Ambiguous Persuasion

We study a persuasion game à la Kamenica and Gentzkow (2011) where players are ambiguity averse with maxmin expected utility (Gilboa and Schmeidler, 1989). With no prior ambiguity, a Sender may choose to use ambiguous communication devices. Our main result characterizes the value of optimal ambiguous persuasion, which is often higher than what is feasible under Bayesian persuasion. We characterize posteriors that are potentially plausible when they are generated by ambiguous devices. One way to construct an optimal ambiguous communication device is by using synonyms, messages that lead to the same posteriors, in which Sender can hedge himself against ambiguity while inducing actions from Receiver that would not be possible under standard Bayesian persuasion. We also show that the use of synonyms are a necessary property of optimal and beneficial ambiguous persuasion. We consider two applications, including the well-known uniform-quadratic example. Our analysis provides a justification for how ambiguity may emerge endogenously in persuasion.