Solow (1956) develops a production function with substitutability between factors of production, modeling output growth as a function of capital, labor, and knowledge. Knowledge or technology is Harrod neutral, since it only affects the productivity of labor. The model assumes an exogenous and homogenous technology across countries. As countries accumulate technology at the same rate, cross-country differences in output growth rates represent differences in capital accumulation. The model also implies that a country’s real GDP per capita