《金融投资学》教学资源（英文文献）A Survey of Behavioral Finance.pdf_大学文库

Ch.18:A Survey of Behavioral Finance 1055 1.Introduction The traditional finance paradigm.which underlies many of the other articles in this handbook,seeks to understand financial markets using models in which agents are "rational".Rationality means two things.First,when they receive new information agents update their beliefs correctly,in the manner described by Bayes'law.Second, given their beliefs,agents make choices that are normatively acceptable,in the sense that they are consistent with Savage's notion of Subjective Expected Utility(SEU). This traditional framework is appealingly simple,and it would be very satisfying if its predictions were confirmed in the data.Unfortunately,after years of effort,it has become clear that basic facts about the aggregate stock market,the cross-section of average returns and individual trading behavior are not easily understood in this framework. Behavioral finance is a new approach to financial markets that has emerged,at least in part,in response to the difficulties faced by the traditional paradigm.In broad terms, it argues that some financial phenomena can be better understood using models in which some agents are not fully rational.More specifically,it analyzes what happens when we relax one,or both,of the two tenets that underlie individual rationality. In some behavioral finance models,agents fail to update their beliefs correctly.In other models,agents apply Bayes'law properly but make choices that are normatively questionable,in that they are incompatible with SEU1. This review essay evaluates recent work in this rapidly growing field.In Section 2, we consider the classic objection to behavioral finance,namely that even if some agents in the economy are less than fully rational,rational agents will prevent them from influencing security prices for very long,through a process known as arbitrage.One of the biggest successes of behavioral finance is a series of theoretical papers showing that in an economy where rational and irrational traders interact,irrationality can have a substantial and long-lived impact on prices.These papers,known as the literature on"limits to arbitrage",form one of the two buildings blocks of behavioral finance. 1 It is important to note that most models of asset pricing use the Rational Expectations Equilibrium framework (REE),which assumes not only individual rationality but also consistent beliefs [Sargent (1993)].Consistent beliefs means that agents'beliefs are correct:the subjective distribution they use to forecast future realizations of unknown variables is indeed the distribution that those realizations are drawn from.This requires not only that agents process new information correctly,but that they have enough information about the structure of the economy to be able to figure out the correct distribution for the variables of interest. Behavioral finance departs from REE by relaxing the assumption of individual rationality.An alternative departure is to retain individual rationality but to relax the consistent beliefs assumption:while investors apply Bayes'law correctly,they lack the information required to know the actual distribution variables are drawn from.This line of research is sometimes referred to as the literature on bounded rationality,or on structural uncertainty.For example,a model in which investors do not know the growth rate of an asset's cash flows but learn it as best as they can from available data,would fall into this class.Although the literature we discuss also uses the term bounded rationality,the approach is quite different

Ch. 18: A Survey of Behavioral Finance 1055 1. Introduction The traditional finance paradigm, which underlies many of the other articles in this handbook, seeks to understand financial markets using models in which agents are “rational”. Rationality means two things. First, when they receive new information, agents update their beliefs correctly, in the manner described by Bayes’ law. Second, given their beliefs, agents make choices that are normatively acceptable, in the sense that they are consistent with Savage’s notion of Subjective Expected Utility (SEU). This traditional framework is appealingly simple, and it would be very satisfying if its predictions were confirmed in the data. Unfortunately, after years of effort, it has become clear that basic facts about the aggregate stock market, the cross-section of average returns and individual trading behavior are not easily understood in this framework. Behavioral finance is a new approach to financial markets that has emerged, at least in part, in response to the difficulties faced by the traditional paradigm. In broad terms, it argues that some financial phenomena can be better understood using models in which some agents are not fully rational. More specifically, it analyzes what happens when we relax one, or both, of the two tenets that underlie individual rationality. In some behavioral finance models, agents fail to update their beliefs correctly. In other models, agents apply Bayes’ law properly but make choices that are normatively questionable, in that they are incompatible with SEU1. This review essay evaluates recent work in this rapidly growing field. In Section 2, we consider the classic objection to behavioral finance, namely that even if some agents in the economy are less than fully rational, rational agents will prevent them from influencing security prices for very long, through a process known as arbitrage. One of the biggest successes of behavioral finance is a series of theoretical papers showing that in an economy where rational and irrational traders interact, irrationality can have a substantial and long-lived impact on prices. These papers, known as the literature on “limits to arbitrage”, form one of the two buildings blocks of behavioral finance. 1 It is important to note that most models of asset pricing use the Rational Expectations Equilibrium framework (REE), which assumes not only individual rationality but also consistent beliefs [Sargent (1993)]. Consistent beliefs means that agents’ beliefs are correct: the subjective distribution they use to forecast future realizations of unknown variables is indeed the distribution that those realizations are drawn from. This requires not only that agents process new information correctly, but that they have enough information about the structure of the economy to be able to figure out the correct distribution for the variables of interest. Behavioral finance departs from REE by relaxing the assumption of individual rationality. An alternative departure is to retain individual rationality but to relax the consistent beliefs assumption: while investors apply Bayes’ law correctly, they lack the information required to know the actual distribution variables are drawn from. This line of research is sometimes referred to as the literature on bounded rationality, or on structural uncertainty. For example, a model in which investors do not know the growth rate of an asset’s cash flows but learn it as best as they can from available data, would fall into this class. Although the literature we discuss also uses the term bounded rationality, the approach is quite different

