Most researchers tried to solve the fat tail problem by studying the behavior of individual traders. Johnson, instead, looked at clusters. He asked what would happen if we assumed traders acted in cliques: small groups whose members all behave the same way, that is, they make the same buy or sell decisions. (The evidence for groupthink in markets, from tulip mania to the internet bubble, is strong.) The clusters need not be permanent. Just like cliques in high school, members come and go, trading cliques form and dissolve, they merge with other cliques or split into two. Imagine bringing a pot
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