329 | Steven Pinker on Rationality and Common Knowledge

What It Covers

Steven Pinker explains common knowledge—when everyone knows that everyone knows something—and how this concept shapes human coordination, social relationships, political movements, financial markets, and why rational people still disagree despite shared evidence.

Key Questions Answered

• •Aumann's Agreement Theorem: Two perfectly rational agents with identical prior beliefs who share their posterior probabilities (not raw evidence) must converge to agreement, though not necessarily meeting halfway—their positions can leapfrog randomly before reaching the same conclusion, challenging the war-like metaphor of argument.
• •Common Knowledge Generation: Public, conspicuous events create common knowledge instantly—demonstrations in public squares coordinate resistance against dictators because participants see everyone else participating simultaneously, explaining why autocrats suppress gatherings and control media to prevent coordinated opposition from forming.
• •Indirect Communication Strategy: People use euphemism and innuendo to convey messages while avoiding common knowledge of those messages, preserving social relationships—saying "want to come up for coffee" maintains plausible deniability about intentions even when both parties understand the actual meaning.
• •Financial Bubbles and Bank Runs: Speculative investing follows Keynes's beauty contest logic—investors pick assets they think others will pick, not based on fundamental value—creating recursive mentalizing that generates bubbles, crashes, and runs through common expectation rather than rational asset evaluation.
• •Self-Conscious Emotions: Embarrassment intensifies dramatically when transgressions become common knowledge rather than merely observed—experiments show people singing karaoke feel significantly more mortified when they know judges know they're being watched, explaining why avoiding eye contact reduces awkwardness in compromising situations.