Human capital, because of its special role in innovative activity and technological progress, has formed the bedrock of the new theories of endogenous growth. Human capital, however, not only serves as an engine of growth, but also as a productive input along with labor and physical capital. In this study, we distinguish between these two roles of human capital and find evidence of the importance of both. We also find that the relationship between growth and the external effects of human capital vary according to trade regime. When literacy rates are relatively high, open economies grow about 0.65 to 1.75 percentage points more than closed economies. Replaced by "Human capital, trade and economic growth"