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....