Handelshochschule Leipzig (HHL)
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Handelshochschule Leipzig (HHL)
Handelshochschule Leipzig (HHL) Decision Theory in the Presence of Uncertainty and Risk Pierfrancesco La Mura HHL Working Paper No. 68 Comments are welcome and may be sent to [email protected] © Copyright: 2005 Jede Form der Weitergabe und Vervielfältigung bedarf der Genehmigung des Herausgebers Decision theory in the presence of risk and uncertainty Pierfrancesco La Mura Department of Microeconomics and Information Systems Leipzig Graduate School of Management (HHL) April 2, 2005 1 Introduction John von Neumann (1903-1957) is widely regarded as a founding father of many fields, including game theory, decision theory, quantum mechanics and computer science. Two of his contributions are of special importance in our context. In 1932, von Neumann proposed a mathematical foundation for quantum mechanics, based on a calculus of projections in Hilbert spaces. In 1944, together with Oskar Morgenstern, he gave the first axiomatic foundation for the expected utility hypothesis (Bernoulli 1738). In both cases, the frameworks he contributed to establish are still at the core of the respective fields. In particular, the expected utility hypothesis is still the de facto foundation of fields such as finance and game theory. The von Neumann - Morgenstern axiomatization of expected utility was immediately greeted as simple and intuitively compelling. Yet, in the course of time, a number of empirical violations and paradoxes (Allais 1953, Ellsberg 1961, Rabin and Thaler 2001) came to cast doubt on the validity of the hypothesis as a foundation for the theory of rational decisions in conditions of risk and uncertainty. In economics and in the social sciences, the empirical and prescriptive shortcomings of the expected utility hypothesis are generally well known, but often tacitly accepted in view of the great tractability and usefulness of the corresponding mathematical framework. In fact, the hypothesis postulates that preferences can be represented by way of a utility functional which is linear in probabilities, and linearity makes expected utility representations particularly tractable in models and applications. Interestingly, the empirical violations and paradoxes which motivated the introduction of the quantum mechanical framework in physics bear a certain resemblance with some of the violations and paradoxes which came to challenge the status of expected utility in the social sciences. In physics, quantum mechanics was introduced as a tractable and empirically accurate mathematical framework in the presence of such violations. In economics, the importance of 1 accounting for violations of the expected utility hypothesis has long been recognized, but so far none of its numerous alternatives has emerged as dominant, for reasons which range from lack of mathematical tractability to the ad-hoc nature of some axiomatic proposals. Motivated by these considerations, we would like to introduce a decision-theoretic framework which accommodates the dominant paradoxes while retaining significant simplicity and tractability. As we shall see, this is obtained by weakening the expected utility hypothesis to its projective counterpart, in analogy with the quantum-mechanical generalization of classical probability theory. The structure of the paper is as follows. The next section briefly reviews the von Neumann-Morgenstern framework. Sections 3 and 4, respectively, present Allais’ and Ellsberg’s paradoxes. Section 5 introduces a mathematical framework for projective expected utility, and our main result. Section 6 contains a brief discussion. Sections 7 and 8 show how Allais’ and Ellsberg’s paradoxes, respectively, can be accommodated within the new framework. Section 9 discusses applications and extensions, then concludes. 2 von Neumann - Morgenstern expected utility Let S be a finite set of outcomes, and ∆ be the set of probability functions defined on S, taken to represent risky prospects (lotteries). Next, let % be a complete and transitive binary relation defined on ∆×∆, representing a decisionmaker’s preference ordering over lotteries. As usual, indifference of p, q ∈ ∆ is defined as [p % q and q % p] and denoted as p ∼ q, while strict preference of p over q is defined as [p % q and not q % p], and denoted by p  q. The preference ordering is assumed to satisfy the following two conditions. Axiom 1 For all p, q, r ∈ ∆ with p  q  r, there exist α, β ∈ (0, 1) such that αp + (1 − α)r  q  βp + (1 − β)r. Axiom 2 For all p, q, r ∈ ∆, p % q if, and only if, αp + (1 − α)r % αq + (1 − α)r for all α ∈ [0, 1]. A functional u : ∆ → R is said to represent % if, for all p, q ∈ ∆, p % q if and only if u(p) ≥ u(q). Theorem 3 Axioms 1 and 2 are jointly equivalent to the existence of a functional u : ∆ → R which represents % such that, for all p ∈ ∆, X u(p) = u(s)p(s). s∈S 2 3 The “double-slit paradox”of decision theory The following paradox is due to Allais (1953). First, please choose between: A: A chance of winning 4000 dollars with probability 0.2 (expected value 800 dollars) B: A chance of winning 3000 dollars with probability 0.25 (expected value 750 dollars). Next, please choose between: C: A chance of winning 4000 dollars with probability 0.8 (expected value 3200 dollars) B: A chance of winning 3000 dollars with certainty. If you chose A over B and D over C then you are in the modal class of respondents. The paradox lies in the observation that A and C are special cases of a two-stage lottery E which in the first stage either returns zero dollars with probably (1 − α) or, with probability α, it leads to a second stage where one gets 4000 dollars with probability 0.8 and zero otherwise. In particular, if α is set to 1 then E reduces to C, and if α is set to 0.25 it reduces to A. Similarly, B and D are special cases of a two-stage lottery F which again with probability (1 − α) returns zero, and with probability α continues to a second stage where one wins 3000 dollars with probability 1. Again, if α = 1 then F reduces to D, and if α = 0.25 it reduces to B. Then it is easy to see that the [A  B, D  C] pattern violates the Independence axiom, as E can be regarded as a lottery αp + (1 − α)r, and F as a lottery αq + (1 − α)r, where p and q represent lottery C and D respectively and r represents the lottery in which one gets zero dollars with certainty. Why should it matter what is the value of α? Yet, experimentally one finds that it does. Allais’ paradox bears a distinct resemblance with the double-slit paradox in physics. Imagine cutting two parallel slits in a sheet of cardboard, shining light through them, and observing the resulting pattern as particles going through the two slits scatter on a wall behind the cardboard barrier. Experimentally, one finds that when both slits are open, the overall scattering pattern is not the sum of the two scattering patterns produced when one slit is open and the other closed. The effect does not go away even if particles of light are shone one by one, and this is paradoxical: why should it matter to an individual particle which happens to go through the left slit, when determining where to scatter later on, with what probability it could have gone through the right slit instead? In a sense, each particle in the double-slit experiment behaves like a decision-maker who violates the Independence axiom in Allais’ experiment. 3 4 Ellsberg’s paradox Another disturbing violation of the expected utility hypothesis was pointed out by Ellsberg (1961). Suppose that a box contains 300 balls of three possible colors: red (R), green (G), and blue (B). You win if you guess which color will be drawn. Do you prefer to bet on red or on green? Many respondents choose red, on grounds that the probability of drawing a red ball is known (1/3), while the only information on the probability of drawing a green ball is that it is between 0 and 2/3. Now suppose that you win if you guess which color will not be drawn. Do you prefer to bet that red will not be drawn (R) or that green will not be drawn (G)? Again many respondents prefer to bet on R, as the probability is known (2/3) while the probability of G is only known to be between 1 and 1/3. The pattern [R  G, R  G] is incompatible with von Neumann - Morgenstern expected utility, which only deals with known probabilities, and is also incompatible with the Savage (1954) formulation of expected utility with subjective probability as it violates its Sure Thing axiom. Hence, the paradox suggests that subjective and objective uncertainty (risk) should be handled as distinct notions. 5 Projective expected utility Let S+ be the positive orthant of the unit sphere in Rn , where n is the cardinality of the set of relevant outcomes S := {s1 , s2 , ..., sn } . Then von Neumann - Morgenstern lotteries, regarded as elements of the unit simplex, are in oneto-one correspondence with elements of S+ , which can therefore be interpreted as prospects in which the probabilities are fully known. Observe that, while the projections of elements of the unit simplex (and hence, L1 unit vectors) on the basis vectors can be naturally associated with probabilities, if we choose to model von Neumann - Morgenstern lotteries as elements of the unit sphere (and hence, as unit vectors in L2 ) probabilities are naturally associated with squared projections. The advantage of such move is that L2 is the only Lp