Subjective Beliefs and Ex-Ante Agreeable Trade

We find a necessary and sufficient condition for ex-ante trade when agents are non-expected utility maximizers. The condition is that they share subjective beliefs. Our result holds for a class of convex preferences that contains many functional forms used in applications. In a special case of expected utility, the condition becomes exactly the common prior assumption. It can also be articulated in the language of other functional forms, confirming results existing in the literature, generating new results, and providing a useful tool for applications. Another contribution of this paper is a characterization of a general definition of beliefs for convex preferences. We show that this definition can be characterized in terms of market behavior. Moreover, it coincides with the usual one for an important class of convex invariant biseparable preferences.