Drawing upon socioanalytic theory of personality, we hypothesize and test inverted U-shaped relationships between team members' assertiveness and warmth (labeled as the "getting ahead" and "getting along" facets of extraversion) and peers' reactions (i.e., advice seeking by peers and peer liking, respectively) that, in turn, predict members' emergence as informal leaders in self-managed teams. Integrating research on prosocial motivation, we also examine whether prosocially motivated members have more enhanced positive curvilinear influences of assertiveness and warmth on peer reactions. Based on 223 members in 69 student project teams (Study 1) and 337 employees in 79 self-managed work teams (Study 2), we found support for the inverted U-shaped relationships between assertiveness and advice seeking by peers, and between warmth and peer liking. Further, prosocial motivation enhances the inverted U-shaped effect of assertiveness in Study 2 and those effects of warmth in both studies. Advice seeking by peers and peer liking, in turn, were positively related to leadership emergence in both studies. Our findings have important theoretical and practical implications for dispositional and motivational factors that shape peer reactions and facilitate leadership emergence in teams. (PsycINFO Database Record (c) 2019 APA, all rights reserved).