International student mobility has increased rapidly since 1975. This article examined country differences in outbound student mobility and the correlates of those differences. Previous studies have addressed this question but differ in their conclusions depending on whether they focus on factors associated with student outflows or inflows. UNESCO data were used to examine student outflows from 190 countries and evaluate the merits of the constrained-schooling thesis, which holds that students go abroad because they lack study opportunities at home. Ordinary least squares regression models estimated the relative importance of country structural factors in shaping the country's outbound mobility ratios. The models showed that outbound student mobility was negatively and significantly related to sending country tertiary supply but not to tertiary demand net of other factors. Countries with large populations had fewer students abroad, but small ones had significantly more students abroad if they had less tertiary supply. Countries with higher gross domestic product per capita as well as North countries, in general, were found to have more students abroad. Spanish and Portuguese language countries have fewer students abroad compared with other countries.