Prospect Theory Applications in Finance Nicholas Barberis Yale

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  • Prospect Theory Applications in Finance

    Nicholas BarberisYale University

    September 2011

    1

  • Overview

    in behavioral finance, we investigate whether certainfinancial phenomena are the result of less than fullyrational thinking

    Preferences

    prospect theory ambiguity aversion

    Beliefs

    representativeness, law of small numbers non-belief in the law of large numbers conservatism, belief perseverance, confirmation bias overconfidence

    in this note, we look at prospect theory applicationsin finance

    2

  • Overview, ctd.

    almost all models of financial markets assume thatinvestors evaluate risk according to expected utility

    but this framework has had trouble matching manyempirical facts

    can we make progress by replacing expected utilitywith a psychologically more realistic preference spec-ification?

    e.g. with prospect theory

    3

  • Prospect Theory, ctd.

    Four key features:

    the carriers of value are gains and losses, not finalwealth levels

    compare v(x) vs. U (W + x)

    v() has a kink at the origin captures a greater sensitivity to losses (even small

    losses) than to gains of the same magnitude

    loss aversion

    inferred from aversion to (110, 12;100, 12) v() is concave over gains, convex over losses

    inferred from (500, 1) (1000, 12) and (500, 1) (1000, 1

    2)

    5

  • Prospect Theory, ctd.

    transform probabilities with a weighting function ()that overweights low probabilities

    inferred from our simultaneous liking of lotteriesand insurance, e.g. (5, 1) (5000, 0.001) and(5, 1) (5000, 0.001)

    Note:

    transformed probabilities should not be thought of asbeliefs, but as decision weights

    6

  • Cumulative Prospect Theory

    proposed by Tversky and Kahneman (1992) applies the probability weighting function to the cu-

    mulative distribution function:

    (xm, pm; . . . ; x1, p1; x0, p0; x1, p1; . . . ; xn, pn),

    where xi < xj for i < j and x0 = 0, is assigned the value

    ni=m

    iv(xi)

    i =

    (pi + . . . + pn) (pi+1 + . . . + pn)(pm + . . . + pi) (pm + . . . + pi1) for

    0 i nm i V (W2) under multivariate Normality, the investors utility is

    F (W, 2W ), which, for fixed

    2W, is increasing in W

    investors choose portfolios on the mean-varianceefficient frontier

    market clearing tangency portfolio is the marketportfolio CAPM

    first proved by De Giorgi, Hens, and Levy (2003)

    15

  • The cross-section, ctd.

    now introduce a small, independent, positively skewedsecurity into the economy, return Rn

    in a representative agent economy with concave EUpreferences, the security would earn an average excessreturn of zero

    we find that, in an economy with CPT investors, thesecurity can earn a negative average excess return

    skewness itself is priced, in contrast to EU models,where only coskewness matters

    equilibrium involves heterogeneous holdings(for now, assume short-sale constraints)

    some investors hold the old market portfolio and alarge, undiversified position in the new security

    others hold the old market portfolio and no posi-tion at all in the new security

    heterogeneous holdings arise from non-unique globaloptima, not from heterogeneous preferences

    16

  • The cross-section, ctd.

    from before, the market return, excluding the newsecurity, is Normally distributed:

    RM N(M, 2M) give the new security a lottery-like binomial distri-

    bution:

    payoff (L, q; 0, 1 q)Rn ( L

    pn, q; 0, 1 q)

    think of L as large, q as small

    set values for preference parameters: = 0.88, = 2.25, =

    0.65

    the risk-free rate, Rf = 1.02, and the market stan-dard deviation, M = 0.15

    the skewed security payoff L = 10, q = 0.09

    search for a market risk premium M and a price of theskewed security pn for which there is a heterogeneousholdings equilibrium

    17

  • The cross-section, ctd.

    equilibrium condition V (RM) = 0 M = 0.075 i.e. a high equity premium

    equilibrium condition sup x>0V (RM + xRn) = 0 pn = 0.925, so that the skewed security earns a neg-ative excess return

    E(Rn) Rf = (0.09)(10)0.925

    1.02= 0.047

    Intuition:

    since it contributes skewness to the portfolios of someinvestors, it is valuable, and so earns a low averagereturn

    not surprising that a CPT investor likes a skewedportfolio

    more surprising that he likes a skewed security,even if it is small

    18

  • 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.1610

    8

    6

    4

    2

    0

    2

    4

    6

    8x 10

    3

    x

    utili

    ty

    FIGURE 3. A HETEROGENEOUS HOLDINGS EQUILIBRIUM. Notes: The figureshows the utility that an investor with cumulative prospect theory preferences derives fromadding a position in a positively skewed security to his current holdings of a Normallydistributed market portfolio. The skewed security is highly skewed. The variable x isthe fraction of wealth allocated to the skewed security relative to the fraction of wealthallocated to the market portfolio. The two lines correspond to different mean returns onthe skewed security.

