Although littering is a significant social problem, there is only a relatively small amount of quantitative research on it; there is little systematic theory concerning the variables which influence it. This study presents a theoretical model of how anti‐littering messages vary in the kind of social pressure against littering they attempt to impose. Three experiments were performed to test the model. It was found that messages making explicit commands against the act of littering (external pressure) actually generated more littering than messages making appeal to social normative standards concerning littering (internal pressure). Other data relevant to the model and areas in which additional research is needed also are discusse