Paolo Ghirardato

Differentiating ambiguity and ambiguity attitude

Abstract The objective of this paper is to show how ambiguity, and a decision maker (DM)s response to it, can be modelled formally in the context of a general decision model. We introduce a relation derived from the DMs preferences, called “unambiguous preference”, and show that it can be represented by a set of probabilities. We provide such set with a simple differential characterization, and argue that it is a behavioral representation of the “ambiguity” that the DM may perceive. Given such revealed ambiguity, we provide a representation of ambiguity attitudes. We also characterize axiomatically a special case of our decision model, the “ α -maxmin” expected utility model.

Risk, Ambiguity, and the Separation of Utility and Beliefs

We introduce a general model of static choice under uncertainty, arguably the weakest model achieving a separation of cardinal utility and a unique representation of beliefs. Most of the nonexpected utility models existing in the literature are special cases of it. Such separation is motivated by the view that tastes are constant, whereas beliefs change with new information. The model has a simple and natural axiomatization.Elsewhere (forthcoming), we show that it can be very helpful in the characterization of a notion of ambiguity aversion, as separating utility and beliefs allows us to identify and remove aspects of risk attitude from the decision makers behavior. Here we show that the model allows us to generalize several results on the characterization of risk aversion in betting behavior. These generalizations are of independent interest, as they show that some traditional results for subjective expected utility preferences can be formulated only in terms of binary acts.

Revealed Ambiguity and Its Consequences: Updating

We study the updating of beliefs under ambiguity for invariant biseparable preferences. In particular, we show that a natural form of dynamic consistency characterizes the Bayesian updating of these beliefs.

Certainty Independence and the Separation of Utility and Beliefs

Economists often operate under an implicit assumption that the tastes of a decision maker are constant, while his beliefs change with the availability of new information. It is therefore customary to seek representations of preferences which cleanly separate the taste component, called ‘utility,’ from the beliefs component. We show that a complete separation of utility from the other components of the representation is possible only if the decision maker’s preferences satisfy a mild but not completely innocuous condition, called ‘certainty independence.’ We prove that the preferences that obtain such separation are a subset of the biseparable preferences.nonatomic probability measures, we extend some of these results to the case of individuals with decreasing marginal evaluations.

Ambiguity in the Small and in the Large

This paper considers local and global multiple-prior representations of ambiguity for preferences that are (i) monotonic, (ii) Bernoullian, i.e. admit an affine utility representation when restricted to constant acts, and (iii) locally Lipschitz continuous. We do not require either Certainty Independence or Uncertainty Aversion. We show that the set of priors identified by Ghirardato, Maccheroni, and Marinacci (2004)’s ‘unambiguous preference’ relation can be characterized as a union of Clarke differentials. We then introduce a behavioral notion of ‘locally better deviation’ at an act, and show that it characterizes the Clarke differential of the preference representation at that act. These results suggest that the priors identified by these preference statements are directly related to (local) optimizing behavior.

Notes, Comments, and Letters to the EditorChoquet Rationality☆

Consider a decision problem under uncertainty for a decision maker with known (utility) payoffs over prizes. We say that an act is Choquet (Shafer, Bernoulli) rational if for some capacity (belief function, probability) over the set of states, it maximizes her “expected” utility. We show that an act may be Choquet rational without being Bernoulli rational, but it is Choquet rational if and only if it is Shafer rational. Journal of Economic Literature Classification Numbers: C72, D81.

The Impossibility Of Compromise: Some Uniqueness Properties Of Expected Utility Preferences

Summary. We focus on the following uniqueness property of expected utility preferences: Agreement of two preferences on one interior indifference class implies their equality. We show that, besides expected utility preferences under (objective) risk, this uniqueness property holds for subjective expected utility preferences in Anscombe-Aumanns (partially subjective) and Savages (fully subjective) settings, while it does not hold for subjective expected utility preferences in settings without rich state spaces. Indeed, when it holds the uniqueness property is even stronger than described above, as it needs only agreement on binary acts. The extension of the uniqueness property to the subjective case is possible because beliefs in the mentioned settings are shown to satisfy an analogous property: If two decision makers agree on a likelihood indifference class, they must have identical beliefs.