Human capital plays an important role in the theory of economic growth, but it has been difficult to measure this abstract concept. We survey the psychological literature on cross-cultural IQ tests, and conclude that modern intelligence tests are well-suited for measuring an important form of a...

I show that in a conventional Ramsey model, between one-fourth and one- half of income differences across countries can be explained by a single factor: The steady-state effect of large, persistent differences in national average IQ on worker productivity. These differences in cognitive...