Interest rate parity (IRP) is a theory stating that the difference in interest rates between two countries equals the difference between their forward and spot exchange rates. It explains why currencies with higher interest rates tend to trade at a forward discount—they are expected to depreciate to offset the interest rate advantage.
Two Forms of Interest Rate Parity
- Covered Interest Rate Parity (CIRP): The forward exchange rate eliminates any risk-free profit opportunity from interest rate differences. If two currencies have different rates, the forward rate adjusts so that hedged returns are equal.
- Uncovered Interest Rate Parity (UIRP): Assumes investors are risk-neutral and willing to invest in the higher-yielding currency without hedging. The expected depreciation of that currency should offset the higher interest rate.
How It Works in Practice
When one country offers higher interest rates than another, an investor earning the higher rate should not ultimately earn more than an investor in the lower-rate country if hedged. The currency of the higher-rate country should appreciate over time to equalize returns. In theory, covered interest rate parity holds fairly well because arbitrage quickly eliminates mispricings. Uncovered parity, however, often fails—currencies don't depreciate as much as rates suggest.
Real-World Challenges
IRP assumes capital mobility and perfect asset substitutability, which don't always exist. Political risk, transaction costs, differing tax treatments, and capital controls can all disrupt the parity relationship. Additionally, UIRP frequently doesn't hold in real markets, creating persistent opportunities in carry trades—a key reality for forex traders.
Why Traders Care
IRP helps explain why interest rate differentials drive exchange rates and why carry trades can be profitable when UIRP fails to hold. Understanding parity allows traders to assess whether forward rates reflect the true interest rate differential and identify potential mispricings.
