Studies of social movements usually concentrate on explanations of participation. Much less attention is given to non-participation. In this article, we develop a social–psychological theory of non-participation. To some extent, non-participation is the flip side of participation, but it also has dynamics of its own. Theoretical notions are applied in an illustrative manner to the dynamics of collective action against nuclear weapons in the Netherlands and two demonstrations of secondary school pupils. In examining reasons to defect any farther, we explore the impact of people's social environment. It turns out that the majority of the respondents adopt the norms of their environment. That is to say, the vast majority of the pupils who participate in collective action are from milieus that approve of their participation. The vast majority of those who did not participate, on the other hand, are from milieus that approve of their non-participation.