Using the NLSY79 and NLSY79 Child and Young Adult Surveys, we investigate the effect of the age difference between siblings (spacing) on educational achievement. Because spacing may be endogenous, we use an instrumental variables strategy that exploits variation in spacing driven by miscarriages. The IV results indicate that a one-year increase in spacing increases test scores for older siblings by about 0.17 standard deviations. These results are larger than the OLS estimates, suggesting that failing to account for the endogeneity of spacing may understate its benefits. For younger siblings, we find no causal impact of spacing on test scores. (Contains 2 figures, 10 tables and 27 footnotes.)