Limits to arbitrage

Limits to arbitrage is a theory which assumes that restrictions placed upon funds, that would ordinarily be used by rational traders to arbitrage away pricing inefficiencies, leave prices in a non-equilibrium state for protracted periods of time.

The efficient market hypothesis assumes that whenever mispricing of a publicly-traded stock occurs as a result of an over-reaction to news, or some similar event, an opportuntity for low-risk profit is created for rational traders. The low-risk profit opportunity exists through the tool of arbitrage, which, briefly, is buying and selling differently priced items of the same value, and pocketing the difference. If a stock falls away from its equilibrium price (let us say it becomes undervalued) due to irrational trading (noise traders), rational investors will (in this case) take a long position while going short on a proxy security, or another stock with similar characteristics.

Rational traders usually work for professional money management firms, and invest other peoples' money. If they engage in arbitrage in reaction to a stock mispricing, and the mispricing persists for an extended period, clients of the money management firm can (and do) formulate the opinion that the firm is incompetent. This results in withdrawal of the clients' funds. In order to deliver funds, the manager must unwind the position at a loss. The threat of this action on behalf of clients causes professional managers to be less prone to take advantage of these opportunities. This has the tendency to exacerbate the problem of pricing inefficiency.

Long-Term Capital Management became a victim of limits to arbitrage in 1998.

Additional reading

Inefficient Markets: An Introduction to Behavioral Finance, Andrei Shleifer, 2000, Oxford University Press.