space which is also a Hilbert space, and the special properties of Hilbert space, and in particular its associated calculus of projections, shall yield tractability to the representation. We call such prospects, whereas the only uncertainty is objective, pure lotteries, and reserve the name mixed lotteries for convex combinations (mixtures) in Rn , which we interpret as situations of subjective uncertainty about the actual pure lottery faced. Let M be the convex closure of S+ in Rn . Observe that pure lotteries cannot be obtained as non-trivial mixtures, and that is why we refer to them as pure. Next, let hx|yi denote the usual inner product in Rn . Axiom 4 (Born’s Rule) There exists an orthonormal basis (z1 , ..., zn ) such that, for all x ∈ M and all si ∈ S, 2 psi (x) = hx|zi i . 4 Axiom 4 requires that there exist an interpretation of elements of M in terms of uncertainty and risk as perceived by the decision-maker. In the von Neumann - Morgenstern treatment, Axiom 4 is tacitly assumed to hold with respect to the natural basis in Rn . By contrast, the preferred basis postulated in Axiom 4 is allowed to vary across different decision-makers, capturing the idea that the relevant dimensions of risk and uncertainty may be perceived differently by different subjects, whereas the natural basis represents those dimensions as perceived by the modeler or an external observer. In our context, orthogonality captures the idea that two events or outcomes are mutually exclusive (for one event to have probability one, the other must have probability zero). The preferred basis captures which, among all the possible ways to partition the relevant uncertainty into a set of mutually exclusive events or outcomes, leads to a meaningful set of payoff-relevant outcomes to which well-defined utility values can be assigned. Axiom 4 postulates that each lottery is evaluated by the decision maker solely by the risk and uncertainty that it induces on the payoff-relevant dimensions. Once an orthonormal basis is given, each mixed lottery x P uniquely corresponds to a function p(x) : S → [0, 1]. If x is a pure lottery, then s∈S ps (x) = 1 and p(x) can therefore be regarded 2 as a probability function. For a general mixed lottery x ∈ M , 1/ kxk can be interpreted as a measure of uncertainty, and qsi (x) := 1 2 hx|zi i ||x||2 as subjective probability. We interpret the vector p(x) as the degree of belief assigned to each outcome by lottery x. Let B be the set of all p(x), for x ∈ M, and let % be a complete and transitive preference ordering defined on B ×B. We postulate the following two axioms, which mirror the ones in the von Neumann - Morgenstern treatment. Axiom 5 (Continuity) For all x, y, z ∈ M with p(x)  p(y)  p(z), there exist α, β ∈ (0, 1) such that αp(x) + (1 − α)p(z)  p(y)  βp(x) + (1 − β)p(z). Axiom 6 (Independence) For all x, y, z ∈ M , p(x) % p(y) if, and only if, αp(x) + (1 − α)p(z) % ap(y) + (1 − α)p(z) for all α ∈ [0, 1]. Observe that the two axioms impose conditions solely on beliefs, and not on the underlying lotteries. In particular, note that a convex combination αp(x) + (1 − α)p(y), where x and y are pure lotteries, also corresponds to a pure lottery, since the convex combination of two probability functions remains a probability function. Hence, the type of mixing that Axioms 5 and 6 are concerned with is objective, while subjective mixing is captured by convex combinations of the underlying lotteries, such as αx + (1 − α)y, which is generally not a pure lottery even when both x and y are. 5 Theorem 7 Axioms 4-6 are jointly equivalent to the existence of a symmetric matrix U such that u(x) := x0 U x for all x ∈ M represents %. Proof. Assume that Axiom 4 holds with respect to a given orthonormal basis (z1 , , zn ). By the von Neumann - Morgenstern result (which applies to any convex mixture set, such as B) Axioms 5 and 6 are jointly equivalent to the existence of a functional u which represents the ordering and is linear in p, i.e. u(x) = n X u(si )psi (x) = i=1 n X 2 u(si ) hx|zi i , i=1 where the second equality is by definition of p as squared inner product with respect to the preferred basis. The above can be equivalently written, in matrix form, as u(x) = x0 P 0 DP x = x0 U x, where D is the diagonal matrix with the payoffs on the main diagonal, P is the projection matrix associated to (z1 , , zn ), and U := P 0 DP is symmetric. Conversely, by the Spectral Decomposition theorem, for any symmetric matrix U there exist a diagonal matrix D and a projection matrix P such that x0 U x = x0 P 0 DP x for all x ∈ M. But this is just expected utility with respect to the orthonormal basis defined by P . Hence, the three axioms are jointly equivalent to the existence of a symmetric matrix U such that u(x) := x0 U x represents the preference ordering. 