The present paper investigates how people identify with groups depending on the clarity of a group's identity content. According to self-categorization theory, self-stereotyping (i.e., projection of group prototypes onto self) should be the cognitive process underlying social identification. We argue, however, that this is only plausible in clearly defined groups. If a group is unclear in terms of its identity content, we argue that self-anchoring (i.e., projection of personal self onto group) accounts for social identification. In line with these hypotheses, two studies (with minimal and real groups) reveal that self-anchoring positively predicts the level of social identification and entitativity in unclearly, but not clearly defined groups. Conversely, self-stereotyping positively predicts the level of social identification in clearly, but not unclearly defined groups. Together, these findings are the first to demonstrate the differential impact of cognitive projection processes on social identification depending on group clarity.