"Not all grandparents invest the same time and resources in their grandchildren," says University of New South Wales psychologist and study co-author, Dr Bill von Hippel.
"Maternal grandmothers are closer to their grandchildren than other grandparents. The next closest relationship between grandparents and grandchildren is by maternal grandfathers, then paternal grandmothers and, last of all, paternal grandfathers."
To be published in Personality & Social Psychology, the study is the first to confirm that preferential investment stems from genetic uncertainty. The finding is based on data from 780 university students who rated their emotional closeness to each biological grandparent from zero (cold or negative feelings) to 100 (warm or positive feelings).
"Not all grandparents are certain that their grandchildren are their own," says von Hippel. "A woman always knows she is the mother of her child but a man has a some uncertainty about his paternity because he might have been cuckolded," he says.
"This issue is compounded for grandparents. A maternal grandmother knows with certainty that her genetic material has passed to her grandchildren through her daughter but a paternal grandfather has double uncertainty -- he has no certainty that either his son or his grandchildren bear his genetic material."
Maternal grandmothers and paternal grandfathers represent two extremes of this genetic uncertainty spectrum says von Hippel.
"Logic suggests these two grandparents share one uncertain genetic link with their grandchildren and should therefore invest the same time and resources in them. Our research confirms that grandchildren feel emotionally closer to their maternal grandfather than their paternal grandmother.
"However, this finding is only true when a paternal grandmother has other grandchildren available to her through her daughter(s), and thereby has genetically more certain grandchildren to invest in rather than her son's children."
"When paternal grandmothers have grandchildren only through their son, their degree of closeness is equal to maternal grandfathers."
Von Hippel's work is a test of the evolutionary explanation for preferential investment.
"Until now, work on grandparent-grandchild relations has ignored the influence that other grandchildren have on grandparental investment. Grandparents must spread their time and resources across grandchildren, and genetic certainty clearly influences this."
Journal
Journal of Personality and Social Psychology