Limits to Arbitrage

Limits to arbitrage is the theoretical and empirical finding that rational traders cannot fully correct mispricing in financial markets, even when the mispricing is apparent and the fundamentals are known. The concept, developed by Andrei Shleifer and others in the 1990s, challenges the efficient market hypothesis by identifying the mechanisms that prevent arbitrage from eliminating irrational pricing.

The central problem is that arbitrage is not riskless. Arbitrageurs face noise-trader risk: the possibility that mispricing will worsen before it corrects, forcing the arbitrageur to close positions at a loss before the long-run correction occurs. Arbitrageurs with finite capital, finite time horizons, and risk-averse principals cannot bet against mispricing indefinitely. The Long-Term Capital Management collapse of 1998 illustrates this: the fund's positions were eventually correct, but the correction came too late.

Additional limits include fundamental risk (the possibility that the arbitrageur's model of fair value is wrong), implementation costs (short-selling constraints, transaction costs, and liquidity limits), and synchronization risk (the difficulty of coordinating arbitrage activity when many rational traders are needed to correct a large mispricing). The combination of these limits implies that markets can remain irrational longer than arbitrageurs can remain solvent — and that apparent profit opportunities can persist because the mechanism to exploit them is itself fragile.