Despite implicating ethnicity in everything from civil war to economic failure, researchers seldom consult psychological research when addressing the most basic question: What is ethnicity? The result is a radical scholarly divide generating contradictory recommendations for solving ethnic conflict. Research into how the human brain actually works demands a revision of existing schools of thought. At its foundation, ethnic identity is a cognitive uncertainty-reduction device with special capacity to exacerbate, but not cause, collective action problems. This produces a new general theory of ethnic conflict that can improve both understanding and practice. A deep study of separatism in the USSR and CIS demonstrates the theory’s potential, mobilizing evidence from elite interviews, three local languages, and mass surveys. The outcome is a significant reinterpretation of nationalism’s role in the USSR’s breakup, which turns out to have been a far more contingent event than commonly recognized. International relations in the CIS are similarly cast in new light.