We develop a theoretical model under which “genuine” or “convincing” smiling is a costly signal that has evolved to induce cooperation in situations requiring mutual trust. Prior to a trust interaction involving a decision by a sender to send money to a recipient, the recipient can emit a signal to induce the sender to trust them. The signal takes the form of a smile that may be perceived as more or less convincing, and that can be made more convincing with the investment of greater effort. Individuals differ in their degree of altruism and in their tendency to display reciprocity. The model generates three testable predictions. First, the perceived quality of the recipient’s smile is increasing in the size of the stake. Secondly, the amount sent by the sender is increasing in the perceived quality of the recipient’s smile. Thirdly, the expected gain to senders from sending money to the recipient is increasing in the perceived quality of the recipient’s smile.