Sibship size and intellectual development: Is the relationship causal?

Previous research has consistently found a negative statistical relationshi p between sibship size and children's intellectual development. Two explana tions have been offered for this finding. The prevailing explanation is tha t the relationship is causal, suggesting that limiting family size would le ad to more intelligent children. A second explanation maintains that the re lationship is spurious-that one or more undetermined factors correlated wit h family size are causally related to intellectual development. Using delta on children from the National Longitudinal Survey of Youth, we reexamine t he issue using change models. These change models allow us to control for s uch unmeasured effects as family intellectual climate, family value system, and family genetic heritage. We begin by replicating in these data the neg ative statistical relationship between three cognitive measures and sibship size. We then apply the change models to siblings measured at two points i n time and to repeated measures of the same individuals. By considering sib ship size as an individual trait that changes over time, we control for eff ects that are shared across siblings and over rime. When these shared effec ts are controlled, the negative relationship between sibship size and intel lectual development disappears, casting doubt on the causal interpretation of the negative relationship conventionally found.