The marginal product of capital, and policy irrelevance

The May 2007 Quarterly Journal of Economics offers up a fun piece on the marginal product of capital, earlier version here.  The bottom line is startling, though it requires only a simple model:

Whether or not the marginal product of capital (MPK) differs across countries is a question that keeps coming up in discussions of comparative economic development and patterns of capital flows.  Attempts to provide an empirical answer to this question have so far been mostly indirect and based on heroic assumptions.  The first contribution of this paper is to present new estimates of the cross-country dispersion of marginal products.  We find that the MPK is much higher on average in poor countries.  However, the financial rate of return from investing in physical capital is not much higher in poor countries, so heterogeneity in MPKs is not principally due to financial market frictions.  Instead, the main culprit is the relatively high cost of investment goods in developing countries.  One implication of our findings is that increased aid flows to developing countries will not significantly increase these countries’ incomes.

The rough equality of MPKs [correction: financial rates of return] means that capital can flow to where it is most productive.  That means if a country receives some aid, and converts that aid into useful capital goods, less capital flows into your country.  A version of neutrality holds.  Of course there is no reason to focus on aid in this argument.  Most one-off improvements (or destructions) wash out in the longer run, due to subsequent adjustments in the capital stock.  The one-off improvements matter only if liquidity and credit imperfections hinder the international mobility of capital; such imperfections would mean that transfers could bring about a permanently higher level of capital.

No, I’m not ready to "press the yes button" on this model (should I be?), but it is a good example of how open economy considerations can overturn our expectations, or how easily economics can generate a counterintuitive conclusion.  You also may have noticed Borjas and Rodrik using a version of this model lately, attempting a Brad DeLong Smackdown.  I am suspicious.  It’s not a model they believe in, or if they do I am waiting for Borjas to stop warning us about capital destruction costs more generally… 


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