Was there a tradeoff between family size and education during the demographic transition in the United States? Exploiting the occurrence of twin births as a source of exogenous variation in family size, I find that an additional sibling reduces the likelihood of attending school by one to two percentage points. To evaluate the persistence of family size effects, I create a linked sample of boys between 1920 and 1940. Individuals raised in larger families accumulate less human capital by adulthood. However, the impact of family size is quantitatively small relative to the average level of education in the population.