Common knowledge is necessary for coordination, for making arbitrary but complementary choices like driving on the right, using paper currency, and coalescing behind a political leader or movement. It's also necessary for social coordination. Humans have a sixth sense for common knowledge, and we create it with signals like laughter, tears, blushing, eye contact, and blunt speech.

But people also go to great lengths to avoid common knowledge—to ensure that even if everyone knows something, they can't know that everyone else knows they know it. And so we get rituals like benign hypocrisy, veiled bribes and threats, sexual innuendo, and pretending not to see the elephant in the room.

Pinker shows how the hidden logic of common knowledge can make sense of many of life's enigmas: financial bubbles and crashes, revolutions that come out of nowhere, the posturing and pretense of diplomacy, the eruption of social media shaming mobs and academic cancel culture, the awkwardness of a first date.

Steven Pinker is the Johnstone Professor of Psychology at Harvard University. He is an elected member of the National Academy of Sciences, a two-time Pulitzer Prize finalist, and one of Time's "100 Most Influential People in the World Today." He has won many prizes for his teaching, his research on language, cognition, and social relations, and his twelve books. His new book is When Everyone Knows That Everyone Knows: Common Knowledge and the Mysteries of Money, Power, and Everyday Life.