Sibship Size and Intellectual Development: Is the Relationship Causal?

Previous research has consistently found a negative statistical relationship between sibship size and children's intellectual development. Two explanations have been offered for this finding. The prevailing explanation is that the relationship is causal, suggesting that limiting family size would lead to more intelligent children. A second explanation maintains that the relationship is spurious—that one or more undetermined factors correlated with family size are causally related to intellectual development. Using data on children from the National Longitudinal Survey of Youth, we reexamine the issue using change models. These change models allow us to control for such unmeasured effects as family intellectual climate, family value system, and family genetic heritage. We begin by replicating in these data the negative statistical relationship between three cognitive measures and sibship size. We then apply the change models to siblings measured at two points in time and to repeated measures of the same individuals. By considering sibship size as an individual trait that changes over time, we control for effects that are shared across siblings and over time. When these shared effects are controlled, the negative relationship between sibship size and intellectual development disappears, casting doubt on the causal interpretation of the negative relationship conventionally found.