Social identity is the part of a person's self-concept that derives from membership in social groups — a nation, a religion, a team, a political faction — together with the value and emotional significance attached to that belonging. Formalized by Henri Tajfel and John Turner in the 1970s and 80s, social identity theory holds that people categorize themselves and others into in-groups and out-groups, and that the very act of categorization generates bias: we favor those we count as our own and differentiate ourselves against those we do not. This drive toward positive distinctiveness means group identity is rarely neutral. It structures status, loyalty, trust, and threat, and under the right conditions it tips into the hostility, dehumanization, and violence that mark intergroup conflict. At its strongest the bond becomes 📝identity fusion, a visceral sense of oneness in which the boundary between personal self and group self dissolves; fused individuals will fight, sacrifice, and die for the group because an attack on it registers as an attack on the self. This is the mechanism that turns ordinary belonging into extremism.