Should we kill granny?

In Paraguay's Ache tribe, aging women used to listen with terror for the footsteps of the young men whose job it was to sneak up on them with an ax and brain them. 

Now most societies don't actually murder their grannies but the fact is that how women manage to attain old age is an evolutionary mystery. In nature, from the point of view of the selfish gene, creatures are supposed to drop dead as soon as they lose the power to reproduce. So why should women survive when they can no longer reproduce?

A man can make babies his whole life, even if the sperm of his old age lacks vigor and genetic fidelity. But a woman outlives her eggs by about 20 years, which almost no other female mammals do. (Only female killer and pilot whales and orcas are known to last as long after the end of their menstrual cycles). Besides being classed among the oddities of the animal kingdom, post-menopausal women lack obvious utility. They tend to be weak. They don't have much sex appeal. They eat food working people might make better use of. 

In the 1980's Hawkes and her colleagues developed the "grandmother hypothesis," which holds that women past childbearing age helped not just their children, but their children's children, and lengthened the human lifespan in the process. Without babies of their own to lug around, grandmothers had both time and a very good reason to be useful. When they eked out food for their daughters' children, they reduced the chance that those children would die. That gave the grandmothers a better chance of passing on their own predisposition to longevity. 

This grandmother hypothesis also explains another conundrum: Why do humans have shorter birth intervals than other primates? Chimp mothers, for instance, wait five or six years to give birth to another neonate. But women can pop out infants as soon as they've weaned previous ones. It turns out that, once humans learned the art of collaborative child-rearing, old women started spending more time with their daughters' toddlers. That freed up the young women to have more. As the grandmother effect spread throughout the population over thousands of generations, this theory goes, it changed humans in another way. It made their brains bigger. As life lengthened, so did each stage of it. Children stayed children longer, which let their brains develop a more complex neural architecture.

Not everyone accepts this triumphantly feminist account of our evolutionary history. When anthropologists first heard it, most of them dismissed it as ridiculous. For one thing, it cuts man-the-hunter out of the picture. What about all the calories needed to grow our oversized brains? Didn't those have to come from the meat brought back from the hunt? Moreover, throughout recorded history, young women left their villages to move in with their men. So how would mothers have had access to their daughters' children?

Two decades later, the grandmother hypothesis has gone from oddball conjecture to one of the dominant theories of why we live so long, breed so fast, and are so smart. The extra calories and care supplied by women in their long post-fertile period subsidized the long pre-fertile period that is childhood. And that's what made us fully smart and human.

In a happy coincidence, the grandmother hypothesis comes along just as Americans enter what might be called the Age of Old Age. America's biggest generation, the baby-boomers, began retiring in 2011. This gerontocracy is expected to drain our wealth. By 2060, more than 20 percent of all Americans will be 65 or older, up from 13 percent in 2010. More than 92 million oldsters will roam the land, if roaming is within their power. People who fret about the federal budget point out that, by 2011, Social Security and Medicare were already eating up a third of it. Looming in the near future is the prospect that both programs' trust funds will vanish as the number of workers paying into the system goes down.

But are senior citizens really "greedy geezers" (a term made popular by Science magazine in 1988) about to bankrupt us? The grandmother hypothesis suggests not. It suggests that we should see the coming abundance of over-65-year-olds as an opportunity, not a disaster. As gerontologist Linda Fried, dean of Columbia University's school of public health, points out, "Older adults constitute the only increasing natural resource in the entire world."

A growing body of research shows how much grandmothers help their grandchildren, even when they aren't giving them hands-on care or food. Often enough, though, they do provide those things, especially in poor families or ones with dysfunctional parents. Unsurprisingly, grandmothers often do more for their grandchildren than grandfathers do. "Older women are the neighborhood watch and the neighborhood glue," says Fried. "They're the community purveyor of norms." 

Not that all grandparents can or want to be useful. As more people in industrialized countries postpone childbearing, parents become grandparents later and have less energy. The divorced ones may have started second or third families of their own. Global mobility puts distance between the generations. Assisted-living facilities segregate the old. Some retirement communities bar children altogether. But children still need the nurture they once got from their mothers' mothers.

So it's worth thinking about institutions that would give parents and children that grandparental boost and of programs that would take advantage of the deepening wellspring of senior talent, which would cut costs, make old people happier, and sew up the threadbare bonds among the generations. If we want to keep enjoying the grandmother effect, we'll just have to broaden our idea of what a grandmother can be. 

Fortunately there is now ample research to buttress the argument that we should revere our grannies and not kill them!