Grandparental investment: The influence of reproductive timing and family size.

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. 2009 Wiley-Liss, Inc.