When Size Matters: IV Estimates of Sibship Size on Educational Attainment in the U.S

Children with additional siblings appear to fare worse on a variety of developmental and educational outcomes across social contexts. Yet, the causal relation between sibship size and later attainment remains dubious, as factors that influence parents’ fertility decisions also shape children’s socioeconomic prospects. We apply instrumental variables methods that treat multiple births (e.g., twins, triplets) and same-sex composition as natural experiments to test whether increases in sibship size have a causal effect on the educational attainment of older siblings in the U.S. We pool several nationally representative datasets, including the Child and Young Adult Cohorts of the NLSY79 and the Panel Study of Income Dynamics, to obtain adequate sample sizes for these methods. Although results indicate that the presence of an additional sibling has a trivial effect on the attainment of older siblings for most families (those with two to four siblings), a large penalty arises with the introduction of a fifth sibling. Our findings imply that the costs associated with sibship size are likely concentrated among the largest families.