Norms are social standards that prescribe what people ought to do. Litter can be reduced by antilittering norms, especially injunctive norms that specify what most others approve or disapprove of. In contrast, descriptive norms, which specify what most people do, seem not effective at reducing littering. Messages that explicitly command people not to litter are less effective than messages that appeal to social norms.This may be due to psychological reactance - the unpleasant emotional response people experience when someone is trying to restrict their freedom to engage in a behavior. Such threats frequently backfire. One reason norms might work is because people feel guilty if they don't follow them. Guilt can have a positive impact on people's behavior, including reducing litter. After a littering campaign, people said they would feel guilty if they littered.