1056 N.Barberis and R.Thaler To make sharp predictions,behavioral models often need to specify the form of agents'irrationality.How exactly do people misapply Bayes law or deviate from SEU?For guidance on this,behavioral economists typically turn to the extensive experimental evidence compiled by cognitive psychologists on the biases that arise when people form beliefs,and on people's preferences,or on how they make decisions, given their beliefs.Psychology is therefore the second building block of behavioral finance,and we review the psychology most relevant for financial economists in Section 32. In Sections 48,we consider specific applications of behavioral finance:to understanding the aggregate stock market,the cross-section of average returns,and the pricing of closed-end funds in Sections 4,5 and 6 respectively;to understanding how particular groups of investors choose their portfolios and trade over time in Section 7; and to understanding the financing and investment decisions of firms in Section 8. Section 9 takes stock and suggests directions for future research3. 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 discounted 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'law and have sensible preferences.In an efficient market,there is "no free lunch":no investment 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 2 The idea,now widely adopted,that behavioral finance rests on the two pillars of limits to arbitrage and investor psychology is originally due to Shleifer and Summers (1990). 3We draw readers'attention to two other recent surveys of behavioral finance.Shleifer(2000)provides a particularly detailed discussion of the theoretical and empirical work on limits to arbitrage,which we summarize in Section 2.Hirshleifer's (2001)survey is closer to ours in terms of material covered, although we devote less space to asset pricing,and more to corporate finance and individual investor behavior.We also organize the material somewhat differently

1056 N. Barberis and R. Thaler To make sharp predictions, behavioral models often need to specify the form of agents’ irrationality. How exactly do people misapply Bayes law or deviate from SEU? For guidance on this, behavioral economists typically turn to the extensive experimental evidence compiled by cognitive psychologists on the biases that arise when people form beliefs, and on people’s preferences, or on how they make decisions, given their beliefs. Psychology is therefore the second building block of behavioral finance, and we review the psychology most relevant for financial economists in Section 32. In Sections 4–8, we consider specific applications of behavioral finance: to understanding the aggregate stock market, the cross-section of average returns, and the pricing of closed-end funds in Sections 4, 5 and 6 respectively; to understanding how particular groups of investors choose their portfolios and trade over time in Section 7; and to understanding the financing and investment decisions of firms in Section 8. Section 9 takes stock and suggests directions for future research 3. 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 discounted 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’ law and have sensible preferences. In an efficient market, there is “no free lunch”: no investment 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 2 The idea, now widely adopted, that behavioral finance rests on the two pillars of limits to arbitrage and investor psychology is originally due to Shleifer and Summers (1990). 3 We draw readers’ attention to two other recent surveys of behavioral finance. Shleifer (2000) provides a particularly detailed discussion of the theoretical and empirical work on limits to arbitrage, which we summarize in Section 2. Hirshleifer’s (2001) survey is closer to ours in terms of material covered, although we devote less space to asset pricing, and more to corporate finance and individual investor behavior. We also organize the material somewhat differently

