When Everyone Knows That Everyone Knows: Common Knowledge and the Mysteries of Money, Power and Everyday Life

It’s not what everyone knows that counts but what everyone admits they know, according to this labyrinthine pop-sci treatise. Harvard cognitive psychologist Pinker (The Better Angels of Our Nature) recaps research—his own and others’—into common knowledge, the store of information that everyone knows and, crucially, is universally acknowledged to be true (as illustrated by the folktale “The Emperor’s New Clothes,” wherein everyone can see the ruler is naked, but it only becomes common knowledge when a naive child publicly declares it). Pinker analyzes how common knowledge enables the coordination of mutually beneficial outcomes: a public protest establishes the common knowledge of political dissatisfaction and allows protestors to take unified action, for example. Elsewhere, a chapter on cancel culture—Pinker has spoken out against its eruptions at Harvard—argues it’s an attempt by ideologues to band together to protect common knowledge from infiltration by politically incorrect ideas. Pinker’s discussions of these findings are intriguing if not earthshaking, but the scientific apparatus he assembles—replete with psych experiments into “recursive mentalizing,” or “thinking about thoughts about thoughts”—can feel overblown and onerous. The result, in a lapse from Pinker’s usual brilliance, is a taxing outing that yields mostly pedestrian insights. (Sept.)