Group living has been essential for our survival since the first humans walked the Earth. Beyond just surviving, people who accurately identify, value, and cooperate with in-group members enjoy numerous material and psychological benefits. However, group life is also a source of social strife and destruction. From Israelis and Palestinians to Red Sox and Yankees fans, group identity has led to innumerous conflicts. Much of the research into this intergroup conflict has focused on group members’ perceptions of the other. However, this focus neglects that when we become part of an “us,” we change as well. Recent evidence from psychology and neuroscience indicates that individuals acting on behalf of a group will often act more ruthlessly than individuals acting alone. Better understanding how we change when we shift from “me versus you” to “us versus them” may generate new avenues for interventions among individuals and groups in conflict.