Even though the Solow growth model is the basic framework for the most following research on growth, it has a number of important weaknesses. Firstly, in the Solow model, the long-run per capita growth rate is determined entirely by the rate of technological progress but the technological growth is outside the model, which is exogenous. Secondly, the evidence shows that income per capita differentials across the countries are much greater than predicted by the model. Differences in capital per labor across countries are too small to account for the observed disparities in real incomes. Thirdly, the model assumes that the marginal product of capital should be much higher in poor countries than in rich countries. The observed tenfold differential in output per worker between rich and poor countries implies a hundredfold difference in the marginal product of capital if output gaps are entirely due to variations in capital intensities. Such differentials in the rate of return to capital are simply not observed between rich and poor countries. Finally, the capital accumulation alone cannot account for either growth of per capita income over the long-run or the enormous disparities in living standards among the nations. In order to overcome this problem, Mankiw et al. (1992) and Mankiw (1995) developed the augmented Solow model by adding the accumulation of human capital to the initial variables.