Promises and Endogenous Reneging Costs∗

We present a novel theoretical mechanism that explains the capacity for non-enforceable communication about future actions to improve efficiency. We explore a two-player partnership game where, before choosing a level of effort to exert on a joint project, each player makes a cheap talk promise to their partner about their own future effort. We allow agents to incur a psychological cost of reneging on their promises. We demonstrate a strong tendency for evolutionary processes to select agents who incur intermediate costs of reneging, and show that these intermediate costs induce second-best optimal outcomes.