Ulrich Berger

Fictitious Play in 2£n Games

It is known that every continuous time fictitious play process approaches equilibrium in every nondegenerate 2x2 and 2x3 game, and it has been conjectured that convergence to equilibrium holds generally for 2xn games. We give a simple geometric proof of this.

Brown's original fictitious play

Abstract What modern game theorists describe as “fictitious play” is not the learning process George W. Brown defined in his 1951 paper. Browns original version differs in a subtle detail, namely the order of belief updating. In this note we revive Browns original fictitious play process and demonstrate that this seemingly innocent detail allows for an extremely simple and intuitive proof of convergence in an interesting and large class of games: nondegenerate ordinal potential games.

Two more classes of games with the continuous-time fictitious play property

Fictitious Play is the oldest and most studied learning process for games. Since the already classical result for zero-sum games, convergence of beliefs to the set of Nash equilibria has been established for several classes of games, including weighted potential games, supermodular games with diminishing returns, and 3×3 supermodular games. Extending these results, we establish convergence of Continuous-time Fictitious Play for ordinal potential games and quasi-supermodular games with diminishing returns. As a by-product we obtain convergence for 3×m and 4×4 quasi-supermodular games.

Learning in games with strategic complementarities revisited

Fictitious play is a classical learning process for games, and games with strategic complementarities are an important class including many economic applications. Knowledge about convergence properties of fictitious play in this class of games is scarce, however. Beyond games with a unique equilibrium, global convergence has only been claimed for games with diminishing returns [V. Krishna, Learning in games with strategic complementarities, HBS Working Paper 92-073, Harvard University, 1992]. This result remained unpublished, and it relies on a specific tie-breaking rule. Here we prove an extension of it by showing that the ordinal version of strategic complementarities suffices. The proof does not rely on tie-breaking rules and provides some intuition for the result.

Best response dynamics for role games

Abstract. In a role game, players can condition their strategies on their player position in the base game. If the base game is strategically equivalent to a zero-sum game, the set of Nash equilibria of the role game is globally asymptotically stable under the best response dynamics. If the base game is 2 ×2, then in the cyclic case the set of role game equilibria is a continuum. We identify a single equilibrium in this continuum that attracts all best response paths outside the continuum.

On the stability of cooperation under indirect reciprocity with first-order information ☆

Indirect reciprocity describes a class of reputation-based mechanisms which may explain the prevalence of cooperation in large groups where partners meet only once. The first model for which this has been demonstrated was the image scoring mechanism. But analytical work on the simplest possible case, the binary scoring model, has shown that even small errors in implementation destabilize any cooperative regime. It has thus been claimed that for indirect reciprocity to stabilize cooperation, assessments of reputation must be based on higher-order information. Is indirect reciprocity relying on first-order information doomed to fail? We use a simple analytical model of image scoring to show that this need not be the case. Indeed, in the general image scoring model the introduction of implementation errors has just the opposite effect as in the binary scoring model: it may stabilize instead of destabilize cooperation.

A Generalized Model Of Best Response Adaptation

We present a generalized model of myopic best response adaptation in large populations. In asymmetric conflicts, individuals can be in the role of the row player or the column player. The idea that an individuals role need not be fixed is introduced explicitly in our model by a process of role switching. The best response dynamics, the symmetrized best response dynamics, and the continuous time fictitious play process are included as special cases. We show that the set of Nash equilibria is attracting for zero-sum games. Moreover, for any base game, convergence to a Nash equilibrium implies convergence to a Nash equilibrium on the Wright manifold in the role game.

Simple scaling of cooperation in donor-recipient games

We present a simple argument which proves a general version of the scaling phenomenon recently observed in donor-recipient games by Tanimoto [Tanimoto, J., 2009. A simple scaling of the effectiveness of supporting mutual cooperation in donor-recipient games by various reciprocity mechanisms. BioSystems 96, 29-34].

Non-algebraic Convergence Proofs for Continuous-Time Fictitious Play

In this technical note we use insights from the theory of projective geometry to provide novel and non-algebraic proofs of convergence of continuous-time fictitious play for a class of games. As a corollary we obtain a kind of equilibrium selection result, whereby continuous-time fictitious play converges to a particular equilibrium contained in a continuum of equivalent equilibria for symmetric 4×4 zero-sum games.

Co-action equilibrium fails to predict choices in mixed-strategy settings

Social projection is the tendency to project one’s own characteristics onto others. This phenomenon can potentially explain cooperation in prisoner’s dilemma experiments and other social dilemmas. The social projection hypothesis has recently been formalized for symmetric games as co-action equilibrium and for general games as consistent evidential equilibrium. These concepts have been proposed to predict choice behavior in experimental one-shot games. We test the predictions of the co-action equilibrium concept in a simple binary minimizer game experiment. We find no evidence of social projection.