6 Properties of the representation The representation introduced in the previous section generalizes von NeumannMorgenstern expected utility in three directions. First, subject-specific uncertainty (from mixed states) and objective uncertainty (from pure states) are treated as distinct notions. Second, within this class of preferences both Allais’ and Ellsberg’s paradoxes are accommodated. Finally, the construction easily 2 extends to the complex unit ball, provided that hx|yi is replaced by | hx|yi |2 in the definition of p, in which case Theorem [??] holds with respect to a Hermitian (rather than symmetric) payoff matrix U , and the result also provides axiomatic foundations for decisions involving quantum uncertainty. Let ei , ej be the degenerate lotteries assigning probability 1 to outcomes si and sj , respectively, and let ei,j be the lottery assigning probability 1/2 to each of the two states. Observe that, for any two distinct si and sj , 1 1 Uij = u(eij ) − ( u(ei ) + u(ej )). 2 2 6 It follows that the off-diagonal entry Uij in the payoff matrix can be interpreted as the discount, or premium, attached to an equiprobable combination of the two outcomes with respect to its expected utility base-line, and hence as a measure of risk aversion along the specific dimension of risk involving outcomes si and sj . Observe that the functional in Theorem 7 is quadratic in x, but linear in p. If U is diagonal, then its eigenvalues coincide with the diagonal elements. In von Neumann - Morgenstern expected utility, those eigenvalues contain all the relevant information about the decision-maker’s risk attitudes. In our framework, risk attitudes are jointly captured by both the diagonal and non-diagonal elements of U . Furthermore, in our setting attitudes towards uncertainty are captured by the concavity or convexity (in x) of the quadratic form x0 U x, and therefore, ultimately, by the definiteness condition of U and the sign of its eigenvalues. Convexity corresponds to the case of all positive eigenvalues, and captures the idea that risk is ceteris paribus preferred to uncertainty, while concavity captures the opposite idea. 7 Example: objective uncertainty Figure [??] presents several examples of indifference maps on pure lotteries which can be obtained within our class of preferences for different choices of U . The first pattern (parallel straight lines) characterizes von Neumann - Morgenstern expected utility. Within our class of representations, it corresponds to the special case of a diagonal payoff matrix U. All other patterns are impossible within von Neumann - Morgenstern expected utility. The representation is sufficiently general to accommodate Allais’ paradox from section [??]. In the context of the example of section [??], let {s1 , s2 , s3 } be the states in which 4000, 3000, and 0 dollars are won, respectively. To accommodate Allais’ paradox, assume a slight aversion to the risk of obtaining no gain (s3 ): s U= 1 s2 s3 s1 13 0 −1 s2 s3 0 −1 . 10 −1 −1 0 Let the four lotteries A, B, C, D be defined, respectively, as unit vectors in S+ : √ √ 0.2 0.8 √0 a = √0 ; b = √0.25 ; c = √0 ; d = 0.8 0.75 0.2 the following 0 1 . 0 Then lottery A is preferred to B, while D is preferred to C, as u(a) = a0 U a = 1.8, 7 8 u(b) = b0 U b = 1.634, u(c) = c0 U c = 9.6, u(d) = d0 U d = 10. 8 Example: subjective uncertainty In the Ellsberg puzzle, suppose that either all the non-red balls are green (R = 100, G = 200, B = 0), or they are all blue (R = 100, G = 0, B = 200), with equal subjective probability. Further, suppose that there are just two payoff-relevant outcomes, Win and Lose. Then the following specification of the payoff matrix accommodates the paradox: U = W in Lose W in Lose 1 0 . 0 0 In fact, let µ p ¶ µ p ¶ 1/3 2/3 p p r = , r = be the lotteries representing R and R, 2/3 µ ¶1/3 µ ¶ 1 0 respectively, and let w = ,l= be the lotteries corresponding to a 0 1 9 sure win and a sure loss, respectively. Further, let g := 21 r + 12 l and g := 12 w + 21 r be the lotteries representing G and G, respectively. Then u(r) = r0 U r = 1/3, u(r) = r0 U r = 2/3, u(g) = g 0 U g = 1/6 u(g) = g 0 U g ≈ 0.62201, and the paradox is accommodated. Conversely, if the payoff matrix is given by U = W in Lose W in Lose 0 0 , 0 −1 the opposite pattern emerges, and subjective uncertainty is preferred to risk. 9 Multi-agent decisions and equilibrium Within the class of preferences characterized by Theorem 7, is it still true that every finite game has a Nash equilibrium? If the payoff matrix U is diagonal we are in the classical case, so we know that any finite game has an equilibrium, which moreover only involves objective risk (in our terms, this type of equilibrium should be referred to as “pure”, as it involves no subjective uncertainty). For the general case, consider that u(p) is still continuous and linear in p ∈ B, and therefore the main steps in Nash’ proof (proving that the best response correspondence is non-empty, convex-valued and upper-hemicontinuous in order to apply Kakutani’s fixed-point theorem) still apply in our case. Then any finite game has an equilibrium even within this larger class of preferences, even though the equilibrium may not be pure (in our sense): in general, an equilibrium will rest on a combination of objective randomization and subjective uncertainty about other players’ decisions. 