What’s so common about common knowledge?

Steven Pinker has long been one of America’s heavyweight thinkers. A cognitive scientist with a flair for popular writing, he has spent decades explaining language, reason, and the strange workings of the human mind. From The Language Instinct (1994) to Enlightenment Now (2018), his books have managed to stir both praise and criticism, often in equal measure. His new work, When Everyone Knows That Everyone Knows… (Scribner, 384 pp.), continues in this vein. It takes a deceptively simple idea—common knowledge—and spins out an exploration of how our lives, from markets to marriages, are shaped by what we think others know we know.

The concept sounds like a trick of words, but Pinker shows it’s much more than that. He opens with Hans Christian Andersen’s fable about the emperor’s new clothes. As he writes, “When the little boy said the emperor was naked, he wasn’t telling anyone anything they didn’t already know. But he added to their knowledge nonetheless.” That little burst of truth, shared in public, transformed private suspicion into collective certainty, and toppled the emperor’s illusion. For Pinker, this is the key: The shift from private knowledge to common knowledge can reorder power, expose hypocrisy, and change behavior in an instant.

From this starting point, he builds outward. Common knowledge, he argues, is the hidden glue of social life. It explains why a Super Bowl ad can launch a technology, why dictators fear blank protest signs, and why reputations can collapse overnight online. It is why a dollar bill holds value, why traffic rules function, and why silence in a meeting can speak louder than words. The book promises, as Pinker puts it, to show how “many peculiarities of public life—its mindless rituals, conventions, and norms—become intelligible as solutions to coordination problems.”

One of the great strengths of the book is Pinker’s knack for making abstract theory feel tangible. Game theory could easily remain dry, but he peppers it with stories of awkward rendezvous, viral math problems, and cultural flash points. He retells the saga of Apple’s 1984 Macintosh ad not as a tale of marketing genius, but as an example of how public events create common knowledge. Millions watched, and millions knew millions more were watching. The machine became a standard not just because of its design, but because a single moment turned it into a shared reference point.

Another notable example is his treatment of cancel culture. Pinker revisits the downfall of Justine Sacco, whose ironic tweet became the top trend on Twitter while she was on a flight to South Africa. A joke with racist undertones to a mere 170 followers turned into global disgrace by the time she landed. Here, the machinery of common knowledge was on brutal display. The world didn’t just know about her tweet; it knew that everyone else knew, and the mob acted mercilessly.

The prose is clear, fast, and often witty. Pinker is a natural teacher, and his explanations rarely drag. Readers will be pulled along by curiosity as one story leads to the next. He writes with confidence, sometimes with charm, and occasionally with a light jab of humor. This accessibility is one of his hallmarks, and the book is no exception.

But the weaknesses are hard to ignore. Pinker sometimes trims messy events so they fit too neatly into his idea. Revolutions, panics, or bubbles rarely spring from a single cause, yet he sometimes presents common knowledge as a universal solvent. A theory slick enough to explain everything often explains too much — or nothing at all — and at times the real world feels shaved down just to fit the model.

There is also Pinker’s familiar strain of optimism. From his perch in the ivory tower, he leans toward airbrushed accounts of progress and order. It can feel naive, even willfully blind, as if darker truths are too inconvenient to face. Readers seeking a harder reckoning with reality will come away disappointed.

Still, the book is not without depth. Pinker acknowledges the limits of common knowledge, and he shows how people often evade it through “rituals of benign hypocrisy,” acting as if they don’t notice what everyone else plainly sees. These passages are some of the most interesting, revealing how silence, euphemism, and doublespeak help us manage multiple relationships. His sections on laughter, blushing, and innuendo are especially sharp, showing that the mechanics of common knowledge seep into even our most intimate moments.

So is it worth reading? Yes, though with caveats. This is not Pinker at his most groundbreaking, but it is Pinker at his most readable. For general readers, it is a lively and thoughtful exploration of a single powerful idea. It will show why despots dread gatherings, why social media shreds reputations overnight, and why even a child’s candor can dethrone an emperor. For specialists, it may feel too sweeping. For everyone else, it offers a fresh lens on the strange ways we coordinate, collide, and sometimes combust together—all filtered through the wild-haired wizard of Harvard who can’t resist a tidy tale.