One answer seems to be a phenomenon called the grandmother effect. For every 10 years a woman survives past the menopause, she gains two additional grandchildren, Lummaa says. It seems that doting on and spoiling grandchildren aids their survival, as well as furthering some of their grandmother’s genes.
Men, by contrast, can reproduce well into their 60s and even 70s and 80s, and most researchers assumed this explained their longevity. But Lummaa and colleague Andy Russell wondered whether other factors explained the long lifespan of men, such as a grandfather effect.
To test this possibility, the team analysed church-gathered records for 25,000 Finns from the 18th and 19th centuries. People tended to move little, no one practiced contraception and the Lutheran Church enforced monogamy.
Only widowed men could remarry, and if they had children with their new wife, they fathered more kids, on average, than men who married once.
But ultimately remarried men "don’t end up with any more grandchildren," Lummaa says. "If anything the presence of a grandfather was associated with decreased survival of grandchildren."
Perhaps, Lummaa adds, the children of the first mother lose out on food and resources that go to the second mother’s kids. "It's kind of the Cinderella effect."
Even fathers with only one wife provided no benefit to their grandchildren, a finding supported by previous research.
With the grandfather effect ruled out, Lummaa and Russell next wondered whether the constraints of human physiology explain male longevity. In the same way that men have nipples that evolved for women to nourish their young, male longevity might be a consequence of biological selection for long-lived women.
To answer this question, the researchers compared the lifespan of men from polygamous countries with those from monogamous nations.
Using data from the World Health Organization, Lummaa and Russell scored 189 countries on a monogamy scale of one to four - totally monogamous to mostly polygamous. They also took into account a country's gross domestic product and average income to minimise the effect of better nutrition and healthcare in monogamous Western nations.
Lummaa stressed that their monogamy score is a crude first stab, and they are working to find multiple ways to assess marriage patterns. The conclusions could evaporate under further analysis, she adds.
If female survival is the main explanation for male longevity, then monogamous and polygamous men would live for about the same length of time. Instead, it seems that fathering more kids with more wives leads to increased male longevity. Men, then, live long because they're fertile well into their grey years.
The explanation could be both social and genetic. Men who continue fathering kids into their 60s and 70s could take better care for their bodies because they have mouths to feed. But evolutionary forces acting over thousands of years could also select for longer-lived men in polygamous cultures.
"It's a valid hypothesis and good prediction," says Chris Wilson, an evolutionary anthropologist at Cornell University in Ithaca, New York, who attended the talk. But the care and attention of several wives who depend on the social status of their ageing husband could explain everything. "It doesn't surprise me that men in those societies live longer than men in monogamous societies, where they become widowed and have nobody to care for them."