Honest signalling of cooperative intentions

Trust can transform conflicting interests into cooperation. But how can individuals know when to trust others? Here, I develop the theory that reputation building may signal cooperative intent, or 'trustworthiness'. I model a simple representation of this theory in which individuals (1) optionally invest in a reputation by performing costly helpful behaviour ('signalling'); (2) optionally use others' reputations when choosing a partner; and (3) optionally cooperate with that partner. In evolutionary simulations, high levels of reputation building; of choosing partners based on reputation; and of cooperation within partnerships emerged. Costly helping behaviour evolved into an honest signal of trustworthiness when it was adaptive for cooperators, relative to defectors, to invest in the long-term benefits of a reputation for helping. I show using game theory that this occurs when cooperators gain larger marginal benefits from investing in signalling than do defectors. This happens without the usual costly signalling assumption that individuals are of two 'types' which differ in quality. Signalling of trustworthiness may help explain phenomena such as philanthropy, pro-sociality, collective action, punishment, and advertising in humans and may be particularly applicable to courtship in other animals.