This article, using longitudinal data from the National Child Development Study, presents new evidence on the effects of family size and birth order on test scores and behavioral development at age 7, 11 and 16. Sibling size is shown to have an adverse causal effect on test scores and behavioral development. For any given family size, first-borns ultimately obtain higher test scores than middle-born or last-born children. First-borns and last-borns tend to be better behaved at school than middle-borns, though last-borns have no test score advantage.