One possible explanation is that having a grandmother around somehow improves the reproductive potential of her grandchildren. In fact, that is exactly what the researchers found when they reviewed stats on 537 Finnish women who had a combined total of 6,002 grandchildren. Adding in data from more than 3,000 French Canadians (who had a modest 100,074 grandchildren) confirmed that having grandma around to help enabled younger women to have more children sooner and with improved chances of surviving into adulthood. "That suggests that perhaps one reason why women do carry on living is because they are able to help," Lummaa says.
Of course, studying humans requires teasing out the confounding cultural effects. For example, the Finnish data indicated that child mortality was much higher in mainland towns than on the islands of Finland's Archipelago Sea. This can be traced back to the fact that mainland women were responsible for farm work, leading to earlier replacement of mother's milk with cow's milk. "That led to infections," Lummaa notes. "In the archipelago this was not the case." Birth rates in both areas also tended to cluster roughly nine months after the period when Finns traditionally married: after the fall harvest.
Studying humans has other pitfalls, most notably that it's very easy to become involved with your subjects, Lummaa says. "We have thousands of people. I can't say I know every one of them but there are some families who pop out," she recalls. "One woman had 18 children and every single one died before adulthood while she lived into her 90s without any of these children." She adds: "If you are studying humans, you can't help feeling more connected to whatever you find out."
Lummaa is learning that first hand these days, having recently given birth to a three-month-old son, Eelis. "It's your own child, you can't have a scientific attitude," she admits, "but you are thinking, 'Well, what in the patterns I see is genetic and is it coming from the mum or dad?' I'm always trying to see my parents' traits in my son." She is thrilled, of course, though her research warns it bodes ill for her life expectancy. Premodern Finnish mothers among the Sami people (famous for their reindeer herding) who bore sons had shorter life spans than those who gave birth to daughters. This has to do with birth weight—male babies are typically larger—but also with that dreaded male hormone, testosterone. "Testosterone can compromise your immune system, it can affect your health," Lummaa says. "Boys are a little bit more costly than girls" to raise, because they drain more physical resources from their mothers. Sons also are not as likely as daughters to stick around to help their mothers out later in life.
Fortunately for Lummaa, she has the benefit of modern medicine. But "I can certainly see that it's taking a lot of energy and I'm sure it's aging me," she says, chuckling. "How on earth these women managed to give birth every year is truly amazing."
Lummaa has now turned her attention to the effect of grandfathers on grandchildren. If grandmothers improve survival odds, what do elderly males contribute? "If anything there's a negative effect," she says. This could be because of the cultural tradition of catering to men, particularly old men. "Maybe if you had an old grandpa, he was eating your food," she speculates. Or it could be that because men can continue to reproduce, they are less vested in anyone other than their own children. Another possible reason is that women can be sure that a grandchild is their genetic descendant, but it is more difficult for grandfathers. This may also have spurred them to seek second and even third wives rather than focusing on their children. "We are comparing men who married once in their lifetime[s] with men who are married several times," Lummaa says.
Lummaa is not alone in using human history to try to enhance evolutionary understanding. A recent study by ethnologist Dustin Penn of the Austrian Academy of Sciences in Vienna and population scientist Ken Smith of the University of Utah, using Mormon church records from the 19th century, discovered that having more children upped women's chances of dying prematurely. Anthropologist Kristen Hawkes, also at Utah, reached the same conclusions as Lummaa about the utility of grandmothers after studying populations of human hunter-gatherers in Africa and South America. "It's most interesting to find out what's causing the differences between human populations," Lummaa says. "How do those general evolutionary theories actually explain the patterns we see in humans? And how much is due to other reasons?" As Lummaa says, "We've got more data than we've got time to analyze." Meaning Lummaa, her colleagues and her scientific descendants will have plenty to study until she is a grandmother herself.