What is the cross-currency basis, and why does it deviate from zero even when covered interest rate parity should hold?

The cross-currency basis represents the additional spread paid or received in a cross-currency basis swap beyond what covered interest rate parity (CIP) would predict. In theory, CIP should hold exactly, making the basis zero. In practice, persistent deviations arise from structural supply-demand imbalances in dollar funding markets.\n\nCovered Interest Rate Parity Recap:\n\nCIP states that the forward exchange rate should fully offset interest rate differentials:\n\nF/S = (1 + r_d) / (1 + r_f)\n\nIf this holds, a cross-currency basis swap (exchanging floating rates in two currencies) should price at flat -- no additional spread. The cross-currency basis (b) is the deviation:\n\nFlat-rate payer receives: Foreign floating rate + b\n\nWhy the Basis Deviates:\n\n`mermaid\ngraph TD\n A[\"Structural USD Demand
Non-US banks need dollars\"] --> B[\"Borrow USD via FX swaps
Supply local currency\"]\n B --> C[\"Excess demand for USD
in FX swap market\"]\n C --> D[\"Negative basis
USD lender earns premium\"]\n E[\"Regulatory Constraints\"] --> C\n F[\"Quarter-end balance sheet
window dressing\"] --> C\n G[\"Central bank USD
swap lines\"] -->|\"Partially offset\"| D\n D --> H[\"Persistent CIP violation
e.g., EUR/USD basis = -20 bps\"]\n`\n\nWorked Example:\nHarborview Asset Management wants to convert EUR-denominated returns into USD for 1 year. The EUR/USD cross-currency basis is -18 bps.\n\nWithout basis: Harborview swaps EUR floating for USD floating at par interest rate differential.\n\nWith -18 bps basis: Harborview receives USD SOFR but pays EUR EURIBOR minus 18 bps. Equivalently, obtaining dollar funding via the FX swap market costs 18 bps more than the direct USD borrowing rate.\n\nOn a EUR 200 million notional:\nAnnual basis cost: 200,000,000 x 0.0018 = EUR 360,000\n\nThis cost reflects the scarcity premium for dollar funding.\n\nKey Drivers of Basis Movements:\n1. Quarter/year-end effects -- Banks reduce balance sheets for regulatory reporting, widening the basis\n2. Central bank policies -- Fed swap lines and dollar liquidity operations compress the basis\n3. Hedging demand -- Japanese life insurers hedging USD bond portfolios create persistent negative JPY/USD basis\n4. Leverage ratio constraints -- Basel III leverage ratios make FX swap intermediation capital-intensive\n\nExam Relevance:\nThe cross-currency basis is a direct measure of dollar funding stress and a violation of a textbook no-arbitrage condition. Understanding why arbitrage fails to eliminate the basis (capital constraints, counterparty limits, regulatory costs) is central to modern fixed-income risk management.\n\nExplore cross-currency risk in our FRM study materials.