Collective action problems arise when individuals must choose between their own self-interest, and the interests of a group or society as a whole. Pursuit of self-interest is tempting, but can have deleterious effects on the group's total welfare. For example, an individual may be tempted to avoid paying his or her taxes. Yet if all did so, government would collapse. Other collective action problems include voting in voluntary elections, supporting public television, and littering. Without concerted effort, these collective endeavors would certainly fail. Social identity has been shown to have important consequences for collective action. When potential contributors identify with other group members, they contribute more to collective action. This positive relationship exists because individual and collective interests become more closely aligned when people recognize that they share a common group identity. But what leads to high levels of social identification within certain groups but not others? We bridge sociological and social psychological literatures to suggest that norm conformity leads to social identification. When individuals conform to perceived norms, they feel similar to other conformers. Perceived similarity, in turn, is a necessary and sufficient prerequisite for the emergence of social identification. We compare predictions from our normative account of social identification and solidarity in collective action groups to the typical rational choice approach to group solidarity. Rational choice approaches generally assume that identification and solidarity are based on the benefits accrued through group membership. In contrast, the argument developed and tested here suggests that the level of identification (and solidarity) is contingent upon "proper" behavior as prescribed by the group (i.e. conformity to group norms). To tease apart these competing views, and to test our basic argument, we propose a new experiment. Participants will be given a monetary endowment, and must decide how much (if any) of that endowment they wish to contribute toward a fund that benefits the group as a whole. Prior to their contribution, participants will be given feedback about the contributions of ostensible group members. In a "norm" condition, others' ostensible contributions vary only slightly. In a "no norm" condition, others' contributions vary a great deal. While the variance differs considerably, mean contributions are held constant across these conditions, thus equating benefits gained through group membership. This allows us to investigate the impact of group norms on social identification. Collective action problems are ubiquitous in social life, and have important social consequences. Thus understanding how to generate cooperation in order to foster successful collective actions is imperative. This research focuses specifically on a new argument about how to solve such problems. The understanding we acquire through this work should be applicable to a host of situations where collective action is threatened. The results of this research will be disseminated through academic journals, professional presentations, and, where appropriate, other media. Such publication and presentation should help to increase understanding and appreciation for experimentally testing basic theories in sociology, and for illustrating the applicability of basic research on collective action to a wide range of real-world cases.