The Macro Policy Trilemma
A country's central bank can have at most two of three things at once — a fixed exchange rate, free capital flows, and an independent monetary policy — never all three simultaneously.
The trilemma, also called the "impossible trinity," says a country can choose at most two of these three: a fixed exchange rate, free movement of capital across its borders, and an independent monetary policy set by its own priorities rather than another country's. Pick any two, and the third has to give.
The logic is arbitrage: if capital moves freely and the exchange rate is fixed, then domestic interest rates are forced to track foreign rates almost exactly — any gap would attract a flood of capital chasing the higher rate, which would break the fixed exchange rate unless the central bank matches rates abroad, surrendering independent monetary policy. Conversely, a country can keep an independent monetary policy and a fixed exchange rate only by restricting capital flows, so no arbitrage flood can happen — the classic capital-controls model. Or it can keep free capital flows and independent monetary policy by letting the exchange rate float freely to absorb whatever pressure would otherwise force a choice.
Real-world regimes sit at these corners: the US runs free capital flows and independent policy with a floating dollar; China has historically run a managed exchange rate and independent policy by restricting capital flows; eurozone members gave up independent monetary policy entirely (a shared currency is the ultimate fixed rate) in exchange for free capital flows within the bloc.
A country cannot simultaneously have a fixed exchange rate, free capital flows, and independent monetary policy — interest rate arbitrage forces a choice of at most two, which is why currency regimes around the world cluster into these three corner solutions rather than spreading evenly across all combinations.
Further reading
- Obstfeld, Shambaugh & Taylor, The Trilemma in History (2005)