most isolated populations in the world), as well as the Yanomamo forager-farmers and an indigenous group in Bolivia called the Tsimane. The research team also looked at farming villages in Gambia and a group of modern Canadians.

    In all six groups, women stopped having children on average by their 50s, while some men continued to reproduce. The age after which men showed no reproduction varied among the groups. Topping the fertility list was Gambia (75 years old), followed by Yanomamo (70), Ache (65), Tsimane (60), Canada and !Kung (55).

    In less developed, traditional societies, males are about 5 to 15 years older than their female partners. In the United States and Europe, men are an average of two years senior to their partners.

    In a study of about 10,000 Swedish men and women, Fieder and his colleagues have found that men had the most children if they were partnered with women about six years younger than themselves.

    (Agencies)