Examined Lives: The Grandmother Hypothesis (Session 39)

November 16th, 12-1pm ET

Examined Lives

Steven P. Gilbert
The grandmother hypothesis is an attempt to explain the origins of reproductive senescence in Homo sapiens.

“You can’t really call women who are past their child-bearing years ‘post-reproductive’ because while they may not be fertile, there is a lot of evidence that they are doing important things for the reproduction of their genes.”  ~ Kristen Hawkes, an anthropologist at the University of Utah who coauthored the grandmother hypothesis.

Menopause and post-menopausal longevity in human females are evolutionary puzzles. What is the purpose of living for many years with no potential to reproduce?  The grandmother hypothesis uses the concepts of alloparenting, kin selection and inclusive fitness to explain and connect these two features of human life.  In 1985, a team of anthropologists studying a hunter-gatherer band in Tanzania noticed that elderly women spent more time foraging than younger women. They hypothesized that food from grandmothers helped their grandchildren survive. This in turn would mean that those grandmothers’ genetic disposition towards longevity would be passed on and increase in the population gene pool.  

Note:  The grandmother hypothesis itself has evolved over time. Initially, menopause was considered an adaptation facilitating grandmothering, i.e. menopause allowed grandmothers to increase investment in existing progeny.  In the revised grandmother hypothesis, grandmothering is considered the adaptation which facilitates increased longevity, and menopause is merely a byproduct.

Do maternal and paternal grandmothers behave similarly toward their progeny? The Wikipedia entry on the grandmother hypothesis notes that “studies have found that not only does the maternal or paternal relationship of the grandparent affect whether or how much help a grandchild receives, but also what kind of help. Paternal grandmothers often had a detrimental effect on infant mortality. Also, maternal grandmothers concentrate on offspring survival, whereas paternal grandmothers increase birth rates. These finding are consistent with ideas of parental investment and paternity uncertainty.”

Critics of the grandmother hypothesis argue that it overlooks other factors that could contribute to increased longevity, including the contributions of other family members.  A common objection is that if fathers' efforts are sufficient, grandmothers are not needed and fathers, who are the hunters, have a strong incentive to provide for their own children (fathers are twice as related to their child as a grandparent). Another objection is that  insofar as the hypothesis relies on benefits maternal grandmothers provide to their adult daughters, it requires that the mother and daughter reside near each other, i.e. that the society is matrilocal — however only 30% of world cultures that have been studied ethnographically are matrilocal.

The grandmother hypothesis is an attempt to explain the origins of reproductive senescence in Homo sapiens. Our discussion can focus on the evidence for and against that hypothesis, but we can also intellectually “forage" a bit more widely and consider whether the grandmother effect could extend beyond kin and improve group fitness and, ranging even further afield, discuss the roles of grandparents in modern societies, including our own.