Impossible Trinity
Quick Definition
The economic trilemma stating a country cannot simultaneously maintain a fixed exchange rate, free capital movement, and independent monetary policy—only two of three are possible.
Key Takeaways
- Cannot have all three: fixed rate, free capital flows, and independent monetary policy
- Only two of the three goals can be achieved simultaneously
- Developed by Mundell and Fleming in the 1960s
- Explains why currency pegs often collapse when capital accounts are liberalized
- Each country chooses which two goals to prioritize based on its circumstances
What Is Impossible Trinity?
The Impossible Trinity (also called the Mundell-Fleming trilemma) is a fundamental concept in international economics stating that a country cannot simultaneously achieve all three of the following policy goals: a fixed exchange rate, free capital mobility, and independent monetary policy. Only two of the three can be maintained at any given time. For example, the U.S. chooses independent monetary policy and free capital flows, sacrificing exchange rate stability (floating dollar). Hong Kong chooses a fixed exchange rate and free capital flows, sacrificing monetary independence (rates follow the Fed). China historically chose a fixed exchange rate and independent monetary policy, restricting capital flows. The trilemma, developed by economists Robert Mundell and Marcus Fleming in the 1960s, explains why currency pegs often fail when countries liberalize capital accounts, and why the eurozone faces challenges with a common monetary policy but divergent national economies.
Impossible Trinity Example
- 1Hong Kong maintains its dollar peg and free capital flows but sacrifices monetary independence—its interest rates must follow the Federal Reserve.
- 2The Asian Financial Crisis of 1997 illustrated the impossible trinity when countries with fixed rates and open capital accounts lost control of monetary conditions.
- 3China maintains some capital controls precisely because of the impossible trinity—to preserve both exchange rate management and monetary policy independence.
Related Terms
Currency Peg
A fixed exchange rate policy where a country ties the value of its currency to another currency or basket of currencies at a set rate.
Exchange Rate
The price of one currency expressed in terms of another, determining how much of one currency is needed to purchase a unit of another.
Monetary Policy
Actions by a central bank to manage the money supply and interest rates to achieve macroeconomic objectives like stable prices and full employment.
Capital Flows
The movement of money for investment, trade, or business operations between countries, including foreign direct investment, portfolio investment, and bank lending.
Central Bank
A national institution responsible for managing a country's monetary policy, regulating banks, maintaining financial stability, and issuing currency.
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