The impact of diversity on group functioning is multifaceted. Exploring the impact of having a newcomer join a group, the authors conducted a 2 (social similarity of newcomer to old-timers; in-group or out-group) × 3 (opinion agreement: newcomer has no opinion ally, one opinion ally, or two opinion allies) interacting group experiment with four-person groups. Groups with out-group newcomers (i.e., diverse groups) reported less confidence in their performance and perceived their interactions as less effective, yet they performed better than groups with in-group newcomers (i.e., homogeneous groups). Moreover, performance gains were not due to newcomers bringing new ideas to the group discussion. Instead, the results demonstrate that the mere presence of socially distinct newcomers and the social concerns their presence stimulates among old-timers motivates behavior that can convert affective pains into cognitive gains.