For years, South Korea presented the puzzling phenomenon of steeply rising sex ratios at birth despite rapid development, including in women's education and formal employment. This paper shows that son preference decreased in response to development, but its manifestation continued until the mid-1990s due to improved sex-selection technology. The paper analyzes unusually rich survey data, and finds that the impact of development worked largely through triggering normative changes across the whole society - rather than just through changes in individuals as their socio-economic circumstances changed. The findings show that nearly three-quarters of the decline in son preference between 1991 and 2003 is attributable to normative change, and the rest to increases in the proportions of urban and educated people. South Korea is now the first Asian country to reverse the trend in rising sex ratios at birth. The paper discusses the cultural underpinnings of son preference in pre-industrial Korea, and how these were unraveled by industrialization and urbanization, while being buttressed by public policies upholding the patriarchal family system. Finally, the authors hypothesize that child sex ratios in China and India will decline well before they reach South Korean levels of development, since they have vigorous programs to accelerate normative change to reduce son preference.