10 Applications and extensions What epistemic conditions are implicit in our construction? What are good measures of risk and uncertainty? Can we increase the empirical accuracy of the CAPM? 11 References Maurice Allais (1953), “Le Comportement de l’Homme Rationnel devant le Risque: Critique des postulats et axiomes de l’École Americaine”. Econometrica 21, 503-546. 10 Daniel Bernoulli (1738), “Specimen theoriae novae de mensura sortis”. In Commentarii Academiae Scientiarum Imperialis Petropolitanae 5, 175-192. Translated as “Exposition of a new theory of the measurement of risk”. Econometrica 22 (1954), 123-136. Daniel Ellsberg (1961), “Risk, Ambiguity and the Savage Axioms”. Quarterly Journal of Economics 75, 643-669. John von Neumann (1932), Mathematische Grundlagen der Quantenmechanic. Berlin, Springer. Translated as Mathematical foundations of quantum mechanics, 1955, Princeton University Press, Princeton NJ. John von Neumann and Oskar Morgenstern (1944), Theory of Games and Economic Behavior. Princeton University Press, Princeton NJ. Matthew Rabin and Richard H. Thaler (2001), “Anomalies: Risk Aversion”. Journal of Economic Perspectives 15, 219-232. Leonard J. Savage (1954), The Foundations of Statistics. Wiley, New York, NY. 11 Handelshochschule Leipzig (HHL) Arbeitspapiere Die Arbeitspapiere der Handelshochschule Leipzig (HHL) erscheinen unregelmäßigen Abständen. Bisher sind folgende Arbeiten erschienen: Nr. 1 Prof. Dr. Dr. h.c. 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Uwe Vielmeyer (2002), 29 Seiten Vom Shareholder Value zum Stakeholder Value? – Möglichkeiten und Grenzen der Messung von stakeholderbezogenen Wertbeiträgen Nr. 53 Carsten Reimund (2002), 34 Seiten Internal Capital Markets, Bank Borrowing and Investment: Evidence from German Corporate Groups Nr. 54 Dr. Peter Kesting (2002), 21 Seiten Ansätze zur Erklärung des Prozesses der Formulierung von Entscheidungsprozessen Nr. 55 Prof. Dr. Wilhelm Althammer / Susanne Dröge (2002), 27 Seiten International Trade and the Environment: The Real Conflicts. - PdfDokument Nr. 56 Prof. Dr. Bernhard Schwetzler / Maik Piehler (2002), 35 Seiten Unternehmensbewertung bei Wachstum, Risiko und Besteuerung – Anmerkungen zum „Steuerparadoxon“ Nr. 57 Prof. Dr. Hagen Lindstädt (2002), 12, 5 Seiten Das modifizierte Hurwicz-Kriterium für untere und obere Wahrscheinlichkeiten - ein Spezialfall des Choquet-Erwartungsnutzens Nr. 58 Karsten Winkler (2003), 25 Seiten Getting Started with DIAsDEM Workbench 2.0: A Case-Based Tutorial Nr. 59 Karsten Winkler (2003), 31 Seiten Wettbewerbsinformationssysteme: Begriff, Anforderungen, Herausforderungen Nr. 60 Prof. Dr. Bernhard Schwetzler / Carsten Reimund (2003), 31 Seiten Conglomerate Discount and Cash Distortion: New Evidence from Germany Nr. 61 Prof. Pierfrancesco La Mura (2003), [10] Seiten Correlated Equilibria of Classical Strategic Games with Quantum Signals Nr. 62 Prof. Manfred Kirchgeorg (2003), 26 Seiten Markenpolitik für Natur- und Umweltschutzorganisationen Nr. 63 Stefan Wriggers (2004), 32, XXV Seiten Kritische Würdigung der Means-End-Theorie im Rahmen einer Anwendung auf M-Commerce-Dienste Nr. 64 Prof. Pierfrancesco La Mura / Matthias Herfert (2004), 18 Seiten Estimation of Consumer Preferences via Ordinal Decision-Theoretic Entropy Nr. 65 Prof. Bernhard Schwetzler (2004), 31 Seiten Mittelverwendungsannahme, Bewertungsmodell und Unternehmenswertung bei Rückstellungen Nr. 66 Prof. Manfred Kirchgeorg / Lars Fiedler (2004), 47 Seiten Clustermonitoring als Kontroll- und Steuerungsinstrument für Clusterentwicklungsprozesse - empirische Analysen von Industrieclustern in Ostdeutschland Nr. 67 Prof. Dr. Manfred Kirchgeorg / Christiane Springer (2005), 39 , XX Seiten UNIPLAN LiveTrends 2004/2005 : Effizienz und Effektivität in der Live Communication ; Eine Analyse auf Grundlage einer branchenübergreifenden Befragung von Marketingentscheidern in Deutschland Nr. 68 Prof. Pierfrancesco La Mura (2005), Decision Theory in the Presence of Uncertainty and Risk (Erscheint demnächst)