Nature plus nature equals love? : A test of the Trivers-Willard hypothesis of differential parental investment on the basis of sociological and biological explanations

Abstract

In this dissertation, I test the Trivers-Willard hypothesis of differential parental investment using the genetic sample of the U.S. National Longitudinal Study of Adolescent Health. Based on general propositions of evolutionary theory, Trivers and Willard predicted that parents of higher socioeconomic status bias their parental investment to favor sons and that parents of lower socioeconomic status bias their investment to favor daughters. Empirical support for this hypothesis has been mixed.I aim to improve previous research in three ways: First, I draw on both biological and sociological explanations for differences in parental investment and reflect these explanations in the analyses. Second, I model parental investment both between and within families, whereas most empirical studies on the Trivers Willard hypothesis have examined parental investment differences only between families. Third, by using data on twins, full siblings, half siblings, and unrelated siblings I compare how biological parenthood and twin status play out in the assumed dynamic between parental status and child gender.The results are mixed and indicate partial support consistent with the TW hypothesis: Support is evident for breastfeeding initiation within families, but not with regard to breastfeeding continuation. Furthermore, I find support for children's perceptions of how much their parents care for them, but only within families and once previous perceptions and parental support are accounted for. There is no support for the Trivers-Willard hypothesis for parental engagement in activities with their children.In the last chapter I conclude with a discussion of reasons why the Trivers-Willard effect may be expressed only for certain types of parental investment and under specific circumstances. Furthermore, I suggest routes for future research that integrates biological and sociological concepts on a more detailed level and that can shed new light on the specific mechanisms that produce the interaction effect between parental status and child gender in parental investment.