Using data from the Programme for International Student Assessment (PISA) and the Trends in International Mathematics and Science Study (TIMSS), we investigate the link between test scores (mathematics and science) and cross-country income differences. We would like to know whether test scores are good indicators of labor-force quality. The analysis suggests that after properly controlling other variables that are typical in cross-country economic growth study, the strong link between test scores and cross-country income differences disappears. Moreover, we show that variables such as Research and Development researchers (per capita) or Scientific and Technical journal articles (per capita) can better account for the cross-country income differences.