Agents communicate; this is one of the defining characteristics of a multiagent system. In traditional linguistic analysis, the communication is taken to have a certain form (syntax), to carry a certain meaning (semantics), and to be influenced by various circumstances of the communication (pragmatics). As we shall see, a closer look at communication adds to the complexity of the story. We can distinguish between purely informational theories of communication and motivational ones. In informational communication, agents simply inform each other of different facts. The theories of belief change, introduced in Chapter 14, look at ways in which beliefs change in the face of new information—depending on whether the beliefs are logical or probabilistic, consistent with prior beliefs or not. In this chapter we broaden the discussion and consider motivational theories of communication, involving agents with individual motivations and possible courses of actions.

We divide the discussion into three parts. The first concerns cheap talk and describes a situation in which self-motivated agents can engage in costless communication before taking action. As we see, in some situations this talk influences future behavior, and in some it does not. Cheap talk can be viewed as “doing by talking”; in contrast, signaling games can be viewed as “talking by doing.” In signaling games an agent can take actions that, by virtue of the underlying incentives, communicate to the other agent something new.