Effective leaders promote cooperation among their followers. Although many studies have focused on approaches to effective leadership, few have studied how social identity impacts a leader’s effectiveness within the context of a controlled laboratory environment. Using a novel laboratory experiment and a guiding theoretical framework enables us to shed new light on how social identity impacts leader–follower interactions in a decontextualized environment. In our experiment, the leader, if present, may or may not share a social identity with the rest of the group. Our main finding is that ingroup leaders are more effective than outgroup leaders, and outgroup leaders are no more effective than groups without a leader. In particular, ingroup leaders are more likely to suggest greater cooperation among followers, and their suggestions are more likely to be followed. Further, we find that outgroup leaders are less credible leaders. These findings suggest that diversity alone does not drive inclusion and provide evidence of an important challenge to achieving the well-documented organizational benefits that stem from a diverse and inclusive workplace.