Why a currency peg limits interest-rate choices

A fixed exchange rate is a promise to keep a country’s currency at a set value against another currency. To make that promise credible while money can move freely across borders, the country’s interest rates generally have to track those of the currency it is pegged to.

The reason is the search for returns. If investors can move money without restriction and the pegged country offers much higher interest rates, funds may pour in; if it offers lower rates, money may flow out. Those movements put pressure on the exchange rate. To defend the peg, the central bank may have to buy or sell currencies and adjust domestic financial conditions—often pushing its interest rates toward the other country’s.

The three goals—and the necessary tradeoff

The three goals are a stable exchange rate, free movement of capital, and independent monetary policy. A country can reliably pursue two, but not all three at once:

  • Peg and open capital flows: Interest rates must largely follow the country whose currency anchors the peg.
  • Open capital flows and independent rates: The exchange rate must be allowed to move, as it does under a floating currency.
  • Peg and independent rates: The country must limit cross-border financial flows, so investors cannot freely exploit the gap between its rates and the anchor country’s.

This is a constraint, not a claim that governments have no choices. They can change which goals they prioritize, and a peg may withstand some pressure for a time. But defending it can become costly if market forces pull strongly in the opposite direction.

What the choice means in practice

The tradeoff matters most when domestic conditions call for a different interest rate than those abroad. For example, a country facing weak demand might want to cut rates, while the country backing its peg is raising them. With capital mobile, the cut could encourage money to leave and put the peg under strain. The government must then accept less freedom over rates, restrict capital movement, or let the currency adjust.

So a currency peg is more than a declaration about exchange rates: paired with open financial borders, it can also narrow the central bank’s room to respond to events at home.