An environmental policy to foster virtuous behaviour does not automatically establish a social norm in a population; that is, the policy might not be socially acceptable or enforceable. Some agents feel compelled to abide by environmental social norms and embrace them, but others do not. Some might want to imitate their peers, while others might prefer not to conform and play the role of a maverick. In this model, we describe the heterogeneity of preferences by proposing a taxonomy of five possible agent types that enrich the traditional triplet presented in the literature. We then employ a random matching model to study how a social norm spreads within a population when its composition changes. Considering three relevant population compositions (scenarios), we show that what is most important for the successful diffusion of social norms is not whether, but why agents abide by it.