Social norms are strongly associated with pro-environmental behaviors, but the evolution and dynamic effects of norms are less well understood. This article builds on the distinction of norms being descriptive, characterizing what people actually do, or injunctive, characterizing what people should do. It identifies four categories of norms with the further distinction of whether the norms arise from the personal beliefs and actions or from the behaviors and judgments of others. The analysis uses five years of longitudinal US data that track household recycling and controls for household characteristics as well as differences in state recycling laws. The results extend previous research by showing that personal norms exhibit cascading dynamics in which norms encourage later changes in recycling, while recycling encourages later changes in personal norms. This mutual reinforcement implies that societal actions encouraging change in either personal norms or recycling will support growth in the other. Recognizing this interdependence can assist in the effective utilization of social norms as a behavioral policy instrument.