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COPYRIGHT NOTICE: Edited by Richard H. Thaler: Advances in Behavioral Finance, Volume II - page 4 / 23





4 / 23



  • 2.

    Limits to Arbitrage

    • 2.1.

      Market Efficiency

In the traditional framework where agents are rational and there are no frictions, a security’s price equals its “fundamental value.” This is the dis- counted sum of expected future cash flows, where in forming expectations, investors correctly process all available information, and where the discount rate is consistent with a normatively acceptable preference specification. The hypothesis that actual prices reflect fundamental values is the Efficient Markets Hypothesis (EMH). Put simply, under this hypothesis, “prices are right,” in that they are set by agents who understand Bayes’s law and have sensible preferences. In an efficient market, there is “no free lunch”: no in- vestment strategy can earn excess risk-adjusted average returns, or average returns greater than are warranted for its risk.

Behavioral finance argues that some features of asset prices are most plausibly interpreted as deviations from fundamental value, and that these deviations are brought about by the presence of traders who are not fully rational. A long-standing objection to this view that goes back to Friedman (1953) is that rational traders will quickly undo any dislocations caused by irrational traders. To illustrate the argument, suppose that the fundamental value of a share of Ford is $20. Imagine that a group of irrational traders becomes excessively pessimistic about Ford’s future prospects and through its selling, pushes the price to $15. Defenders of the EMH argue that ra- tional traders, sensing an attractive opportunity, will buy the security at its bargain price and at the same time, hedge their bet by shorting a “substi- tute” security, such as General Motors, that has similar cash flows to Ford in future states of the world. The buying pressure on Ford shares will then bring their price back to fundamental value.

Friedman’s line of argument is initially compelling, but it has not sur- vived careful theoretical scrutiny. In essence, it is based on two assertions. First, as soon as there is a deviation from fundamental value—in short, a mispricing—an attractive investment opportunity is created. Second, ra- tional traders will immediately snap up the opportunity, thereby correcting the mispricing. Behavioral finance does not take issue with the second step in this argument: when attractive investment opportunities come to light, it is hard to believe that they are not quickly exploited. Rather, it disputes the first step. The argument, which we elaborate on in sections 2.2 and 2.3, is that even when an asset is wildly mispriced, strategies designed to correct the mispricing can be both risky and costly, rendering them unattractive. As a result, the mispricing can remain unchallenged.

It is interesting to think about common finance terminology in this light. While irrational traders are often known as “noise traders,” rational traders are typically referred to as “arbitrageurs.” Strictly speaking, an arbitrage is

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