Empirical research on the effect of family size on child education is complicated by the endogeneity of family size. This study exploits plausibly exogenous changes in family size caused by China’s population control policy to estimate the causal relationship between family size and child education outcomes. The results show that, compared to an only child, a person with an additional sibling will have an approximate seventeen percentage points lower likelihood of completing middle school in China. Separate regressions across individual characteristics reveal that much of this negative effect appears to be driven by the cohorts born in earlier years after the policy, and children with the highest birth order within a family.