Previous research has established that the predictions made by game theory about strategic behavior in incomplete information games are quite sensitive to the assumptions made about the players’ infinite hierarchies of beliefs. We evaluate the severity of this robustness problem by characterizing conditions on the primitives of the model — the players’ hierarchies of beliefs — for the strategic behavior of a given Harsanyi type to be approximated by the strategic behavior of (a sequence of) perturbed types. This amounts to providing characterizations of the strategic topologies of Dekel, Fudenberg, and Morris (2006) in terms of beliefs. We apply our characterizations to a variety of questions concerning robustness to perturbations of higher-order beliefs, including genericity of common priors, and the connections between robustness of strategic behavior and the notion of common p-belief of Monderer and Samet (1989).
We study the robustness of interim correlated rationalizability to perturbations of higher-order beliefs. We introduce a new metric topology on the universal type space, called uniform weak topology, under which two types are close if they have similar first-order beliefs, attach similar probabilities to other players having similar first-order beliefs, and so on, where the degree of similarity is uniform over the levels of the belief hierarchy. This topology generalizes the now classic notion of proximity to common knowledge based on common p-beliefs (Monderer and Samet (1989)). We show that convergence in the uniform weak topology implies convergence in the uniform strategic topology (Dekel, Fudenberg, and Morris (2006)). Moreover, when the limit is a finite type, uniform-weak convergence is also a necessary condition for convergence in the strategic topology. Finally, we show that the set of finite types is nowhere dense under the uniform strategic topology. Thus, our results shed light on the connection between similarity of beliefs and similarity of behaviors in games.