This paper examines the effects of a massive salt iodization program on human capital formation of schoolaged children in China. Exploiting province and time variation, we find a strong positive impact on cognition for girls and no effects for boys. For non-cognitive skills, we find the opposite. We show in a simple model of parental investment that gender preferences can explain our findings. Analyses exploiting within the province, village-level variation in gender attitudes confirm the importance of parental gender preferences. Consequently, large scale programs can have positive (and possibly) unintended effects on gender equality in societies with son preference.