Impossible Trinity

Three goals, two choices

Governments want stable exchange rates to reduce uncertainty for trade and investment, free capital flows to tap foreign funds, and independent monetary policy to manage domestic output and inflation. But as the Mundell–Fleming model shows, with free capital flows a fixed exchange rate forces domestic interest rates to track those of the anchor country: cutting rates independently triggers capital outflows that make the peg impossible to defend. So there are three options: (1) fixed rate plus capital mobility, giving up monetary policy (eurozone members, Hong Kong's currency board); (2) capital mobility plus monetary independence, with a floating rate (the United States, Japan, South Korea); and (3) fixed rate plus monetary independence, with capital controls (the Bretton Woods system, China in the past).

Historical cases

The gold standard from the late nineteenth century to 1914 chose fixed rates and capital mobility and gave up monetary autonomy; leaving gold during the Great Depression was a choice to regain autonomy to stimulate domestic economies. The Bretton Woods system of 1944 combined fixed rates and autonomy through capital controls but collapsed in 1971 as capital mobility rose. In the 1992 European Exchange Rate Mechanism crisis and the 1997 Asian financial crisis, countries pursuing de facto pegs, open capital accounts and independent policy faced speculative attacks (Krugman, 1979). Using 150 years of data, Obstfeld, Shambaugh and Taylor (2005) confirmed that interest rates in pegging countries track base-country rates more closely. Aizenman, Chinn and Ito (2013) constructed indexes showing that a weighted sum of the three goals is roughly constant.

Middle solutions and critiques

Rather than abandon one corner entirely, many countries choose intermediate options such as managed floats, partial capital controls and reserve accumulation (Frankel, 1999). Klein and Shambaugh (2015) found that limited exchange-rate flexibility can buy considerable monetary autonomy. After the global financial crisis the IMF accepted macroprudential measures and capital controls as tools against inflow surges (Ostry et al., 2010). Hélène Rey (2013) argued that because of a global financial cycle driven by US monetary policy, even floating countries with open capital accounts have limited autonomy, making it effectively a "dilemma, not a trilemma." Dani Rodrik (2011) extended the idea to a political trilemma among globalization, national sovereignty and democracy.

FAQ

What is the impossible trinity in simple terms?

A country cannot have a fixed exchange rate, free movement of capital and its own interest-rate policy all at once; it has to give up one.

Which combination does South Korea use?

Since the 1997 crisis South Korea has chosen free capital flows and monetary independence with a floating exchange rate, though it intervenes in currency markets during sharp swings.

Sources reviewed: 2026-10-26

References

1. Mundell, R. A. (1963). Capital mobility and stabilization policy under fixed and flexible exchange rates. Canadian Journal of Economics and Political Science, 29(4), 475–485.
2. Fleming, J. M. (1962). Domestic financial policies under fixed and under floating exchange rates. IMF Staff Papers, 9(3), 369–380.
3. Obstfeld, M., Shambaugh, J. C., & Taylor, A. M. (2005). The trilemma in history. Review of Economics and Statistics, 87(3), 423–438.
4. Obstfeld, M., & Taylor, A. M. (2004). Global Capital Markets: Integration, Crisis, and Growth. Cambridge: Cambridge University Press.
5. Aizenman, J., Chinn, M. D., & Ito, H. (2013). The "impossible trinity" hypothesis in an era of global imbalances. Review of International Economics, 21(3), 447–458.
6. Rey, H. (2013). Dilemma not trilemma: The global financial cycle and monetary policy independence. Jackson Hole Economic Policy Symposium Proceedings, 285–333.
7. Eichengreen, B. (2019). Globalizing Capital: A History of the International Monetary System (3rd ed.). Princeton: Princeton University Press.
8. Frankel, J. A. (1999). No single currency regime is right for all countries or at all times. NBER Working Paper No. 7338.
9. Klein, M. W., & Shambaugh, J. C. (2015). Rounding the corners of the policy trilemma. American Economic Journal: Macroeconomics, 7(4), 33–66.
10. Krugman, P. (1979). A model of balance-of-payments crises. Journal of Money, Credit and Banking, 11(3), 311–325.
11. Obstfeld, M., & Rogoff, K. (1995). The mirage of fixed exchange rates. Journal of Economic Perspectives, 9(4), 73–96.
12. Rodrik, D. (2011). The Globalization Paradox. New York: W. W. Norton.
13. Ostry, J. D., Ghosh, A. R., Habermeier, K., et al. (2010). Capital inflows: The role of controls. IMF Staff Position Note SPN/10/04.