What is covered interest parity?

Covered interest parity (CIP) is a fundamental relationship in international finance stating that the difference between the forward and spot exchange rates between two currencies should equal the interest rate differential between them. If US rates are 5% and European rates are 3%, CIP dictates that the euro should trade at a 2% forward premium against the dollar, so that an investor earns the same return regardless of which currency they invest in, once the currency risk is hedged.

Before 2008, CIP held almost perfectly, deviating by only a few basis points. Any larger deviations were quickly arbitraged away by banks and currency traders. After the financial crisis, persistent and significant CIP violations emerged, with the "cross-currency basis" (the deviation from CIP) widening to 20-60 basis points for major currency pairs and even more for some emerging market currencies. This breakdown of a fundamental arbitrage relationship puzzled economists and became one of the most studied anomalies in post-crisis finance.

The explanation centers on bank balance sheet constraints. Post-2008 regulations (Basel III leverage ratios, supplementary leverage ratio requirements) made it expensive for banks to expand their balance sheets to conduct the arbitrage trades that would restore CIP. Even when the arbitrage profit exceeded the cost of capital, regulatory constraints prevented banks from scaling the trade sufficiently to close the gap. The result is a persistent "shadow cost" of dollar funding for non-US banks that must obtain dollars through FX swap markets rather than direct borrowing.

For market participants, CIP deviations are a barometer of dollar funding stress. When the cross-currency basis widens (CIP deviations increase), it signals that non-US institutions are struggling to obtain dollar funding. This tends to happen during year-end and quarter-end periods when balance sheet constraints bind more tightly, and during genuine financial stress when counterparty concerns limit FX swap activity. Central bank swap lines, which provide dollars directly to foreign central banks, help alleviate this stress but have not eliminated the structural basis, which persists as a permanent feature of the post-2008 financial architecture.