In this ongoing work we test whether family size has a causal effect on children's education in rural Mexico. We use son preference as a source of random variation in the propensity to have more children, and estimate the causal effects using an instrumental variable methodology, using son preferences and twins as instruments. We find no evidence of family size having an adverse effect on education, once we account for the endogeneity of family size. We address a number of concerns about the validity of the instrument, including economies of scale and son preferences having a direct effect on outcomes. We also allow for the possibility that the instrument is imperfect and provide bounds on the effects. We show that the bounds are never lower than the OLS estimates.