Upon joining a new social category, group members strive to establish and maintain high social identification. Thus far, we know relatively little about the cognitive underpinnings of social identification when developing from a new to a well-established group member. This research investigates the differential impact of newcomers' self-stereotyping (i.e., assimilation of the self to group stereotypes) and self-anchoring (i.e., projection of self-attributes onto the ingroup) on the development of social identification over time. Across two time points during the academic year, first year psychology students (N = 123) filled in a questionnaire on their perceptions about the self, psychology students, and social identification. Cross-sectional and longitudinal analyses confirmed our hypotheses that self-anchoring instigated new group members' social identification, while self-stereotyping instigated social identification once group membership was more well-established. This research emphasizes the interactive role of the personal and social self in the development of social identification.