Four experiments explored the interaction of group size, social categorization, and bystander behavior. In Study 1, increasing group size inhibited intervention in a street violence scenario when bystanders were strangers but encouraged intervention when bystanders were friends. Study 2 replicated and extended these findings to social category members. When gender identity was salient, group size encouraged intervention when bystanders and victim shared social category membership. In addition, group size interacted with context-specific norms that both inhibit and encourage helping. Study 3 used physical co-presence and gender identities to examine these social category effects. Increasing group size of women produced greater helping of a female victim, but increasing group size of men did not. Additionally, increasing numbers of out-group bystanders resulted in less intervention from women but more intervention from men. Study 4 replicated these findings with a measure of real-life helping behavior. Taken together, the findings indicate that the bystander effect is not a generic consequence of increasing group size. When bystanders share group-level psychological relationships, group size can encourage as well as inhibit helping.