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Why Do Grandmothers Exist?

Solving an evolutionary mystery

Illustration by Carin Goldberg

Why do grandmothers exist?

The question is not as unfeeling as it sounds.

From the point of view of the selfish gene, creatures are supposed to drop dead as soon as they lose the power to reproduce. A man can make babies his whole life, even if the sperm of his old age lacks vigor and genetic fidelity. A woman outlives her eggs by about 20 years, which almost no other female mammals do.1 (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 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. Most societies don't actually murder their grannies, but that women manage to attain old age is an evolutionary mystery and requires explanation.

Some people deny that women did live past menopause, whether in the Pleistocene era or the nineteenth century. Before modern hygiene and medicine, the argument goes, people just didn't live very long. But most scientists don't think that anymore. It is true that, in the olden days, fewer people reached their golden years. Children dropped dead with disturbing ease, keeping life-expectancy averages low. But humans still had the capacity to live twice as long as our hominid ancestors. Those who got to 15 had about a 60 percent chance of making it to 45, at which point odds were respectable that they'd reach old age. Many anthropologists and biologists now believe that the bodies of Homo sapiens were designed to last about 72 years.

So why should women stop procreating so early? In 1957, the evolutionary biologist George Williams proposed what is called the "stopping-early" hypothesis: Middle-age women need baby-free time to usher their youngest children into adulthood. In the 1980s, an American anthropologist named Kristen Hawkes and two colleagues came up with a different explanation. They had gone to northern Tanzania to study the foraging habits of the Hadza, the last known hunter-gatherers in Africa. While there, the scholars were struck by how strong the tribe's old women were and how, rather than live off the fruits of others' labor, they worked hard digging up the tribe's main starch staple, a deeply-buried tuber. "Their acquisition rates were similar to the rates of younger women," Hawkes told me, "but these old ladies were spending even more time" than their daughters gathering food, leaving camp earlier, coming back later, and bringing back more than they needed. The anthropologists also noticed that many children with grandmothers or great-aunts had faster growth rates than their counterparts.

From these slim clues, 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. (In general, grandmothers appear to have helped daughters' offspring more than sons'; evolutionary theorists explain this by pointing out that a daughter's maternity affords a surer genetic connection than a son's paternity, unless you all but imprison your daughter-in-law.2)

The 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. 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, 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?

The comeback to these objections is that hunter-gatherer families probably made all kinds of arrangements.3 In the tribes that anthropologists have been able to observe, some couples stayed in the wife's village, some moved in with her parents while starting their families, and some women left home. Patrilocality—men staying put—probably became the norm only when our ancestors settled down to farm, which made men unwilling to leave their land and wealth.

And it's not as if hunters alone brought in enough food to let the children thrive. Hawkes argues that, while meat boosted a tribe's overall nutrition, hunters couldn't be counted on to come home with a kill. When they did, the demands of status made them just as likely to share the bounty with the tribe as to hold it back for their children. The food grandmothers provided, on the other hand, was steady and reliable.

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 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 this 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."4

If we are going to exploit this resource in the post-industrial world, we'll have to use the social capital of the old the way the Hadza used them to dig up tubers. Mature people of both sexes have a lifetime's worth of education and experience. We'd be crazy to waste that surplus value, especially when so many people languish after retirement, mortified at no longer being needed. To show how much retirees have to offer, Fried started a program that puts them in at-risk public schools in 19 cities. Early results suggest that children read better and get sent to the principal less often in classrooms where seniors spend 15 hours a week, perhaps because they give teachers support and embarrass students inclined to act up. For their part, the volunteers do better on tests of health and happiness, probably because they like feeling useful.

As for actual grandparents, a growing body of research shows how much they 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. The number of children being raised by their grandparents has been steadily rising since 2000. In 2011, there were 2.75 million such children in the United States.

But grandparents also give grandchildren more intangible gifts. In the mid-'90s, a Stanford University fellow named Luba Botcheva went home to Bulgaria to study how grandparents affected families struggling to survive the fall of communism. In the remote and very traditional region where she did her research, several generations would live under the same roof. The socialist-era factories had been shut down, and jobs were scarce. Botcheva discovered that grandparents' pensions were often the most dependable source of a household's income. In addition to paying the bills, however, grandparents buffered grandchildren against the harsh parenting that comes from acute anxiety. Children who grew up with grandparents in the home reported less depression than those without. "It was the opposite of what we expected," she said. "I called it the 'moderation effect.'" Many of the grandparents had lived through World War II, so when it came to poverty and uncertainty about the future, they had "social wisdom" to share, as Botcheva puts it, which kept tension levels down.5

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." When older black people in South Africa first started getting pensions from the post-apartheid government that were big enough to live on, the grandchildren who lived with grandmothers—especially the granddaughters—got taller and heavier, which observers took as a sign that they were eating better.6 But when it was the grandfathers who got the pensions, the grandchildren didn't grow at all. That wouldn't surprise economists who work with microfinance lending programs. They have discovered that female borrowers use their loans to improve their children's lot, whereas male borrowers, on the whole, do not.

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, along with Fried, about institutions that would give parents and children that grandparental boost. I dream of communal houses or apartment complexes where families could live near grandparents but not right on top of them. That vision gives rise to others, some of them unlikely in our conservative United States, but realities elsewhere: publicly funded day care, better mothers' and children's aid societies, a national version of Fried's experiment of putting older people in schools. These programs would take advantage of our 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.

  1. Don't read this as saying that all other female primates make babies until they die. Lots of them stop before that. But only women stop menstruating so close to the middle of their lives. 

  2. Did you know that, if you're a man, you share no sex chromosomes with your father's mother? That's because you got your Y chromosome from your father, who got his from his father, whereas you got your one X chromosome from your mother. (Males have one X and one Y chromosome. Females have two Xes.) Not sharing sex chromosomes doesn't mean you don't share some genes, but you got a lot less from your paternal grandmother than your sister did. This may be why one study of seven traditional societies found that the survival rates of boys went down when paternal grandmothers loomed large in their lives.

    Read more about this in "Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality."

  3. Plenty of animals share the care and feeding of the young: birds, bees, wild dogs, marmosets. But the primates closest to us—apes—don't. So why is this ape different from all other apes? Anthropologist Sarah Blaffer Hrdy thinks it's precisely because so many hunter-gatherer mothers stayed around their mothers, while other young female apes leave their mother's troupe before they reproduce.

    For more, read Hrdy's "Mothers and Others: The Evolutionary Origins of Mutual Understanding" (2009).

  4. In his notorious "greedy geezer" article (The New Republic, March 28, 1988) Henry Fairlie declared: "Something is wrong with a society that is willing to drain itself to foster such an unproductive section of its population, one that does not even promise (as children do) one day to be productive."

  5. Read Botcheva's "Grandparents as family stabilizers during economic hardship in Bulgaria."

  6. Read Esther Duflo's "Grandmothers and Granddaughters: Old Age Pension and Intra-household Allocation in South Africa."

This article has been corrected. A footnote originally stated that a man shares no chromosomes with his father's mother. In fact, men share no sex chromosomes with their paternal grandmothers.