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How School Shootings Spread

But Granovetter thought it was a mistake to focus on the decision-making processes of each rioter in isolation. In his view, a riot was not a collection of individuals, each of whom arrived independently at the decision to break windows. A riot was a social process, in which people did things in reaction to and in combination with those around them. Social processes are driven by our thresholds—which he defined as the number of people who need to be doing some activity before we agree to join them. In the elegant theoretical model Granovetter proposed, riots were started by people with a threshold of zero—instigators willing to throw a rock through a window at the slightest provocation. Then comes the person who will throw a rock if someone else goes first. He has a threshold of one. Next in is the person with the threshold of two. His qualms are overcome when he sees the instigator and the instigator’s accomplice. Next to him is someone with a threshold of three

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Finally, Granovetter’s model suggests that riots are sometimes more than spontaneous outbursts. If they evolve, it means they have depth and length and a history. Granovetter thought that the threshold hypothesis could be used to describe everything from elections to strikes, and even matters as prosaic as how people decide it’s time to leave a party. He was writing in 1978, long before teen-age boys made a habit of wandering through their high schools with assault rifles. But what if the way to explain the school-shooting epidemic is to go back and use the Granovetterian model—to think of it as a slow-motion, ever-evolving riot, in which each new participant’s action makes sense in reaction to and in combination with those who came before?