Trade-offs in modern parenting: a longitudinal study of sibling competition for parental care

Evolutionary and economic models of the family propose that parents face a fundamental trade-off between fertility and investment per offspring. However, tests of this hypothesis have focused primarily on offspring outcomes rather than direct measures of parental investment. Existing studies of parenting also suffer a number of methodological problems now recognized as common sources of error in sociodemographic studies. Here, we present a more definitive picture of the effects of family structure on parental care by analyzing an extensive longitudinal dataset of contemporary British families (the Avon Longitudinal Study of Parents and Children). Unlike other studies, we simultaneously track maternal and paternal behaviors within the same family and consider variation both across time and between distinct population subgroups. Parental investment was measured as frequency of engagement in key care activities over the first decade of life. For both parents, larger family size was traded off against investment per offspring, representing the strongest explanatory variable considered in our analysis. However, contrary to the predictions of traditional quantity–quality trade-off models, increasing family socioeconomic status did not alleviate this effect. In fact, for paternal care in particular, increases in wealth and education created stronger trade-offs. We also demonstrate that large sibships were particularly costly for later-born offspring. Sex of siblings did not influence parental care, however maternal investment was biased towards daughters and paternal investment biased towards sons. Unrelated father figures were also associated with lower investment from both parents. Results are discussed in relation to parental investment theory and evolutionary models of modern low fertility.