At first, it seems difficult to grasp: global capital is flowing from poor to rich countries. Emerging-market countries run current-account surpluses, while advanced economies have deficits.

One would expect fast-growing, capital-scarce (and young) developing countries to be importing capital from the rest of world to finance consumption and investment. So, why are they sending capital to richer countries, instead?

China is a case in point. With its current-account surplus averaging 5...