<p><p>This paper tests whether family size has a causal effect on girls' education in Mexico. It exploits son preference as the main source of random variation in the propensity to have more children, and estimates causal effects using instrumental variables. Overall, it finds no evidence of family size having an adverse effect on education, once the endogeneity of family size is accounted for. Results are robust to another commonly used instrument in this literature, the occurrence of twin births. A divisive concern throughout this literature is that the instruments are invalid, so that inferences including policy recommendations may be misleading. An important contribution of this paper is to allow for the possibility that the instruments are invalid and to provide an answer to the question of just how much the assumption of instrument exogeneity drives findings. It concludes that the assumption of exogeneity does not affect the results that much, and the effects of family size on girls' schooling remain extremely modest at most.
