Interest Rate Parity
Interest rate parity is a condition linking interest rates and exchange rates across countries. It holds that the difference between interest rates on assets denominated in two currencies should be offset by the expected change in the exchange rate between them, so that investors earn the same expected return regardless of the currency in which they invest. Parity conditions are central to the analysis of international financial markets and to the determination of exchange rates.
Covered Interest Parity
Covered interest parity applies when the exchange rate risk is hedged using a forward contract. It states that the interest rate differential between two currencies equals the forward premium or discount. Because the forward rate is known, covered parity involves no exchange rate risk and arbitrage should enforce it closely, subject to transaction costs and capital mobility.
Empirically, covered interest parity holds closely for major currencies in normal times, and deviations are limited to small amounts reflecting transaction costs and credit risk. During periods of financial stress, however, deviations can widen substantially, reflecting limits to arbitrage arising from balance sheet constraints, regulatory costs, and the dollar funding needs of non-US institutions.
Uncovered Interest Parity
Uncovered interest parity applies when the investor bears exchange rate risk. It states that the interest differential equals the expected rate of depreciation. Unlike covered parity, uncovered parity involves expectations and risk, and its empirical performance is poor.
The forward premium puzzle, the finding that currencies with high interest rates tend to appreciate rather than depreciate as uncovered parity would predict, is one of the most robust anomalies in international finance. Explanations include risk premiums that vary over time, expectations that are not rational or that involve learning, and behaviour that differs from the optimising benchmark.
Real Interest Parity and the Fisher Relationship
Real interest parity combines interest rate parity with the Fisher relationship, which links nominal and real interest rates through expected inflation. It holds that real interest rates should be equal across countries, adjusted for expected changes in exchange rates and inflation. Its failure reflects the same factors that cause uncovered parity to fail.
Applications and Implications
Parity conditions are used to interpret movements in interest rates and exchange rates, to assess the arbitrage relationships that link markets, and to test the efficiency of international financial markets. They also inform policy, since deviations from parity can signal capital controls, market segmentation, or risk premiums, and their magnitude affects the cost of external finance and the incentives for cross-border investment.
Deviations and Limits to Arbitrage
Covered interest parity, which should hold when arbitrage is costless, deviates during times of financial stress. These deviations reflect the balance sheet constraints of arbitrageurs, regulatory costs, and the funding needs of institutions that operate in multiple currencies.
When arbitrageurs face capital constraints, they cannot fully exploit deviations, and the resulting limits to arbitrage allow spreads to persist. The deviations are important because they affect the cost of cross-border funding and can signal stress in financial markets, as was evident in the global financial crisis and in later episodes of dollar funding pressure.
Expectations and the Forward Premium
The failure of uncovered interest parity is one of the most studied anomalies in international finance. The forward premium puzzle, in which high interest rate currencies tend to appreciate, contradicts the prediction that they should depreciate to equalise expected returns.
Explanations include time-varying risk premiums that compensate investors for bearing currency risk, expectational errors that persist, and the use of information that is not captured by standard models. The puzzle has motivated substantial research on the determinants of exchange rate expectations and on the behaviour of international investors.
Carry Trades and Currency Risk
The failure of uncovered parity underpins carry trade strategies, which borrow in low interest rate currencies and invest in high interest rate currencies. These strategies earn returns when currencies do not depreciate as predicted, but they are exposed to sudden reversals, particularly during periods of market stress when high yield currencies depreciate sharply.
The returns to carry trades compensate for risk, including the risk of crashes, and their behaviour illustrates the interplay between risk premiums, expectations, and market frictions. The analysis of carry trades informs the broader understanding of currency risk and international portfolio allocation.
Policy Implications
Parity conditions inform the assessment of capital mobility and market integration. Deviations from covered parity may signal capital controls, market segmentation, or credit risk, and their magnitude affects the cost of external finance. For policymakers, the conditions help interpret the relationship between interest rates and exchange rates, the effects of intervention, and the transmission of monetary policy across borders.