We explore how emotion expression impacts emergence of cooperation in social dilemmas. Three experiments are described where participants play the prisoners dilemma with (computer) players that display emotion. Experiment 1 compares a cooperative player, whose displays reflect mutual cooperation, with a control player that shows no emotion. Experiment 2 compares a competitive player, whose displays reflect earning more points than the participant, and the control player. Experiment 3 compares the cooperative and competitive players. Results show that people: cooperate more with the cooperative than the control player; do not cooperate differently with the competitive and control players; and, cooperate more with the cooperative than the competitive player, when playing the latter first. We argue people infer, from emotion displays, the opponents propensity to cooperate by reversing the emotion appraisal process. Post-game surveys show that people interpret the displays according to appraisal variables consistently with predictions from appraisal theories of emotion.