Ch.18:A Survey of Behavioral Finance 1057 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 rational traders, sensing an attractive opportunity,will buy the security at its bargain price and at the same time,hedge their bet by shorting a"substitute"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 survived 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,rational 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 an investment strategy that offers riskless profits at no cost.Presumably,the rational traders in Friedman's fable became known as arbitrageurs because of the belief that a mispriced asset immediately creates an opportunity for riskless profits.Behavioral finance argues that this is not true:the strategies that Friedman would have his rational traders adopt are not necessarily arbitrages;quite often,they are very risky. An immediate corollary of this line of thinking is that"prices are right"and"there is no free lunch"are not equivalent statements.While both are true in an efficient market,"no free lunch"can also be true in an inefficient market:just because prices are away from fundamental value does not necessarily mean that there are any excess risk-adjusted average returns for the taking.In other words, prices are right'”→"no free lunch" but no free lunch”“prices are right". This distinction is important for evaluating the ongoing debate on market efficiency. First,many researchers still point to the inability of professional money managers to beat the market as strong evidence of market efficiency [Rubinstein(2001),Ross (2001)].Underlying this argument,though,is the assumption that "no free lunch" implies"prices are right."If,as we argue in Sections 2.2 and 2.3,this link is broken,the

Ch. 18: A Survey of Behavioral Finance 1057 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 rational traders, sensing an attractive opportunity, will buy the security at its bargain price and at the same time, hedge their bet by shorting a “substitute” 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 survived 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, rational 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 an investment strategy that offers riskless profits at no cost. Presumably, the rational traders in Friedman’s fable became known as arbitrageurs because of the belief that a mispriced asset immediately creates an opportunity for riskless profits. Behavioral finance argues that this is not true: the strategies that Friedman would have his rational traders adopt are not necessarily arbitrages; quite often, they are very risky. An immediate corollary of this line of thinking is that “prices are right” and “there is no free lunch” are not equivalent statements. While both are true in an efficient market, “no free lunch” can also be true in an inefficient market: just because prices are away from fundamental value does not necessarily mean that there are any excess risk-adjusted average returns for the taking. In other words, “prices are right” ⇒ “no free lunch” but “no free lunch” “prices are right”. This distinction is important for evaluating the ongoing debate on market efficiency. First, many researchers still point to the inability of professional money managers to beat the market as strong evidence of market efficiency [Rubinstein (2001), Ross (2001)]. Underlying this argument, though, is the assumption that “no free lunch” implies “prices are right.” If, as we argue in Sections 2.2 and 2.3, this link is broken, the

1058 N.Barberis and R.Thaler performance of money managers tells us little about whether prices reflect fundamental value. Second,while some researchers accept that there is a distinction between "prices are right"and "there is no free lunch",they believe that the debate should be more about the latter statement than about the former.We disagree with this emphasis.As economists,our ultimate concern is that capital be allocated to the most promising investment opportunities.Whether this is true or not depends much more on whether prices are right than on whether there are any free lunches for the taking 2.2.Theory In the previous section,we emphasized the idea that when a mispricing occurs, strategies designed to correct it can be both risky and costly,thereby allowing the mispricing to survive.Here we discuss some of the risks and costs that have been identified.In our discussion,we return to the example of Ford,whose fundamental value is $20,but which has been pushed down to $15 by pessimistic noise traders. Fundamental risk.The most obvious risk an arbitrageur faces if he buys Ford's stock at $15 is that a piece of bad news about Ford's fundamental value causes the stock to fall further,leading to losses.Of course,arbitrageurs 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+. 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 arbitrageur 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 4Another 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

1058 N. Barberis and R. Thaler performance of money managers tells us little about whether prices reflect fundamental value. Second, while some researchers accept that there is a distinction between “prices are right” and “there is no free lunch”, they believe that the debate should be more about the latter statement than about the former. We disagree with this emphasis. As economists, our ultimate concern is that capital be allocated to the most promising investment opportunities. Whether this is true or not depends much more on whether prices are right than on whether there are any free lunches for the taking. 2.2. Theory In the previous section, we emphasized the idea that when a mispricing occurs, strategies designed to correct it can be both risky and costly, thereby allowing the mispricing to survive. Here we discuss some of the risks and costs that have been identified. In our discussion, we return to the example of Ford, whose fundamental value is $20, but which has been pushed down to $15 by pessimistic noise traders. Fundamental risk. The most obvious risk an arbitrageur faces if he buys Ford’s stock at $15 is that a piece of bad news about Ford’s fundamental value causes the stock to fall further, leading to losses. Of course, arbitrageurs 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 arbitrageur 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 4 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

