The purpose of this paper is to explain differences in the productivity of capital across countries taking 84 rich and poor countries over the period 1980-2011, and to test the orthodox neoclassical assumption of diminishing returns to capital. The marginal product of capital is measured as the...

A country's growth of output is identically equal to its ratio of investment to output and the productivity of investment. In "new" growth theory regressions, which include the investment ratio, all other included variables pick up why the productivity of investment differs between countries....

In ‘new growth theory' equations that include the investment ratio, all other variables included are determinants of the productivity of investment. We convert a ‘new growth theory' equation into a productivity of investment equation by dividing the equation through by the investment ratio....