Beliefs and rationalizability in games with complementarities

We propose two characteristics of players' beliefs and study their role in shaping the set of rationalizable strategy profiles in games with incomplete information. The first characteristic, type-sensitivity, is related to how informative a player thinks his type is. The second characteristic, optimism, is related to how "favorable" a player expects the outcome of the game to be. The paper has two main results: the first result provides an upper bound on the size of the set of rationalizable strategy profiles, the second gives a lower bound on the change of location of this set. These bounds have explicit and relatively simple expressions that feature type-sensitivity, optimism, and properties of the payoffs. Our results generalize and clarify the well-known uniqueness result of global games (Carlsson and van Damme (1993)). They imply new uniqueness results and allow to study rationalizability in new environments. We provide applications to supermodular mechanism design (Mathevet (2010)) and non-Bayesian updating (Epstein (2006)).Complementarities, rationalizability, beliefs, type-sensitivity, optimism, global games, equilibrium uniqueness

Beliefs and rationalizability in games with complementarities

We propose two characteristics of players' beliefs and study their role in shaping the set of rationalizable strategy profiles in games with incomplete information. The first characteristic, type-sensitivity, is related to how informative a player thinks his type is. The second characteristic, optimism, is related to how "favorable" a player expects the outcome of the game to be. The paper has two main results: the first result provides an upper bound on the size of the set of rationalizable strategy profiles, the second gives a lower bound on the change of location of this set. These bounds have explicit and relatively simple expressions that feature type-sensitivity, optimism, and properties of the payoffs. Our results generalize and clarify the well-known uniqueness result of global games (Carlsson and van Damme (1993)). They imply new uniqueness results and allow to study rationalizability in new environments. We provide applications to supermodular mechanism design (Mathevet (2010)) and non-Bayesian updating (Epstein (2006))

Sentiments and rationalizability

Sentiments are characteristics of players' beliefs. I propose two notions of sentiments, confidence and optimism, and I study their role in shaping the set of rationalizable strategy profiles in (incomplete information) games with complementarities. Confidence is related to a player's perceived precision of information; optimism is the sentiment that the outcome of the game will be ``favorable.'' I prove two main results on how sentiments and payoffs interact to determine the size and location of the set of rationalizable profiles. The first result provides an explicit upper bound on the size of the set of rationalizable strategy profiles, relating complementarities and confidence; the second gives an explicit lower bound on the change of location, relating complementarities and optimism. I apply these results to four areas. In models of currency crisis, the results suggest that the most confident investors may drive financial markets. In models of empirical industrial organization, the paper provides a classification of the parameter values for which the model is identified. In non-Bayesian updating, the results clarify the strategic implications of certain biases. Finally, the results generalize and clarify the uniqueness result of global games.Rationalizable strategy profiles; complementarities; sentiments; confidence; optimism

Expectational coordination in a class of economic models: Strategic substitutabilities versus strategic complementarities

We consider an economic model that features: 1. a continuum of agents 2. an aggregate state of the world over which agents have an infinitesimal influence. We first propose a review, based on work by Jara (2007), of the connections between the eductive viewpoint that puts emphasis on Strongly Rational Expectations equilibrium and the standard game-theoretical rationalizability concepts. We explore the scope and limits of this connection depending on whether standard rationalizability versus point-rationalizability, or the local versus the global viewpoint, are concerned. In particular, we define and characterize the set of Point-Rationalizable States and prove its convexity. Also, we clarify the role of the heterogeneity of beliefs in general contexts of expectational coordination (see Evans and Guesnerie, 2005). Then, as in the case of strategic complementarities the study of some best response mapping is a key to the analysis, in the case of unambiguous strategic substitutabilities the study of some second iterate, and of the corresponding two-period cycles, allows to describe the point-rationalizable states. We provide application in microeconomic and macroeconomic contexts.expectational coordination ; rational expectations ; iterative expectational stability ; eductive stability ; strong rationality ; strategic complementarities ; strategic substitutabilities

Expectational coordination in simple economic contexts: concepts and analysis with emphasis on strategic substitutabilities.

We consider an economic model that features : 1. a continuum of agents 2. an aggregate state of the world over which agents have an infinitesimal influence. We first review the connections between the eductive viewpoint that puts emphasis for example on strongly rational expectations equilibrium and the standard game-theoretical rationalizability concepts. Besides the Cobweb tâtonnement outcomes, which mimic an eductive reasoning subject to homogenous expectations, we define, characterize (and prove the convexity of) the sets of rationalizable states and point-rationalizable states, which respectively incorporate heterogenous point-expectations and heterogenous stochastic expectations. In the case where our model displays strategic complementarities, we find unsurprisingly that all the eductive criteria under scrutiny support rather similar conclusions, particularly when the equilibrium is unique. With strategic susbstitutabilities, the success of expectational coordination, in the case where a unique equilibrium does exists, relates with the absence of cycles of order 2 of the Cobweb mapping : in this case, again, heterogenity of expectations does not matter. However, when cycles of order 2 do exist, our different criteria predict different set of outcomes, although all are tied with cycles of order 2. Under differentiability assumptions, the Poincaré-Hopf method leads to global results for strong rationality of equilibrium. At the local level, the different criteria under scrutiny can be adapted to the analysis of expectational coordination. They leads to the same stabilty conclusions, only when there are local strategic complementarities or strategic substitutabilities. However, so far as the analysis of local expectational coordination is concerned, it is argued and shown that the stochastic character of expectations can most often be forgotten.expectational coordination ; rational expectations ; iterative expectational stability ; eductive stability ; strong rationality ; strategic complementarities ; strategic substitutabilities

Strategic substitutabilities versus strategic complementarities: Towards a general theory of expectational coordination?

This paper contrasts the views of expectational coordination in a stylised economic model under two polar assumptions: Strategic Complementarities (StCo) dominate or on the contrary are dominated by Strategic Substitutabilities (StSu). Although in the StCo case, "uniqueness" often "buys" "eductive stability", the two issues are strikingly different in the second case. Furthermore if, in the first case, incomplete information often improves "expectational coordination", it may have the converse effect in the StSu case. It is finally argued that, in macroeconomic contexts, StSu often unambiguously dominate StCo, even in a large class of models with Keynesian features, and even in an aggregate framework that magnifies the StCo effects. The "remains" of StCo in general cases are discussed.Strategic Complementarities (StCo) ; Strategic Substitutabilities (StSu)

Dominance-Solvable Lattice Games

This paper derives sufficient and necessary conditions for dominance-solvability of so-called lattice games whose strategy sets have a lattice structure while they simultaneously belong to some metric space. The argument combines and extends Moulin's (1984) approach for nice games and Milgrom and Roberts' (1990) approach for supermodular games. The analysis covers - but is not restricted to - the case of actions being strategic complements as well as the case of actions being strategic substitutes. Applications are given for n-firm Cournot oligopolies, auctions with bidders who are optimistic - respectively pessimistic - with respect to an imperfectly known allocation rule, and Two-player Bayesian models of bank runs.

Epistemic Analysis of Strategic Games with Arbitrary Strategy Sets

We provide here an epistemic analysis of arbitrary strategic games based on the possibility correspondences. Such an analysis calls for the use of transfinite iterations of the corresponding operators. Our approach is based on Tarski's Fixpoint Theorem and applies both to the notions of rationalizability and the iterated elimination of strictly dominated strategies.Comment: 8 pages Proc. of the 11th Conference on Theoretical Aspects of Rationality and Knowledge (TARK XI), 2007. To appea