Yesterday a passage in Niall Ferguson’s new column brought me up short. Ferguson was ruminating on the causes of America’s mental health crisis and found himself unsatisfied with the usual explanations—pandemic isolation, substance abuse, despair at the culture’s recurring failure to solve problems like mass shootings that everyone agrees are horrific. Bad vibes aren’t a simple case of individuals reacting similarly yet independently to different stressors, Ferguson reasoned. They’re infectious.

It’s a contagion.

Contagion is the key concept if we are to understand our modern malaise, and you cannot understand contagion until you understand the structure of networks. In my 2017 book The Square and the Tower, I quoted Stanford University biology professor Deborah M. Gordon’s argument that online social networks were replicating on a vast scale many of the more insidious features of friendship circles among girls in a middle school. Malicious gossip spreads far faster and further on Facebook than it ever did in the schoolyard.

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