The authors propose that when people become fused with a group, their personal and social identities become functionally equivalent. Two hypotheses follow from this proposition. First, activating either personal or social identities of fused persons should increase their willingness to endorse extreme behaviors on behalf of the group. Second, because personal as well as social identities support group-related behaviors of fused persons, the 2 forms of identity may combine synergistically, fostering exceptionally high levels of extreme behavior. Support for these hypotheses came from 5 preliminary studies and 3 experiments. In particular, fused persons were more willing to fight or die for the group than nonfused persons, especially when their personal or social identities had been activated. The authors conclude that among fused persons, both the personal and social self may energize and direct group-related behavior. Implications for related theoretical approaches and for conceptualizing the relationship between personal identities, social identities, and group processes are discussed.