Ch.18:A Survey of Behavioral Finance 1059 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 trying to exploit worsens in the short run,generating negative returns,investors may decide that he is incompetent, and withdraw their funds.If this happens,the arbitrageur will be forced to liquidate his position prematurely.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 worsening of the mispricing itself.This need not always be the case.For example,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 cannot find other shares to borrow.The risk that this occurs during a temporary worsening of the mispricing makes the arbitrageur more cautious from the start. Implementation costs.Well-understood transaction costs such as commissions,bid- ask spreads and price impact can make it less attractive to exploit a mispricing. Since shorting is often essential to the arbitrage process,we also include short-sale constraints in the implementation costs category.These refer to anything that makes it less attractive to establish a short position than a long one.The simplest such constraint is the fee charged for borrowing a stock.In general these fees are small-D'Avolio (2002)finds that for most stocks,they range between 10 and 15 basis points-but they can be much larger;in some cases,arbitrageurs may not be able to find shares to borrow at any price.Other than the fees themselves,there can be legal constraints:for a large fraction of money managers-many pension fund and mutual fund managers in particular-short-selling is simply not allowed5. We also include in this category the cost of finding and learning about a mispricing, as well as the cost of the resources needed to exploit it [Merton (1987)].Finding 5 The presence of per-period transaction costs like lending fees can expose arbitrageurs to another kind of risk,horizon risk,which is the risk that the mispricing takes so long to close that any profits are swamped by the accumulated transaction costs.This applies even when the arbitrageur is certain that no outside party will force him to liquidate early.Abreu and Brunnermeier (2002)study a particular type of horizon risk,which they label synchronization risk.Suppose that the elimination of a mispricing requires the participation of a sufficiently large number of separate arbitrageurs.Then in the presence of per-period transaction costs,arbitrageurs may hesitate to exploit the mispricing because they don't know how many other arbitrageurs have heard about the opportunity,and therefore how long they will have to wait before prices revert to correct values

Ch. 18: A Survey of Behavioral Finance 1059 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 trying to exploit worsens in the short run, generating negative returns, investors may decide that he is incompetent, and withdraw their funds. If this happens, the arbitrageur will be forced to liquidate his position prematurely. 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 worsening of the mispricing itself. This need not always be the case. For example, 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 cannot find other shares to borrow. The risk that this occurs during a temporary worsening of the mispricing makes the arbitrageur more cautious from the start. Implementation costs. Well-understood transaction costs such as commissions, bid– ask spreads and price impact can make it less attractive to exploit a mispricing. Since shorting is often essential to the arbitrage process, we also include short-sale constraints in the implementation costs category. These refer to anything that makes it less attractive to establish a short position than a long one. The simplest such constraint is the fee charged for borrowing a stock. In general these fees are small – D’Avolio (2002) finds that for most stocks, they range between 10 and 15 basis points – but they can be much larger; in some cases, arbitrageurs may not be able to find shares to borrow at any price. Other than the fees themselves, there can be legal constraints: for a large fraction of money managers – many pension fund and mutual fund managers in particular – short-selling is simply not allowed 5. We also include in this category the cost of finding and learning about a mispricing, as well as the cost of the resources needed to exploit it [Merton (1987)]. Finding 5 The presence of per-period transaction costs like lending fees can expose arbitrageurs to another kind of risk, horizon risk, which is the risk that the mispricing takes so long to close that any profits are swamped by the accumulated transaction costs. This applies even when the arbitrageur is certain that no outside party will force him to liquidate early. Abreu and Brunnermeier (2002) study a particular type of horizon risk, which they label synchronization risk. Suppose that the elimination of a mispricing requires the participation of a sufficiently large number of separate arbitrageurs. Then in the presence of per-period transaction costs, arbitrageurs may hesitate to exploit the mispricing because they don’t know how many other arbitrageurs have heard about the opportunity, and therefore how long they will have to wait before prices revert to correct values

