The innovation of this paper is the proof of a relationship between human capital and growth, using a sample of 83 countries and six time periods between 1960 and 1990. Nonetheless, calculations from both interactive and variable-coefficient models based on these same panel data suggest that the contribution of education to growth depends on the level of commercial opening. This result could be explained by the variation in factor returns linked, on one hand, to trade-induced changes in labour demand and, on the other, to changes in relative factor supply which are amplified in closed economies. Distortions of human-capital allocation are therefore likely to influence the rate of growth ...