    56

  • The cross-section, ctd.

    the skewed security only earns a negative excess returnif it is highly skewed

    e.g. for q = 0.15, there is no heterogeneous hold-ings equilibrium

    security cant contribute enough skewness to over-come lack of diversification

    for moderately skewed securities, there is a homoge-neous agent equilibrium

    here, the expected excess return is zero

    the result that a (highly) positively skewed securityearns a negative average excess return holds:

    even if there are many skewed securities

    even if short sales are allowed

    qualitatively, even if expected utility agents arepresent

    19

  • 0 0.05 0.1 0.15 0.2 0.250.2

    0.1

    0

    0.1

    x

    utili

    ty

    FIGURE 4. A HOMOGENEOUS HOLDINGS EQUILIBRIUM. Notes: The figureshows the utility that an investor with cumulative prospect theory preferences derivesfrom adding a position in a positively skewed security to his current holdings of a Nor-mally distributed market portfolio. The skewed security is only moderately skewed. Thevariable x is the fraction of wealth allocated to the skewed security relative to the fractionof wealth allocated to the market portfolio. The three lines correspond to different meanreturns on the skewed security.

    57

  • 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.1420

    15

    10

    5

    0

    5

    10

    15

    20

    q

    expe

    cted

    exc

    ess

    retu

    rn (

    perc

    ent)

    FIGURE 5. SKEWNESS AND EXPECTED RETURN. Notes: The figure shows theexpected return in excess of the risk-free rate earned by a small, independent, positivelyskewed security in an economy populated by cumulative prospect theory investors, plottedagainst a parameter of the securitys return distribution, q, which determines the securitysskewness. A low value of q corresponds to a high degree of skewness.

    58

  • The cross-section, ctd.

    Empirical evidence and applications

    several papers test the models basic prediction thatskewness is priced in the cross-section

    Zhang (2006) uses returns on stocks similar to stockX to predict the skewness of stock X

    Boyer, Mitton, Vorkink (2010) use a regressionmodel to predict future skewness

    Conrad, Dittmar, Ghysels (2010) use option pricesto infer the perceived (risk-neutral) distribution ofthe underlying stock

    all three studies find supportive evidence

    20

  • The cross-section, ctd.

    Empirical evidence and applications, ctd.

    low average return on IPOs Green and Hwang (2011) show that IPOs predicted

    to be more positively skewed have lower long-termreturns

    overpricing of out-of-the-money options Boyer and Vorkink (2011) find that stock options

    predicted to be more positively skewed have lowerreturns

    low average return on stocks with high idiosyncraticvolatility (Ang et al., 2006; Boyer, Mitton, Vorkink,2010)

    low average return on OTC stocks (Eraker and Ready,2011)

    diversification discount (Mitton and Vorkink, 2008) under-diversification

    Mitton and Vorkink (2010) find that undiversifiedindividuals hold stocks that are more positivelyskewed than the average stock

    21

  • The cross-section, ctd.

    Remarks:

    an example of how psychology can lead us to usefulnew predictions

    the model manages to capture both the high and lowrisk premia we observe

    skewness, or the lack of it, is key

    alternative framing assumptions? the pricing of skewness should follow even more

    directly under stock-level narrow framing

    alternative reference points? not clear that the reference point matters very

    much here

    22

  • Prospect theory applications

    [1]

    the cross-section of stock returns one-period models

    new prediction: the pricing of skewness

    probability weighting plays the most critical role

    [2]

    the aggregate stock market intertemporal representative agent models

    try to address the equity premium, volatility, pre-dictability, and non-participation puzzles

    loss aversion plays a key role; but probability weight-ing also matters

    [3]

    trading behavior multi-period models

    try to address the disposition effect and other trad-ing phenomena

    all aspects of prospect theory play a role

    23

  • The aggregate stock market

    can pr