High ratios of males to females in China have concerned researchers (Sen 1990, Yi et al. 1993) and the recent increase has alarmed policymakers worldwide. This paper presents an analysis of China's census data that indicates that the "missing girls" phenomenon is causally linked to enforcement of the One Child Policy. Fertility is lower and sex ratios are higher among those under stricter fertility control, and the overall increase in the sex ratio is driven by an increase in the prevalence of sex selection among first and second births. By exploiting regional and temporal variation in fines levied for unauthorized births, I find that higher fine regimes discourage fertility, but are associated with higher ratios of males to females. (Contains 35 footnotes, 10 tables, and 4 figures.)