329 | Steven Pinker on Rationality and Common Knowledge

Episode Summary

Executive Summary: Sean Carroll and Steven Pinker discuss Pinker’s new book on common knowledge: not just what people know, but what they know that others know. Using examples from boating rules, euphemism, bank runs, finance, embarrassment, and political protest, they show how common knowledge enables coordination, social norms, and collective action—yet also how humans often avoid it to preserve deniability and social equilibrium.

Main Topics: What common knowledge is (Priority: 5/5): The conversation defines common knowledge as recursive mutual awareness: I know that you know, you know that I know, and so on. This technical meaning underlies coordination in social life. Language, euphemism, and plausible deniability (Priority: 5/5): Pinker argues that indirect speech, innuendo, and weasel words work by conveying meaning without fully generating common knowledge, preserving relationships and face. Coordination, conventions, and social institutions (Priority: 5/5): Rules like driving on the right, respecting authority, and trusting currency all depend on shared public understanding that others also recognize. Finance, panic, and recursive expectations (Priority: 4/5): Bank runs, hoarding, and speculative bubbles arise when people anticipate what others expect and do; common expectations can create self-fulfilling cascades. Alman’s agreement theorem and rationality (Priority: 4/5): The theorem shows that ideally rational agents with shared priors and common knowledge of each other’s rationality cannot agree to disagree, illustrating epistemic humility and the limits of argument. Embarrassment, eye contact, and nonverbal signals (Priority: 4/5): Self-conscious emotions intensify when mistakes become common knowledge; eye contact, laughter, blushing, and staring often function as public knowledge generators. Protest, repression, and political coordination (Priority: 4/5): Public demonstrations, blank signs, and shared signals can create the common knowledge needed for mass resistance, which autocrats try to prevent through censorship.

Key Arguments: Common knowledge is not merely shared information; it is mutual awareness of shared information, and that recursive structure is what makes coordination possible. Human social life depends on common knowledge for conventions, institutions, and relationships, so language likely co-evolved with sociality. People often avoid generating common knowledge because direct acknowledgment can threaten relationships, remove plausible deniability, or increase embarrassment. Financial panics and bubbles are driven less by fundamentals than by expectations about others’ expectations, making them examples of common expectation. Alman’s theorem implies that two perfectly rational agents with the same priors should converge on the same posterior once their beliefs are made common knowledge. Real-world disagreement often persists because people have different priors, doubt others’ rationality, or selectively use evidence, so the theorem is idealized but normatively important. Public signals—from deposit insurance signs to demonstrations—work because they make beliefs and intentions visible to everyone at once, creating coordination. Common knowledge can be politically powerful: regimes fear it because once people know that others know dissent is widespread, collective action becomes safer. Pluralistic ignorance and conspiracy thinking show how people can misread how many others share a belief, which distorts coordination and public discourse.

Data Points: Book title: When Everyone Knows That Everyone Knows - Steven Pinker’s new book on common knowledge Auman agreement theorem paper length: 3 pages - Sean Carroll notes the theorem’s famous brevity and surprising conclusion Bank-run coordination example: 17th clink - In the spinach-in-the-teeth puzzle, 17 people with spinach would all act on the 17th clink Spinach puzzle example: 3 clinks - With 3 people having spinach, all clean their teeth on the third clink Great Barrier Reef spawning timing: 5 days after the full moon - Pinker’s example of coral synchronization through a conspicuous public cue Cross-cultural eye contact warning: More than 6 seconds - Attributed to Irv Devor: prolonged eye contact may signal sex or violence Survey-style posterior example: 0.7 vs 0.4 - Illustrative probabilities used to explain agreement under rational updating Public coordination scale: 100,000 Englishmen vs 350 million Indians - Gandhi example of why control fails without coordination of the many Conspiracy belief misperception: 5% believed as 60% believed - Sean cites the tendency to wildly overestimate how many others believe a conspiracy theory

Pivotal Quotes: "Common knowledge is knowledge that lots of people have, and they all know each other has it." — Sean Carroll: Introductory explanation of the concept and why it matters for coordination "We often try to avoid common knowledge in order to preserve the relationship that we have." — Steven Pinker: On why people use euphemism, innuendo, and indirect speech "That is, it is, they cannot agree to disagree." — Sean Carroll: Summarizing the force of Auman’s agreement theorem under shared priors and rationality

Implications: Common knowledge shapes coordination, markets, institutions, protest, and everyday social life. For listeners, it highlights why signaling, framing, and publicness matter—and why arguments, norms, and movements succeed or fail based on what everyone knows everyone knows.