Prior studies of the effect of group identification on cooperation in social dilemmas have advanced 2 competing accounts: the goal-transformation hypothesis, which holds that identification makes personal and collective goals interchangeable; and the goal-amplification hypothesis, which states that identification induces positive expectations about others’ cooperative behavior. However, prior studies have neglected to assess the process measures necessary to pit the one account against the other. The present study showed that the effect of identification was moderated by participants’ social value orientation. Identification influenced proselfs’ cooperation more than prosocials’ cooperation. Mediational analyses further showed that the effect of our identification manipulation was mediated by participants’ sense of collective self, and not by their expectations.