Efficient communication, common knowledge, and consensus

We study a model of pairwise communication in a finite population of Bayesian agents. We show that, in contrast with claims to the contrary in the existing literature, communication under a fair protocol may not lead to common knowledge of signals. We prove that commonly known signals are achieved if the individuals convey, in addition to their own message, the information about every individual’s most recent signal they are aware of. If the signal is a posterior probability about some event, common knowledge implies consensus.