Covered interest rate parity links the prices of currency forwards to spot exchange rates and interest rates so that no riskless profit is available from arbitrage. In its simplest discrete form the parity implies that the forward exchange rate equals the spot rate adjusted by the ratio of one plus the domestic interest rate to one plus the foreign interest rate. John Hull at the University of Toronto outlines this relation in his textbook on derivatives, showing how a covered arbitrage strategy using deposits and a forward contract eliminates systematic profit if the equality holds. This connection makes the forward price a market reflection of relative interest returns, not a pure prediction of future spot movements.
Mechanism and causes
The operational cause of the parity is arbitrage. If a forward is mispriced relative to spot and interest rates, traders can borrow in the low-rate currency, convert to the high-rate currency, invest, and lock the future conversion via a forward contract to earn a guaranteed spread. Covered interest rate parity therefore relies on arbitrage enforcement and on the ability to transact in spot, forward, and money markets. Market frictions such as transaction costs, capital controls, differential credit risk, or regulatory constraints weaken enforcement. The Bank for International Settlements documents how these frictions and balance sheet constraints can create persistent, measurable deviations in short-term periods, especially around crises or in emerging market contexts.
Relevance and consequences
When covered interest rate parity holds closely, forward prices serve as reliable hedging instruments: corporations and investors can lock future exchange rates with minimal expectation of arbitrage distortions. This underpins cross-border trade and corporate risk management and affects pricing of derivatives. Conversely, departures from parity create practical consequences. Persistent deviations can signal funding stress, regulatory segmentation, or limits to arbitrage and may invite speculative strategies such as the carry trade. Such strategies can amplify currency moves, with social and territorial nuance: economies with limited capital mobility may see higher local borrowing costs and greater exchange rate volatility, affecting importers, exporters, and household purchasing power.
In practice, central bank policy, market liquidity, and institutional constraints shape how closely parity holds. For market participants and policymakers alike, monitoring forward-spot-implied interest differentials offers insight into funding conditions and the degree to which forward prices are driven by interest-rate fundamentals versus market stress or regulation.