So If There’s Parental Favoritism, What About Grandparental Favoritism?

There's been a lot of attention paid lately to parental favoritism and what factors might influence who of a set of siblings is the favorite (if there is one). Jeffrey Kluger's The Sibling Effect and my own (co-authored with Katrin Schumann) The Secret Power of Middle Children both touch on issues and research on parental investment and how birth order and gender, among other traits, can influence how much attention and investment children receive. And of course, no discussion of such a topic can take place without also considering the role of sibling competition and how it also shapes personalities and preferences. But if the evidence suggests that parents invest differentially (in other words, play favorites), what about grandparents? Are they also influenced by the same factors that are in play for parents?

From an evolutionary perspective, children are the vehicles by which parental genes get transported into future generations, but not all children are equally likely to survive and reproduce in turn. And some may benefit more from a certain amount of parental investment than others. As a result, selection has favored mechanisms of parental care that have the effect of increasing the fitness of the parent and that result in the favoring of offspring who are likely to provide a higher reproductive return on their parents' investment (this is one of many papers written by Martin Daly and Margo Wilson on discriminative parental solicitude). Some of the parental variables that influence the costs and benefits of parental investment include parental age (older parents will have less future opportunities to invest), the number of children at any given time (which obviously has an impact on the amount of resources to go around), and parental resource circumstances (when resources are in short supply, you might be best off to invest highly in the most promising one, when resources are more abundant, you may be able to invest highly in several offspring). There are also a number of factors related to the specific offspring as well that can influence the return on investment. They include age of child (older being more valuable than younger by virtue of survival to that point and investment already given), a child's expected future prospects (ability to convert parental care into future reproductive success, indicated by attractiveness, intelligence, gender under some conditions, etc.), and birth order (firstborns and lastborns being more frequent parental favorites and middles more likely to lose out in terms of investment).

Do the same factors influence grandparental investment or is something else at work? A recent study by Tanskanen, Rotkirch and Danielsbacka (2011), examined whether the sex of grandchildren (paternal favoring of granddaughters and maternal favoring of grandsons, based on sex chromosomal selection) or paternity uncertainty (which focused on the uncertainty paternity links male experience so that maternal grandmothers invest the most, followed by maternal grandfathers, paternal grandmothers, and paternal grandfathers) play a bigger role in grandparental resource allocation. The study used data from a nationally representative sample of British and Welsh adolescents which contained 17 variables related to grandparental investment. While the results clearly demonstrated that grandparents bias their investment toward certain grandchildren, sex of the grandchild was not a relevant influence on grandparental favoritism of this kind. Rather, their results were consistent with the effects predicted by paternity uncertainty (mother's mother, the heaviest investor and father's father the least due to the two uncertain paternity links btw grandfather and son and son and grandson), favoring the matrilineal line. Seems to be back to "momma's baby and poppa's maybe" again, just the intergenerational version this time. And this study doesn't speak to whether or not the previously mentioned factors (birth order, potential, etc.) influencing parental investment play a role for grandparents too. For that you'll need to stayed tuned for the next post.