This thesis by Charles Irving Jones, submitted to the Department of Economics at MIT in 1993, proposes a general test of endogenous growth models based on the observation that per capita growth rates in the U.S. appear to be stationary over the last century. The test is used to provide time series evidence against both the "AK"-style models and R&D-based models, which are major alternatives to the neoclassical growth model. A modified version of the Romer [1990] R&D model is introduced, which is not rejected by the stationarity test but alters a key implication of endogenous growth theory. In this extended model, growth is generated endogenously through R&D undertaken by profit-maximizing agents, but the long-run growth rate depends only on exogenous parameters, particularly population growth. Empirical evidence from TFP growth and R&D in the U.S., France, Germany, and Japan supports the extended model. The thesis concludes with a discussion of the implications of the augmented R&D model and a cross-section analysis of growth in different countries, focusing on the relative price of capital.This thesis by Charles Irving Jones, submitted to the Department of Economics at MIT in 1993, proposes a general test of endogenous growth models based on the observation that per capita growth rates in the U.S. appear to be stationary over the last century. The test is used to provide time series evidence against both the "AK"-style models and R&D-based models, which are major alternatives to the neoclassical growth model. A modified version of the Romer [1990] R&D model is introduced, which is not rejected by the stationarity test but alters a key implication of endogenous growth theory. In this extended model, growth is generated endogenously through R&D undertaken by profit-maximizing agents, but the long-run growth rate depends only on exogenous parameters, particularly population growth. Empirical evidence from TFP growth and R&D in the U.S., France, Germany, and Japan supports the extended model. The thesis concludes with a discussion of the implications of the augmented R&D model and a cross-section analysis of growth in different countries, focusing on the relative price of capital.