MRW augment the Solow textbook model by a third factor of production, namely human capital. Their empirical results are meant to support the augmented Solow model, not the textbook Solow model, but this does not make a difference for the point to be discussed here. As an aside, human capital per output, which equals the conditional share of human capital investment in GDP, should be used as an explanatory variable in the augmented Solow model (as in MRW). Using the level of human capital per worker as an explanatory variable, as in some other empirical studies, cannot be motivated with reference to the Solow model.