Cross-posted to the EA forum. Summary * History’s most destructive ideologies—like Nazism, totalitarian communism, and religious fundamentalism—exhibited remarkably similar characteristics: * epistemic and moral certainty * extreme tribalism dividing humanity into a sacred “us” and an evil “them” * a willingness to use whatever means necessary, including brutal violence. * Such ideological fanaticism was a major driver of eight of the ten greatest atrocities since 1800, including the Taiping Rebellion, World War II, and the regimes of Stalin, Mao, and Hitler. * We focus on ideological fanaticism over related concepts like totalitarianism partly because it better captures terminal preferences, which plausibly matter most as we approach superintelligent AI and technological maturity. * Ideological fanaticism is considerably less influential than in the past, controlling only a small fraction of world GDP. Yet at least hundreds of millions still hold fanatical views, many regimes exhibit concerning ideological tendencies, and the past two decades have seen widespread democratic backsliding. * The long-term influence of ideological fanaticism is uncertain. Fanaticism faces many disadvantages including a weak starting position, poor epistemics, and difficulty assembling broad coalitions. But it benefits from greater willingness to use extreme measures, fervent mass followings, and a historical tendency to survive and even thrive amid technological and societal upheaval. Beyond complete victory or defeat, multipolarity may persist indefinitely, with fanatics permanently controlling a non-trivial fraction of the universe, potentially using superintelligent AI to entrench their rule. * Ideological fanaticism increases existential risks and risks of astronomical suffering through multiple mutually-reinforcing pathways. * Ideological fanaticism exacerbates most common causes of war. Fanatics' sacred values and outgroup hostility often preclude compromise,