Lesson preview · Exchange Rate Regimes
Exchange Rate Regimes & the Impossible Trinity
~12 min · Free to read
Every country has to decide one thing about its money: who sets its price against other currencies? Some let the market decide. Others pin it in place. This single choice shapes how a country trades, how it fights recessions, and whether its central bank is free to act at all.
Start with the two extremes. A floating exchange rate is set by supply and demand in the currency market — it moves freely, day to day, as buyers and sellers trade. The US dollar, the British pound, and the Japanese yen all float. Nobody pins their value; it is whatever the market says it is right now.
A fixed (or pegged) exchange rate is the opposite. The central bank holds the currency at a set value and defends it by buying and selling its own money. If the currency drifts too high, the bank sells it; if it drifts too low, the bank buys it back. The Hong Kong dollar is pegged to the US dollar — about 7.8 Hong Kong dollars per US dollar — and the central bank stands ready to trade at that rate to keep it there.
Most countries sit between the two. A managed float lets the currency move with the market most of the time, but the central bank steps in to smooth big swings — it leans against a crash or a spike without committing to one fixed number. Think of it as a float with guardrails.
Key Term
Floating vs fixed: the trade-off
Each regime buys you something and costs you something.
- A floating rate absorbs shocks. If demand for a country’s exports collapses, its currency simply falls, making exports cheaper again and cushioning the blow. It also frees the central bank to set interest rates for the home economy. The cost: the rate can be volatile, which makes planning trade and investment harder.
- A fixed rate gives stability and predictability. Firms importing and exporting know exactly what the currency will be worth, which is great for trade. The cost: defending the peg ties the central bank’s hands — it must spend its effort holding the rate rather than steering the home economy.
That trade-off hints at something deeper. A country might want it all: a stable, predictable currency; the freedom for money to flow in and out across its border; and a central bank free to set interest rates for its own economy. It turns out you cannot have all three. This is the impossible trinity.
Key Term
The impossible trinity (the trilemma)
A country wants three things from its currency policy. The impossible trinity says it can pick at most two — keeping all three at once is impossible.
The three goals:
- A fixed exchange rate — a stable, pegged currency.
- Free movement of capital — money can flow freely in and out across the border, with no controls.
- Independent monetary policy — the central bank sets interest rates to suit the home economy, whatever other countries do.
Every real country picks two and gives up the third. There is no way around it.
Key Term
The three valid combinations
Each choice keeps two goals and sacrifices one.
- Keep a fixed rate + free capital flows → give up independent monetary policy. You import another country’s interest-rate policy. Example: Hong Kong’s peg to the US dollar means Hong Kong effectively follows US rates. The classical gold standard and members of the euro work the same way.
- Keep a fixed rate + independent monetary policy → give up free capital flows. You impose capital controls — limits on money crossing the border. Example: China through the 1990s and 2000s; also the post-war Bretton Woods system.
- Keep free capital flows + independent monetary policy → give up a fixed rate. You let the currency float. Example: the US, the UK, Japan, and most large developed economies today.
Trap · Common Misconception
Why can't a country just have all three?
It seems like you should be able to peg the currency, leave the borders open, AND set your own interest rates. Here is why you cannot. Suppose a country pegs to the US dollar but tries to set a higher interest rate than the US. With money free to move, investors rush in to earn that higher rate. All that incoming money pushes the currency up — which breaks the peg. To hold the peg, the central bank would have to push its rate back down to match the US. So it has given up its independent policy after all. Free capital flows plus a different home rate forces the exchange rate to move. Something has to give.
Tip
So what?
A country can have at most two of {a fixed exchange rate, free capital flows, an independent monetary policy} — wanting all three is impossible, so every country’s currency policy is really a choice about which one to give up.
Worked Example
Hong Kong pegs its dollar firmly to the US dollar and keeps its borders wide open to money flowing in and out. Using the impossible trinity, work out what Hong Kong must give up — and what that means in practice.
Check yourself · no marks
A country pegs its currency, keeps its borders open to money, but then tries to set an interest rate well above the country it is pegged to. Walk through why this cannot last.
Commit to one first. Guessing and being wrong beats reading the answer cold.
Practice · 1 / 4
What is the difference between a floating exchange rate and a fixed (pegged) exchange rate?
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