Interest Rate, Growth And Debt Sustainability

Frequently, discussions about debt sustainability have discussions about the importance of the interest rate and growth in debt sustainability analysis. See for example, today’s Paul Krugman’s post on his blog. It is concluded that as long as the rate of interest is below the rate of growth, the ratio public debt/gdp doesn’t explode. Unfortunately, this result is erroneous.

John Maynard Keynes’ biggest disservice to the profession is to not start with the open economy. In my view, debt sustainability is tightly connected to balance of payments.

Imagine a nation whose exports is constant. If output rises, it will have adverse effects on the current account balance of payments because of income induced increase in imports. This will have an adverse effect on the international investment position of the nation: the net international investment position will keep deteriorating unless output is slowed down or some measure is taken to improve exports. In the case of rising exports, there is a similar constraint, except it is weaker but dependent on the rate of growth of exports.

If the ratio net international investment position/gdp keeps deteriorating, either the public debt to non-residents or private indebtedness to non-residents or both have to keep rising, all unsustainable.

There are some complications. A nation’s balance of payments also depends on how assets held abroad and liabilities to foreigners affect the primary income account of balance of payments. Also, the exchange rate can depreciate (or be devalued in fixed-exchange rate regimes) improving exports and reducing imports. However assuming that exchange rate movements do the trick is believing in the invisible hand. Foreign trade doesn’t just depend on price competitiveness but also on non-price competitiveness. These complications are highly interesting but do not affect the fundamental fact that a nation’s success is dependent on the success of corporations to compete in international markets for goods in services.

Even the conclusion that the government should contract fiscal policy and aim for a primary surplus in its budget balance or else the ratio public debt/gdp keeps rising if the rate of interest is greater than the rate of growth is erroneous. Consider a closed economy. An expansion in fiscal policy will automatically raise output and gdp and hence tax collections to prevent the ratio public debt/gdp from exploding. The public sector balance may hit primary surpluses but not due to contraction of fiscal policy or targeting a primary surplus in its budget balance.

In short, although the rate of interest and the rate of growth are important in debt sustainability analysis, it is not as easy as is usually presenting in macroeconomics textbooks and in the blogosphere. For a more detailed analysis see the reference below.


  1. Godley, W. and B. Rowthorn (1994) ‘Appendix: The Dynamics of Public Sector Deficits and Debt.’ In J. Michie and J. Grieve Smith (eds.), Unemployment in Europe (London: Academic Press), pp. 199–206

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