When extreme outcomes happen much more frequently than you expect, the probability distribution develops what statisticians call a “fat tail.” Physicists love fat tails. Random systems with no hidden connections, like coin tosses, have thin tails. They’re kind of boring. Fat tails signal interesting dynamics in a network. That might be a network of trees through which a fire spreads. Or it might be a network of people trading stocks, through which an idea spreads—in other words, a financial market. Physicists, including Johnson, had been studying the fat tails in financial markets for years,
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