"Agreeing to Disagree" Type Results under Ambiguity

In this paper we show that unlike in Bayesian frameworks asymmetric information does matter and can explain differences in common knowledge decisions due to ambiguous character of agents' private information. Agents share a common-but-not-necessarily-additive prior beliefs represented by capacities. It is shown that, if each agent's information partition is made up of unambiguous events in the sense of Nehring (1999, Mat. Soc. Sci. 38, 197-213), then it is impossible that the agents disagree on their commonly known decisions, whatever these decisions are : whether posterior beliefs or conditional expectations. Conversely, an agreement on conditional expectations, but not on posterior beliefs, implies that agents' private information must consist of Nehring-unambiguous events. The results obtained allow to attribute the existence of a speculative trade to the presence of agents' diverse and ambiguous information.