This study examines a crucial assumption in much of the recent work on endogenous growth, namely, constant returns to scale in the production of human capital. A simple model is constructed to show that the returns to scale in human capital production can be inferred from the relationship between the wage rate and years of schooling. A large international micro dataset is used to estimate this relationship. The empirical evidence is decisive. There are decreasing returns to scale in human capital production; that is, the micro-level evidence is not supportive of endogenous growth driven by human capital accumulation.