Finding ways to encourage collective action in contexts where only a minority adopts the desired behaviour is central to solving many of today's global environmental problems. We study how correcting people's beliefs about social norms and behavioural trends encourages collective action in a setting where the desired behaviour is not yet prevalent. In a field experiment, we test whether low sign-up rates for a municipal recycling program in urban Peru can be increased by providing information (1) that most people regard participation in the program as important, i.e., on the “injunctive norm," (2) on an increasing recent trend in sign-up rates. We find that correcting inaccurate beliefs increases sign-up decisions significantly among people who either substantially underestimate the injunctive norm or who underestimate the positive trend. We further find that the effects increase in the level of underestimation. This evidence demonstrates that belief updating can be used effectively to encourage collective action where it is currently weak.