The hypothesis that happy persons are more cooperative than sad persons has become a popular presumption in social and applied psychology. However, empirical evidence for this notion is less clear than often assumed. We argue that mood affects the process of decision making rather than (or in addition to) affecting the level of cooperation, increasing heuristic processing when persons feel good or secure, but leading to more systematic processing when persons feel sad or insecure. As a consequence, feeling states should moderate persons' reactions to heuristic cues, as for example the expected or perceived behaviour of others. Two experiments are reported varying feeling states, descriptive social norms, and the perceived behaviour of other group members in a chicken dilemma game. As expected, happy (Experiment 1) or secure participants (Experiment 2) showed shorter decision latencies and heuristically imitated others' behaviour in the chicken dilemma, whereas sad or insecure participants exhibited more systematic and rational behaviour, tending to defect when others' cooperation was high, but to increase their investment for the common when others' cooperation was low. No main effect of mood on cooperation occurred in either experiment.