High sex ratios in China and India have historically concerned researchers (Sen 1990) and their recent increase has alarmed policymakers worldwide. This paper identifies sex selection via infanticide and abortion as the principal explanation for the sex ratio distortion, and rules out competing explanations such as biology (Oster 2005) or differential mortality rates. Consistent with recent work (Jha et al. 2006), I find that the sex ratio of first-order births is close to the natural rate and steeply rising following the birth of low-order daughters, indicating that mothers are practicing pre-natal sex selection or immediate infanticide. Sex ratios are found to be higher among those anticipating lower fertility, such as those under stricter government fertility limits. I present a model of a mother's fertility choice when she has access to a sex-selection technology and faces a mandated fertility limit. By exploiting variation in fines levied in China for unsanctioned births, I demonstrate that higher fine regimes discourage fertility but are associated with higher sex ratios among those who choose to have an additional child. I then estimate a structural model of parental preferences using China's 2000 census data that indicates that a son is worth 2.90 years of income more than a daughter, and the premium is highest among less educated mothers and rural families. I conclude with a set of simulations to model the effect on sex ratios and total fertility of a proposed subsidy to families who fail to have a son, and find that such a policy would reduce sex ratios and lower overall fertility.