Poison In The Ink: Darwinian Grandparenting

Most grandparents would never admit it, but studies consistently reveal that they treat some grandchildren better than others.

When surveyed, adults said they felt closest to their maternal grandmothers, followed by their maternal grandfathers then paternal grandmothers and finally paternal grandfathers.

The pattern was the same whether the researchers tested for emotional closeness, the amount of time spent per week with a grandchild or the money spent on them each month.

It was also the same whether the adults surveyed were from America, Germany, Greece or Australia and even when such things as the grandparents’ age, the distance they lived away from the grandchild and the number of living grandparents were controlled for.

One of the most intriguing explanations for this trend comes from evolutionary biology. The idea is that the investment a grandparent makes in a grandchildren reflects how certain they are that they are actually related to them.

Biologists refer to an organism's ability to survive and produce offspring as “fitness.” From a Darwinian point of view, the goal of grandparents is to help their children have as many children of their own as possible. By doing so, the grandparents not only increase their children’s fitness, but their own as well.

Evolutionary theory therefore predicts that a maternal grandmother will be most likely to invest in her grandchild because in nearly all cases, she can be 100 percent sure that the grandchild born of her daughter is really related to her.

It also predicts that a paternal grandfather will have the least incentive to invest in his grandchild because not only is he unsure of whether his son is really his grandchild’s father (the daughter-in-law may have cheated on her husband), he also can’t be sure of whether his son is really his son (his wife may have cheated on him).

But while evolutionary theory does a good job of explaining why maternal grandmothers invest the most in their grandchildren and paternal grandfathers the least, it doesn't explain why adults consistently said they felt closer to their maternal grandfathers than their paternal grandmothers.

If all that matters is relatedness, both these grandparents should show similar levels of investment since both have an uncertain genetic link to their grandchildren: the paternal grandmother can’t be completely sure that her son was really the father of her grandchild and the maternal grandfather can’t be completely sure that the mother of his grandchild is really his daughter.

A possible explanation for this anomaly was suggested by William von Hippel, a psychologist from the University of New South Wales in Sydney, Australia and colleagues in a paper published last year in the journal for the Society for Personality and Social Psychology.

According to von Hippel, paternal grandmothers are more distant than maternal grandfathers because they have another, safer, bet when it comes to the investment of their time and resources: your cousins.

The reasoning behind this idea is simple: while your paternal grandmother may be uncertain about the genetic link between her son (your father) and you, she can be 100 percent sure of her relatedness to her daughter’s (your aunt) child (your cousin).

This hypothesis therefore predicts that your paternal grandmother will invest more time in your cousins if they are the children of her daughter than in you. Your maternal grandfather, on the other hand, is as clueless about his relation to you as to your cousins and therefore has no incentive to prefer one over another. The researchers also predicted that in cases where the maternal grandmother had no grandchildren through daughters, this effect would dissapear.

To test their hypothesis, the researchers surveyed 787 students from the University of South Wales. They asked the students to rate their emotional closeness with their grandparents and to also indicate whether they had cousins, and if so, whether they were from paternal or maternal aunts and uncles. The results followed the exact pattern that the researchers predicted; however, the effect was only marginally significant.

The researchers were unfazed though. “Rather than being unimpressed by the small size of these effects, one might instead be impressed that such an effect emerges at all,” they write.

“Of all the reasons to feel close or distant to a grandparent, the fact that genetic uncertainty and preferred investment outlets led to predicted differences in closeness testifies to the potency of evolutionary principles.”

The researchers hope to replicate the experiment in non-Western cultures and to use more direct measures of grandparental investment, such as gifts given.