1060 N.Barberis and R.Thaler mispricing,in particular,can be a tricky matter.It was once thought that if noise traders influenced stock prices to any substantial degree,their actions would quickly show up in the form of predictability in returns.Shiller(1984)and Summers (1986) demonstrate that this argument is completely erroneous,with Shiller(1984)calling it "one of the most remarkable errors in the history of economic thought".They show that even if noise trader demand is so strong as to cause a large and persistent mispricing,it may generate so little predictability in returns as to be virtually undetectable In contrast,then,to straightforward-sounding textbook arbitrage,real world arbitrage entails both costs and risks,which under some conditions will limit arbitrage and allow deviations from fundamental value to persist.To see what these conditions are,consider two cases. Suppose first that the mispriced security does not have a close substitute.By definition then,the arbitrageur is exposed to fundamental risk.In this case,sufficient conditions for arbitrage to be limited are(i)that arbitrageurs are risk averse and(ii)that the fundamental risk is systematic,in that it cannot be diversified by taking many such positions.Condition(i)ensures that the mispricing will not be wiped out by a single arbitrageur taking a large position in the mispriced security.Condition(ii) ensures that the mispricing will not be wiped out by a large number of investors each adding a small position in the mispriced security to their current holdings. The presence of noise trader risk or implementation costs will only limit arbitrage further. Even if a perfect substitute does exist,arbitrage can still be limited.The existence of the substitute security immunizes the arbitrageur from fundamental risk.We can go further and assume that there are no implementation costs,so that only noise trader risk remains.De Long et al.(1990a)show that noise trader risk is powerful enough,that even with this single form of risk,arbitrage can sometimes be limited.The sufficient conditions are similar to those above,with one important difference.Here arbitrage will be limited if:(i)arbitrageurs are risk averse and have short horizons and(ii)the noise trader risk is systematic.As before,condition (i)ensures that the mispricing cannot be wiped out by a single,large arbitrageur,while condition(ii)prevents a large number of small investors from exploiting the mispricing.The central contribution of Shleifer and Vishny (1997)is to point out the real world relevance of condition (i): the possibility of an early,forced liquidation means that many arbitrageurs effectively have short horizons. In the presence of certain implementation costs,condition (ii)may not even be necessary.If it is costly to learn about a mispricing,or the resources required it to exploit it are expensive,that may be enough to explain why a large number of different individuals do not intervene in an attempt to correct the mispricing It is also important to note that for particular types of noise trading,arbitrageurs may prefer to trade in the same direction as the noise traders,thereby exacerbating the mispricing,rather than against them.For example,De Long et al.(1990b)

1060 N. Barberis and R. Thaler mispricing, in particular, can be a tricky matter. It was once thought that if noise traders influenced stock prices to any substantial degree, their actions would quickly show up in the form of predictability in returns. Shiller (1984) and Summers (1986) demonstrate that this argument is completely erroneous, with Shiller (1984) calling it “one of the most remarkable errors in the history of economic thought”. They show that even if noise trader demand is so strong as to cause a large and persistent mispricing, it may generate so little predictability in returns as to be virtually undetectable. In contrast, then, to straightforward-sounding textbook arbitrage, real world arbitrage entails both costs and risks, which under some conditions will limit arbitrage and allow deviations from fundamental value to persist. To see what these conditions are, consider two cases. Suppose first that the mispriced security does not have a close substitute. By definition then, the arbitrageur is exposed to fundamental risk. In this case, sufficient conditions for arbitrage to be limited are (i) that arbitrageurs are risk averse and (ii) that the fundamental risk is systematic, in that it cannot be diversified by taking many such positions. Condition (i) ensures that the mispricing will not be wiped out by a single arbitrageur taking a large position in the mispriced security. Condition (ii) ensures that the mispricing will not be wiped out by a large number of investors each adding a small position in the mispriced security to their current holdings. The presence of noise trader risk or implementation costs will only limit arbitrage further. Even if a perfect substitute does exist, arbitrage can still be limited. The existence of the substitute security immunizes the arbitrageur from fundamental risk. We can go further and assume that there are no implementation costs, so that only noise trader risk remains. De Long et al. (1990a) show that noise trader risk is powerful enough, that even with this single form of risk, arbitrage can sometimes be limited. The sufficient conditions are similar to those above, with one important difference. Here arbitrage will be limited if: (i) arbitrageurs are risk averse and have short horizons and (ii) the noise trader risk is systematic. As before, condition (i) ensures that the mispricing cannot be wiped out by a single, large arbitrageur, while condition (ii) prevents a large number of small investors from exploiting the mispricing. The central contribution of Shleifer and Vishny (1997) is to point out the real world relevance of condition (i): the possibility of an early, forced liquidation means that many arbitrageurs effectively have short horizons. In the presence of certain implementation costs, condition (ii) may not even be necessary. If it is costly to learn about a mispricing, or the resources required it to exploit it are expensive, that may be enough to explain why a large number of different individuals do not intervene in an attempt to correct the mispricing. It is also important to note that for particular types of noise trading, arbitrageurs may prefer to trade in the same direction as the noise traders, thereby exacerbating the mispricing, rather than against them. For example, De Long et al. (1990b)

