‘Grandmother effect’ helps explain human longevity

Not long after our first child was born, my wife and I contacted my mother in Pakistan to see if she could come and stay with us for a while in Durham, N.C., where I was training to be a cardiologist. We were overjoyed when she agreed. But when she arrived at the airport counter to collect her boarding pass, she learned that her valid visa had been unceremoniously cancelled without any reason given.

That she couldn’t come see her only granddaughter (and help out her parents) was devastating for all of us. But as two recent articles published in Current Biology show, the presence of grandmothers goes far beyond sentimental implications: They may be responsible for the success of the human species.

First, some background:

Evolutionary biologists have long been struck by two unique features of humans. The first is that we enjoy some of the longest life spans in the animal kingdom. In just the past 200 years, there has been an unprecedented increase in how long we live, not just in the richest countries but also in the poorest. We have moved so far away from our hunter-gatherer ancestors that their life spans are more similar to those of apes and chimpanzees than to modern human beings.

This feature is coupled with another. To a biologist, an organism that cannot reproduce is of little value in evolutionary terms, an unmitigated oddity. So scientists have been puzzled by the long postmenopausal life of human females. From a purely evolutionary perspective, grandmothers can be considered a failure of reproductive fitness: What good is an organism that cannot reproduce and cannot help perpetuate the species?

Yet as newly published studies are confirming, grandmothers may not only help perpetuate the species but also be one of the reasons why humans enjoy long lives to begin with.

Human females were once thought to be the only mammals with a long postreproductive phase of life. Scientists have since identified other mammals, like orca whales, with long postreproductive life spans. It turns out that these two features, humans’ long life spans and women’s long postreproductive lives, might be connected through “the grandmother effect.”

Evolutionary biologists have shown that the very women who were once thought to be an evolutionary aberration may be a key to human longevity. By helping raise their grandchildren, grandmothers in hunter-gatherer (and even more modern) societies allowed their daughters to have more babies. Mathematical simulations have demonstrated that the grandmother effect, first posited in 1966, is an important reason human life spans moved from an apelike pattern to our current one in both modern and more primitive societies.

By relieving a mother of some of her child-raising responsibilities, so the thinking goes, grandmothers make it easier for their daughters to have more children and also make it possible for those children to have longer lives by helping them during the difficult early years of life.

Two new studies, both published in Current Biology, strengthen this notion. An analysis of church birth and death records in Finland for individuals born between 1731 and 1890 showed that having a maternal grandmother between 50 and 75 years of age while a grandchild was 5 years old or younger increased the child’s survival. The effect disappeared for grandmothers older than 75 years of age, possibly because they may not have been be able to help with child rearing. The effect on longevity was seen mostly with maternal grandmothers and not paternal grandmothers.

The second study drills down into this effect, showing that it is not just the existence of a grandmother, but her proximity that matters. The shorter the distance between grandmother and grandchild, the more involved the grandmother can be and the more benefits that accrue to her daughter and grandchildren.

These new studies strengthen the pre-existing case for grandmothers, indicating just how important they might be to the success of our species.

In some ways, though, we are at a crossroads. For centuries, grandmothers have helped their family units grow. Yet more families now live by themselves, without grandparents, than at any time in the past. Over the course of the 20th century, the nuclear family has transformed, with some of the greatest changes coming from geographic dispersion. At the start of the 20th century, only 7 percent of older women lived alone. That peaked at 38 percent in the 1990s and is now at about 32 percent. If this dispersion continues, we could effectively reverse the evolutionary role that grandmothers have played and may lose the benefits we have achieved from this effect.

What must change is not only a greater understanding of the role grandmothers can play but also an increased effort to return to multigenerational family units. Such a movement would help both mothers and grandmothers, particularly as the latter grow older and require care for themselves.

Modern families ought not look just to grandmothers. Grandfathers also need to be part of the equation. Just as fathers have taken on an increasing role in parenting, so should grandfathers.

Children who grow up in multigenerational homes have better high school graduation and college enrollment rates and fewer emotional and behavioral problems than those who don’t. After Barack Obama’s parents divorced, he spent several years living with his grandparents in Hawaii. His upbringing represents a prominent example of grandparents doing a wonderful job helping raise a grandchild, especially when the parents’ situation is complicated.

Even as we think of new ways to enhance the upbringing of our children, we should hold to some old ways and do what we can to prevent a reversal of the grandmother effect that has been so central to the growth of the human species.

Our daughter, Eva, is now 3 years old and my wife and I are still not ready to have another child. Eva takes up everything we have: our love and attention, but also time and our energy. One thing that might change the equation, though, would be if my mom was able to get a visa and come to the U.S.

While our family and others stand at the unexpected intersection of schizophrenic immigration policies and evolutionary biology, and we look for what is best for the future of our children, science would suggest that we should begin by looking to the past.

Haider Warraich, M.D., is a fellow in advanced heart failure and transplantation at Duke University Medical Center and author of the forthcoming “State of the Heart: Exploring the History, Science & Future of Cardiac Disease” (St. Martin’s Press, July 2019). The views expressed in this article are those of the author and do not necessarily reflect those of his employer.