Collective action researchers have focused on injunctive norms that specify approved behavior as a panacea for collective action problems. We investigate whether descriptive norms (similar behavior) can also solve these problems. We argue that descriptive norms generate social identification, which then sustains conformity to expectations. Consequently, descriptive norms can characterize both cooperation and non-cooperation, such that cooperative norms sustain successful collective action while noncooperative norms result in collective action failure. Results from two laboratory experiments supported the hypothesis that descriptive norms can sustain collective action success and failure. Further, while normative non-cooperation eroded cooperation for high contributors, normative cooperation had little affect on low contributors. This asymmetry points to a paradox: because they promote group identification, noncooperative descriptive norms can be self-sustaining, with deleterious outcomes.