Before boarding a flight to South Africa in 2013, Justine Sacco sent a sardonic joke to her 170 Twitter followers: “Going to Africa. Hope I don’t get AIDS. Just kidding—I’m white!” By the time her plane landed, she was the number-one trending topic worldwide, fired from her job, and branded a racist by millions of strangers.

What happened wasn’t simply outrage at bad humor. It was the sudden creation of “common knowledge.”

What Is Common Knowledge?

As Steven Pinker argues in his newest book, When Everyone Knows That Everyone Knows: Common Knowledge and the Mysteries of Money, Power, and Everyday Life, “It’s not enough that everyone knows something; it becomes common knowledge only when everyone knows that everyone knows it, and everyone knows that everyone knows that everyone knows it, and so on.”

That recursive certainty transforms diffuse awareness into coordinated action. And “the dynamic of punitive mobbing,” as Pinker notes, “is a recurrent vulnerability of human societies.”

“The leap is not from ignorance to knowledge,” Pinker explains, “but from private knowledge to common knowledge.” In other words, from what everyone knows but everyone doesn’t know that everyone knows it, to what everyone knows that everyone knows.

The former is often marked by self-censoring, preference falsification and pluralistic ignorance. Self-censoring happens when fear of speaking freely results in staying silent. Preference falsification is professing to hold opinions one doesn’t hold out of fear of repercussions. Pluralistic ignorance, Pinker explains, is “when people misinterpret others’........

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