Grandparental investment: The influence of reproductive timing and family size

Abstract

The influence that grandparents have on the life history traits of their descendants has been studied extensively. However, no attention has been paid to the potential influence a grandparent's own reproductive history has on the investment they make in their grandchildren. We use data from 658 Swiss grandchildren and 591 of their grandparents to investigate whether grandparents' reproductive scheduling and family size influence the amount of investment grandparents make in a focal grandchild (shared contacts, occasions to meet, activities, discussions, interests, and important roles the grandparent plays). Grandparents who were younger when they had their first child had more children and grandchildren; this relationship strengthened after controlling for grandparental age, sex, lineage, and education (all P < 0.001). Generally, having more children or grandchildren was associated with reduced levels of grandparental investment. After adjustment for a wide range of factors known to influence investment, having more children or grandchildren and having a first child or grandchild at a younger age were associated with reduced investment in 14 of 24 analyses (all P < 0.09). The association between reproductive scheduling and investment was partially mediated by the grandparent's family size. Interestingly, these relationships were only present in data reported from the grandchild's point of view, not the grandparent's. This analysis provides preliminary evidence that grandparents' reproductive strategies have consequences for the amount of investment they make in their grandchildren. These results are examined in terms of the trade-offs between current and future reproduction and offspring quality and quantity. Am. J. Hum. Biol. 21:455-463, 2009. Grandparents can play a crucial role in the lives of their grandchildren. However, as with any relationship, the extent of grandparental investment is highly variable (e.g., Euler and Weitzel, 1996;Laham et al., 2005;Michalski and Shackelford, 2005;Pashos and McBurney, 2008). Studies examining the underlying factors that contribute to this variability have been carried out by behavioral ecologists, psychologists, sociologists, and economists. To date, grandparental investment research has focused on the potential influences of genetic relatedness (e.g., Aldous, 1995), paternity certainty (e.g., Pollet et al., 2006), sex-specific reproductive strategies (e.g., Euler and Weitzel, 1996), the moderating role of parents (e.g., Pashos and McBurney, 2008), including mothers as links maintaining family ties (e.g., Monserud, 2008), grandparental age (Lahdenperä et al., 2004), physical resemblance (e.g., Michalski and Shackelford, 2005), the grandchild's reproductive value (e.g., Smith, 1991), and the grandchild's family size and birth order (Leonetti et al., 2005). Although grandparental investment, like parental investment, is often conceptualized as part of an individual's reproductive strategy (see Coall and Hertwig, submitted for publication), no attention has been pai...

The Place of Proximity

“…Previous research has shown that larger sibships lead to dilution of resources directed toward children and grandchildren (Coall et al 2009;Lawson and Mace 2009), reflecting the dynamics of a quality-quantity trade-off (Stearns 1992). Several studies have clearly demonstrated the negative effects of large family size on child health outcomes (Hagen et al 2006;Lawson and Mace 2008).…”

Parental investment and the optimization of human family size

“…Cooney & Uhlenberg [86] for example, have reported that number of siblings is negatively related to a range of later investments including the direct receipt of money or gifts, giving advice in difficult decisions and direct assistance with childcare (see also [87]). Keister [88] has also demonstrated that number of siblings has a strong negative influence on the likelihood of receiving a trust fund or an inheritance.…”

The impact of grandparental investment on mothers’ fertility intentions in four European countries

“…This possible grandparental effect can be expected to vary with the ecological and the socioeconomic circumstances of the family, and also with the sex and the lineage of the grandparents. Based on Hamilton's rule, inclusive fitness can be increased through the individual's own reproduction or by supporting reproduction among genetically closely related kin (Coall et al 2009). …”