The effects of nesting on grandparent investment

Grandparental investment in grandchildren is a topic of interest within the field of evolutionary psychology. Studies have shown a consistent pattern of differences when it comes to each grandparent’s investment levels (Danielsbacka, Tanskanen, Jokela, & Rotkirch, 2011; DeKay, 1995; Laham, Gonsakorale, & Von Hippel, 2005; Michaski & Shackelford, 2005; Pollet, Nettle, & Nelisson, 2006). The pattern is as follows: the maternal grandmother invests the most, followed by the maternal grandfather, the paternal grandmother, and finally the paternal grandfather. The current hypotheses and theories behind this pattern are based on previous evolutionary theories of relatedness (Hamilton, 1964) and relational certainty (Trivers, 1972). The large and consistent difference between maternal grandmothers and paternal grandfathers can be explained relatively directly based on relational uncertainty, as the maternal grandmother is the most certain that the grandchild is her genetic relative and the paternal grandfather is least certain. This explanation is widely accepted. However, the smaller but consistent differences in investment patterns by maternal grandfathers versus paternal grandmothers, in favor of the former, is not as easily explained. The two currently competing theories are as follows: (1) although maternal grandfathers and paternal grandmothers are equally uncertain of their relatedness, they invest differently depending on which generation their uncertainty lies in – the grandparent generation for the grandfather and the parent generation for the grandmother; and (2) paternal grandparents have other grandchildren for which they are maternal grandparents, and they will choose to invest more in these more certain relatives. The current research was designed to test a new hypothesis, that patterns of grandparent investment are affected by whether or not grandparents are nested (coupled and sharing resources). A survey measuring grandparent investment across four dimensions was administered and found no significant differences between certain nested and unnested grandparents in terms of investment. Results instead revealed a different overall pattern of investment entirely, indicating multiple design flaws and providing some direction for future research