Ch.18:A Survey of Behavioral Finance 1061 consider an economy with positive feedback traders,who buy more of an asset this period if it performed well last period.If these noise traders push an asset's price above fundamental value,arbitrageurs do not sell or short the asset.Rather,they buy it,knowing that the earlier price rise will attract more feedback traders next period,leading to still higher prices,at which point the arbitrageurs can exit at a profit. So far,we have argued that it is not easy for arbitrageurs like hedge funds to exploit market inefficiencies.However,hedge funds are not the only market participants trying to take advantage of noise traders:firm managers also play this game.If a manager believes that investors are overvaluing his firm's shares,he can benefit the firm's existing shareholders by issuing extra shares at attractive prices.The extra supply this generates could potentially push prices back to fundamental value. Unfortunately,this game entails risks and costs for managers,just as it does for hedge funds.Issuing shares is an expensive process,both in terms of underwriting fees and time spent by company management.Moreover,the manager can rarely be sure that investors are overvaluing his firm's shares.If he issues shares,thinking that they are overvalued when in fact they are not,he incurs the costs of deviating from his target capital structure,without getting any benefits in return. 2.3.Evidence From the theoretical point of view,there is reason to believe that arbitrage is a risky process and therefore that it is only of limited effectiveness.But is there any evidence that arbitrage is limited?In principle,any example of persistent mispricing is immediate evidence of limited arbitrage:if arbitrage were not limited,the mispricing would quickly disappear.The problem is that while many pricing phenomena can be interpreted as deviations from fundamental value,it is only in a few cases that the presence of a mispricing can be established beyond any reasonable doubt.The reason for this is what Fama(1970)dubbed the"joint hypothesis problem".In order to claim that the price of a security differs from its properly discounted future cash flows,one needs a model of"proper"discounting.Any test of mispricing is therefore inevitably a joint test of mispricing and of a model of discount rates,making it difficult to provide definitive evidence of inefficiency. In spite of this difficulty,researchers have uncovered a number of financial market phenomena that are almost certainly mispricings,and persistent ones at that. These examples show that arbitrage is indeed limited,and also serve as interesting illustrations of the risks and costs described earlier. 2.3.1.Twin shares In 1907,Royal Dutch and Shell Transport,at the time completely independent companies,agreed to merge their interests on a 60:40 basis while remaining separate entities.Shares of Royal Dutch,which are primarily traded in the USA and in the

Ch. 18: A Survey of Behavioral Finance 1061 consider an economy with positive feedback traders, who buy more of an asset this period if it performed well last period. If these noise traders push an asset’s price above fundamental value, arbitrageurs do not sell or short the asset. Rather, they buy it, knowing that the earlier price rise will attract more feedback traders next period, leading to still higher prices, at which point the arbitrageurs can exit at a profit. So far, we have argued that it is not easy for arbitrageurs like hedge funds to exploit market inefficiencies. However, hedge funds are not the only market participants trying to take advantage of noise traders: firm managers also play this game. If a manager believes that investors are overvaluing his firm’s shares, he can benefit the firm’s existing shareholders by issuing extra shares at attractive prices. The extra supply this generates could potentially push prices back to fundamental value. Unfortunately, this game entails risks and costs for managers, just as it does for hedge funds. Issuing shares is an expensive process, both in terms of underwriting fees and time spent by company management. Moreover, the manager can rarely be sure that investors are overvaluing his firm’s shares. If he issues shares, thinking that they are overvalued when in fact they are not, he incurs the costs of deviating from his target capital structure, without getting any benefits in return. 2.3. Evidence From the theoretical point of view, there is reason to believe that arbitrage is a risky process and therefore that it is only of limited effectiveness. But is there any evidence that arbitrage is limited? In principle, any example of persistent mispricing is immediate evidence of limited arbitrage: if arbitrage were not limited, the mispricing would quickly disappear. The problem is that while many pricing phenomena can be interpreted as deviations from fundamental value, it is only in a few cases that the presence of a mispricing can be established beyond any reasonable doubt. The reason for this is what Fama (1970) dubbed the “joint hypothesis problem”. In order to claim that the price of a security differs from its properly discounted future cash flows, one needs a model of “proper” discounting. Any test of mispricing is therefore inevitably a joint test of mispricing and of a model of discount rates, making it difficult to provide definitive evidence of inefficiency. In spite of this difficulty, researchers have uncovered a number of financial market phenomena that are almost certainly mispricings, and persistent ones at that. These examples show that arbitrage is indeed limited, and also serve as interesting illustrations of the risks and costs described earlier. 2.3.1. Twin shares In 1907, Royal Dutch and Shell Transport, at the time completely independent companies, agreed to merge their interests on a 60:40 basis while remaining separate entities. Shares of Royal Dutch, which are primarily traded in the USA and in the