Does the marginal productivity of capital decrease with per capita capital?

It is usually assumed that the higher a country’s capital per worker , the lower its marginal productivity of capital. As a result, rich countries should invest in poor countries since the marginal productivity of capital is normally higher in poor countries than in rich countries. But the opposite is true: poor countries have current account surpluses, while rich countries have current account deficits: capital is flow ing from poor to rich countries. We return to the basic assumption: the marginal productivity of capital is higher in countries with low per capita capital. We examine the relevance of this assumption. In reality, there are two groups of countries: OECD countries and emerging countries. We see that the marginal productivity of capital is no t higher in emerging countries with low capital per worker than in OECD countries with high capital per worker . This explains why capital is not flowing from OECD countries to emerging countries.