depends only on the saving, growth, and depreciation rates. This is the Solow–Swan model's version of the golden rule saving rate. Since , at any time the marginal product of capital in the Solow–Swan model is inversely related to the capital/labor ratio. If productivity is the same across countries, then countries with less capital per worker have a higher marginal product, which would provide a higher return on capital investment. As a consequence, the model predicts that in a world of open market economies and global financial capital, investment will flow from rich countries to poor countries, until capital/worker and income/worker equalize across countries. Since the marginal product of physical capital is not higher in poor countries than in rich countries, the implication is that productivity is lower in poor countries. The basic Solow model cannot explain why productivity is lower in these countries. Lucas suggested that lower levels of human capital in poor countries could explain the lower productivity. Because the marginal product of capital equals the rate of return so that is the fraction of income appropriated by capital. Thus, the Solow–Swan model assumes from the beginning that the labor-capital split of income is constant. Mankiw–Romer–Weil version of model Addition of human capital N. Gregory Mankiw, David Romer, and David Weil created a human capital augmented version of the Solow–Swan model that can explain the failure of international investment to flow to poor countries. In this model output and the marginal product of capital (K) are lower in poor countries because they have less human capital than rich countries. Similar to the textbook Solow–Swan model, the production function is of Cobb–Douglas type: where is the stock of human capital, which depreciates at the same rate as physical capital. For simplicity, they assume the same function of accumulation for both types of capital. Like in Solow–Swan, a fraction of the outcome, , is saved each period, but in this case split up and invested partly in physical and partly in human capital, such that . Therefore, there are two fundamental dynamic equations in this model: The balanced (or steady-state) equilibrium growth path is determined by , which means and . Solving for the steady-state level of and yields: In the steady state, . Econometric estimates Klenow and Rodriguez-Clare cast doubt on the validity of the augmented model because Mankiw, Romer, and Weil's estimates of did not seem consistent with