Finite Order Implications of Common Priors in Infinite Models

Lipman [2003] shows that in a finite model, the common prior assumption has weak implications for finite orders of beliefs about beliefs. In particular, the only such implications are those stemming from the weaker assumption of a common support. To explore the role of the finite model assumption in generating this conclusion, this paper considers the finite order implications of common priors in a countable model. I show that in countable models, the common prior assumption also implies a tail consistency condition regarding beliefs. More specifically, I show that in a countable model, the finite order implications of the common prior assumption are the same as those stemming from the assumption that priors have a common support and have tail probabilities converging to zero at the same rate.