Consider the following facts. In 1950 the richest ten-percent of countries attained an average of 8.1 years of schooling whereas the poorest ten-percent of countries attained 1.3 years, a 6-fold difference. By 2005, the difference in schooling declined to 2-fold. The fact is that schooling has increased faster in poor than in rich countries even though the per-capita income gap has generally not decreased. What explains educational attainment across countries and their evolution over time? We develop a model of human capital accumulation that emphasizes productivity and life expectancy differences across countries and time. Calibrating the parameters of the model to reproduce historical data for the United States, we find that the model accounts for 95 percent of the difference in schooling levels between rich and poor countries in 1950 and 78 percent of the increase in schooling over time in poor countries. The model generates a faster increase in schooling in poor than in rich economies even when their income gap does not decrease. These results have important implications for educational policy.