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Paternity uncertainty has proven to be a robust ultimate hypothesis for predicting the higher investment in grandchildren observed among maternal grandparents compared to that of the paternal grandparents. Yet, the proximate mechanisms for generating such preferred biases in grandparental investment remain unclear. Here we address two different questions for better understanding the proximate mechanisms leading to the observed bias in grandparental investments: (i) is there a larger emphasis on resemblance descriptions (between grandchildren and grandparent) among daughters than among sons, and (ii) do mothers really believe that their offspring more resemble their parents, i.e., the children’s grandparents, than fathers do? From questioning grandparents, we find that daughters more often and more intensely than sons express opinions about resemblance between their children and their parents, i.e., about grandchild-grandparent resemblance. Moreover, daughters also seem to believe that their children more resemble their grandmother than sons do. The latter is however not the case for beliefs about children’s resemblance to grandfathers. In sum, our results suggest that even in a population of Norwegians, strongly influenced by ideas concerning gender equality, there exist a sexual bias among parents in opinions and descriptions about grandparent-grandchild resemblance. This resemblance bias, which echoes that of mothers biasing resemblance descriptions of newborns to putative fathers, does not seem to represent a conscious manipulation. Yet, it could be instrumental for influencing grandparental investments. We believe that a “manipulative mother hypothesis” might parsimoniously account for many of the results relating to biased alloparenting hitherto not entirely explained by “the paternity uncertainty hypothesis”.