For years, sex ratios at birth kept rising in South Korea despite rapid development. We show that this was not an anomaly: underlying son preference fell with development, but the effect of son preference on sex ratios at birth rose until the mid-1990s as a result of improved sex-selection technology. Now South Korea leads Asia with a declining sex ratio at birth. We explore how son preference was affected by development and by public policy. Decomposition analysis indicates that development reduced son preference primarily through triggering normative changes across society—rather than just in individuals whose socioeconomic circumstances had changed. The cultural underpinnings of son preference in preindustrial Korea were unraveled by industrialization and urbanization even as public policies sought to uphold the patriarchal family system. Our results suggest that child sex ratios in China and India may decline before those countries reach South Korean levels of development, since the governments of both countries vigorously promote normative change to reduce son preference.