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COPYRIGHT NOTICE: Edited by Richard H. Thaler: Advances in Behavioral Finance, Volume II - page 6 / 23





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are well aware of this risk, which is why they short a substitute security such as General Motors at the same time that they buy Ford. The problem is that substitute securities are rarely perfect, and often highly imperfect, making it impossible to remove all the fundamental risk. Shorting General Motors protects the arbitrageur somewhat from adverse news about the car industry as a whole, but still leaves him vulnerable to news that is specific to Ford—news about defective tires, say.4

Noise Trader Risk. Noise trader risk, an idea introduced by De Long et al. (1990a) and studied further by Shleifer and Vishny (1997), is the risk that the mispricing being exploited by the arbitrageur worsens in the short run. Even if General Motors is a perfect substitute security for Ford, the arbi- trageur still faces the risk that the pessimistic investors causing Ford to be undervalued in the first place become even more pessimistic, lowering its price even further. Once one has granted the possibility that a security’s price can be different from its fundamental value, then one must also grant the possibility that future price movements will increase the divergence.

Noise trader risk matters because it can force arbitrageurs to liquidate their positions early, bringing them potentially steep losses. To see this, note that most real-world arbitrageurs—in other words, professional portfolio managers—are not managing their own money, but rather managing money for other people. In the words of Shleifer and Vishny (1997), there is “a separation of brains and capital.”

This agency feature has important consequences. Investors, lacking the specialized knowledge to evaluate the arbitrageur’s strategy, may simply evaluate him based on his returns. If a mispricing that the arbitrageur is try- ing to exploit worsens in the short run, generating negative returns, in- vestors may decide that he is incompotent, and withdraw their funds. If this happens, the arbitrageur will be forced to liquidate his position prema- turely. Fear of such premature liquidation makes him less aggressive in combating the mispricing in the first place.

These problems can be severely exacerbated by creditors. After poor short-term returns, creditors, seeing the value of their collateral erode, will call their loans, again triggering premature liquidation.

In these scenarios, the forced liquidation is brought about by the wors- ening of the mispricing itself. This need not always be the case. For exam- ple, in their efforts to remove fundamental risk, many arbitrageurs sell securities short. Should the original owner of the borrowed security want it back, the arbitrageur may again be forced to close out his position if he can- not find other shares to borrow. The risk that this occurs during a temporary

Another problem is that even if a substitute security exists, it may itself be mispriced. This can happen in situations involving industry-wide mispricing: in that case, the only stocks with similar future cash flows to the mispriced one are themselves mispriced. 4

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