Abstract:

This paper extends the concept of coalitional rationalizability of Ambrus(01) to incorporate sequential rationality in multi-stage games with observable actions and incomplete information. Agreements among players are implicit, it is assumed that players cannot communicate with each other during the game. They reflect a reasoning procedure which entails restricting strategies in a mutually advantegous way. They can be conditional on observed histories and players' types, which corresponds to allowing players to make agreements ex post and along the course of play. An agreement that is conditioned on a history is evaluated from the point of view of that history. This introduces a dynamic interaction among coalitional agreements with features of both backward and forward induction. Coalitional agreements iteratively define the set of extensive form coalitionally rationalizable strategies. This solution concept has a number of analogous properties with normal form coalitionally rationalizability. It is always nonempty. The set of outcomes consistent with it is a subset of the outcomes consistent with extensive form rationalizability, and it is robust to the order in which agreements are made. In games of perfect information extensive form coalitional rationalizability is outcome equivalent to extensive form rationalizability. Perfect coalition-proof Nash equilibria and renegotiation-proof Nash equilibria do not have to be contained in the solution set, even in two-player games, because those concepts do not imply forward induction reasoning. An alternative notion of extensive form coalitional rationalizability is also provided, assuming that coalitional agreements can only be made ex ante, but sequential individual rationality is maintained.