Why do people engage in collective actions, such as demonstrations? We suggest that intentions to engage in protest activities come from the perception that the action is an efficient way to affect policy but is also dependent upon the level of others' engagement. Specifically, lower support should spur intentions to engage if the individual believes that the collective act is an efficient means to bring about social change. In two experiments, manipulating social support, efficacy increased intentions to participate in collective actions but mainly for participants with low social support (Experiments 1 and 2). In Experiment 3, manipulating efficacy, high social support was related to decreased intentions to engage. However, in this study, social support did not interact with efficacy.