Identity fusion describes a visceral feeling of oneness with a group in which the personal and social self become functionally equivalent. Where ordinary group identification — 📝social identity in the conventional sense — keeps the "I" and the "we" distinct, fusion theory, developed by William Swann, Harvey Whitehouse, and colleagues, argues that for strongly fused people the boundary between them grows porous. Personal agency flows into the group and the group is felt as an extension of the self, so a threat to the collective registers as a threat to one's own body. The theory distinguishes local fusion, the relational bond with a small group of known others such as family or a combat unit, from extended fusion, in which those same family-like ties are projected onto a large abstract collective like a nation, faith, or movement. What makes fusion consequential is its capacity to drive extreme pro-group behavior: the strongly fused are far more willing to fight, sacrifice, and even die for their group, treating fellow members as fictive kin. One of the most reliable pathways into this state is shared dysphoric experience — painful, frightening events endured together, which, reflected on personally, weld individuals to one another and to the cause. This is the mechanism beneath suicide terrorism, holy wars, and sectarian violence.