Abstract:Collective efficacy-the belief that one's group is capable of affecting relevant aspects of its environment-has been highlighted as an important predictor of sustainable behavior. It increases people's collective action tendencies, and is important for fostering environmental behavioral change beyond self-efficacy beliefs. The current study addresses two primary goals. First, we tested whether the difficulty of a task increased collective efficacy, and thereby environmental intentions. Second, we explored how collective and self-efficacy in concert predict such intentions. In a combined field-and-survey study, 165 voluntary participants took part in a plastic reduction challenge that was pretested as easy, moderate, or difficult. After being confronted with the task, participants completed an online questionnaire in which, among other variables, specific and general self-efficacy, collective efficacy, and pro-environmental intentions were measured-both general and plastic-reduction specific. Results revealed that (a) collective efficacy was significantly stronger when task difficulty was moderate rather than easy or difficult; and (b) that through specific collective and self-efficacy perceptions, sustainable intentions were gauged-even when controlling for attitudes and social norms. These findings suggest that collective efficacy beliefs are particularly relevant for attaining environmental goals that are neither too easy nor too difficult, and could thus be valuable for communication and policy strategies.