Population growth, A, and income shares going to labor, 1 - P, do of course differ across countries, but neither varies in such a way as to provide an account of income differentials. Countries with rapid population growth are not systematically poorer than countries with slow-growing populations, as the theory predicts, either cross-sectionally today or historically. There are, cer- tainly, interesting empirical connections between economic variables (narrowly defined) and birth and death rates, but I am fully persuaded by the work of Becker (1981) and others that these connections are best understood as arising from the way decisions to maintain life and to initiate it respond to economic conditions. Similarly, poor countries have lower labor shares than wealthy countries, indicating to me that elasticities of substitution in production are below unity (contrary to the Cobb-Douglas assumption I am using in these examples), but the prediction (9) that poorer countries should therefore grow more rapidly is not confirmed by experience.