Past research on the social identity approach to attitude–behaviour relations has operationalized group norms as a mixture of both descriptive information (i.e. what most people do themselves) and injunctive information (i.e. what most people approve of). Two experiments (Study 1 = 185 participants; Study 2 = 238 participants) were conducted to tease apart the relative effects of descriptive and injunctive group norms. In both studies, university students’ attitudes towards current campus issues were obtained, the descriptive and injunctive group norms were manipulated, and participants’ post-manipulation attitudes, behavioural willingness, and behaviour were assessed. Study 2 also examined the role of norm source (i.e. in-group vs. out-group injunctive and descriptive norms). In both studies, the injunctive and descriptive in-group norms interacted significantly to influence attitudes, behavioural willingness, and behaviour. Study 2 revealed that out-group norms were largely ineffective. The research illustrates that in-groups interactively influence decisions, not only by what they say, but also by what they do, and asserts the value of considering the interaction of descriptive and injunctive norms in accounts of normative influence.