REALITY CHECK: But would you say today black and white thinking is causing this very thing to happen? It is a major fuel. It is not the engine all by itself.Black-and-white thinking—people are good or evil, a policy is salvation or treason, a party is the country or the enemy—makes mega-identities work. Once D or R is a moral brand, nuance looks like betrayal. That is how “are you one of us?” eats tax policy, foreign policy, and whether someone is allowed to be wrong in public.What it does well at explaining Affective polarization is closer to a purity rule than to a spreadsheet of issues. Thermometer scores, “they’re immoral,” and approval gaps of 70–80 points are what you get when the other side is a type, not a bundle of tradeoffs. Social media and nationalized news reward the same binary: one clip, one villain, one team. Binary cognition and binary media fit each other.What it does not explain by itself People were not suddenly worse at seeing gray in 2008. Institutions changed: close elections, sorted parties, primary voters, donor media, phones that multiply same-side friends. You can be a subtle thinker and still live in a market that pays you to pick a side. Elites often perform black-and-white while holding more complicated private views. Mass publics still hold mixed opinions; the display of politics got simpler.Direction of causality It runs both ways. Binary thinking makes partisan identity sticky. Partisan identity then trains binary thinking: once the team is sacred, exceptions feel dangerous. That loop is why “just think in shades of gray” rarely fixes it. The identity has to matter less, or the incentives for purity have to get weaker.So today Yes: all-or-nothing moral sorting is a big reason party feels like the main issue. No: it is not a personality epidemic that appeared from nowhere and caused polarization while institutions sat still. The thing happening is binary identity under high-stakes, high-volume politics—not that humans forgot how to count to three.