Emotions displayed by others are pivotal ingredients of the decisions we make in social contexts. However, most of the research to date has focused on the subjective emotion of the decider rather than on the emotional expressions of the partners in the interaction. The present investigation was designed to explore how happy and angry facial expressions modulate cooperative responses in multi-round Trust Games. Our results show that happy partners generate higher levels of trust than angry partners even after repeated experience in a context in which emotional displays are not predictive of the partners' cooperation rates. This effect disappears once the social meaning of emotional displays is removed from the game. An additional study shows that participants are able to learn specific associations between discrete emotions and positive or negative cooperative tendencies, although they need more evidence when the associations counteract prior expectations. Overall, our results stress the reliability of discrete emotions as cues in interpersonal interactions and the resilience of the effect of these positive and negative cues in contexts in which they lack real predictive power.