Nature + Nurture = Love? A Within-Family Analysis of the Trivers-Willard Effect in Breastfeeding Initiation

Sebastian Schnettler, Universität Konstanz

Using data from the sibling pairs subset of the Add Health Study, I test the Trivers-Willard hypothesis (TWH) of differential parental investment with regard to breastfeeding initiation. The TWH predicts parents of higher SES to favor sons and those of lower SES to favor daughters. Previous empirical support has been mixed. This contribution improves previous research in three ways: (1) I draw on biological and sociological explanations for differences in parental investment. (2) I model parental investment both between and within families using family fixed-effects models. (3) By using data on twins, full, half, and unrelated siblings I compare how biological parenthood and twin status play out in the dynamic between parental status and child gender. Results indicate partial support consistent with the TWH: Support is evident for breastfeeding initiation within families, but not with regard to breastfeeding continuation.