Sean Carroll's Mindscape Podcast, Episode 329
Guest: Steven Pinker
Topic: Rationality and Common Knowledge
Date: September 22, 2025

Episode Overview

In this engaging episode, Sean Carroll welcomes cognitive psychologist and renowned author Steven Pinker to discuss the intricate concept of common knowledge—the subtle, often-overlooked foundation that underlies much of human rationality, cooperation, and social life. Centered around Pinker’s new book When Everyone Knows That Everyone Knows, the conversation explores how common knowledge shapes social conventions, communication, economics, embarrassment, collective action, and more. The discussion seamlessly blends real-world examples, striking logic puzzles, and psychological theory—highlighting both the power and the pitfalls of shared understanding.

Key Discussion Points & Insights

1. Introduction to Common Knowledge

• Definition: Common knowledge isn't just information most people know—it's information that everyone knows everyone else knows, “and so on, ad infinitum.”
  • Quote: "Universal private knowledge is not the same as common knowledge; at least in this technical sense...it refers to...not everyone knows something, but everyone knows that everyone knows that everyone knows it." — Steven Pinker, [17:06]
• Illustrative Example: Boating collision avoidance rules require not just that all boaters know the rules, but that they know the other boaters know them too ([03:09]).

2. Common Knowledge in Language & Social Life

• Pinker's interest stems from language: Why do humans often communicate indirectly through innuendo, euphemism, or implication? Much of this, Pinker argues, is about managing when to generate or avoid common knowledge to maintain social relationships ([07:37]).
  • Quote: "The solution I proposed...is that the difference [between innuendo and directness] is generating common knowledge." — Steven Pinker, [10:13]
• Common knowledge is foundational for:
  • Social conventions (e.g., driving on a side of the road, currency value)
  • Relationships ("Two people are friends when each knows the other knows they’re friends." [11:34])
  • Indirect communication ("We often try to avoid common knowledge in order to preserve the relationship that we have." [11:48])

3. Philosophical & Mathematical Foundations

• Bayesian Reasoning & The Aumann Agreement Theorem: If two rational agents—with common priors—share their credences openly so that their reasoning is common knowledge, they are mathematically required to reach agreement ([04:29], [33:50]).
  • Quote: "Two rational agents with the same priors...who then share their posteriors as common knowledge...must be the same—they cannot agree to disagree." — Steven Pinker, [33:50]
• Disagreement persists in real life due to differences in priors and lack of common knowledge of each other's rationality ([42:10]).

4. Logic Puzzles & Illustrations of Common Knowledge

• Three Logicians in a Bar ([26:43]): Highlights recursive mentalizing—reasoning about what others are reasoning—which is distinct from full common knowledge.
• The Muddy Children Problem ([27:47]): Demonstrates how an announcement ("At least one of you has spinach in your teeth") and successive non-acts generate recursive knowledge culminating in common knowledge. This only works if everyone is aware of others’ awareness.
  • Quote: "It crucially depends on common knowledge. It wouldn’t work if the department chair whispered." — Steven Pinker, [32:54]

5. Common Knowledge in Economics & Social Psychology

• Financial bubbles, bank runs, and phenomena like hoarding are driven by recursive expectations and common knowledge (or the lack thereof).
  • Example: Bank runs pre-empted by conspicuous bank displays or federal insurance ("The purpose of that seal is...to reassure them that other people know that they're insured." [20:14])
• Speculation & Keynes’ Beauty Contest: Investors try to anticipate others’ anticipations—a self-reinforcing, recursive loop.
  • Quote: "That would often involve...the second, third, and fourth order of anticipation...can lead to runaway behavior." — Steven Pinker, [23:10]

6. Cooperation, Coordination, and Evolutionary Perspectives

• Cooperation (potentially altruistic) and coordination (mutual benefit) both rest on common knowledge but differ in their biological/evolutionary logic ([49:02]).
  • Example: Coral spawning is coordinated using a conspicuous event (the full moon), solving the knowledge problem even without cognition ([54:32]).
• Many social conventions (gatherings, demonstrations, currency) exist and function due to common knowledge.

7. Social Rituals, Signals, and Indirectness

• Common knowledge shapes nonverbal behaviors and emotional responses (“eye contact,” “blushing,” “laughter”) ([58:21]).
  • Avoiding eye contact or blushing is a strategy to manage or evade common knowledge’s social consequences.

8. Psychology Experiments & Evidence

• Pinker’s lab demonstrated, both with surveys and controlled (karaoke) experiments, that embarrassment and self-conscious emotions peak when faux pas becomes common knowledge, not merely when others observe it ([62:06]).
  • Example: Participants were most embarrassed when they knew judges knew they knew they were being watched during karaoke ([62:37]).

9. Applications: Protests, Political Change, and Controversial Signaling

• Public demonstrations generate common knowledge essential for collective action (referencing Gandhi and the Soviet joke about leaflets, [73:25], [75:39]).
• Authoritarian states suppress free press and mass gatherings precisely to thwart common knowledge and, thus, resistance coordination.
  • Quote: "In the end, you will leave, because there is simply no way that 100,000 Englishmen control 350 million Indians if they refuse to cooperate." — Pinker paraphrasing Gandhi, [74:57]
• The ambiguity of signals (e.g., the “OK” hand sign’s appropriation and misinterpretation) hinges on different communities’ common knowledge ([72:08]).

10. Pitfalls, Misconceptions, and Pluralistic Ignorance

• Pluralistic ignorance: Many people privately reject a belief but incorrectly assume most others accept it, impeding organization or change ([78:02]).
• Overvaluing iconoclastic discovery in science (and media’s focus on challenger findings) can be a result of neglecting the rational role of consensus and priors ([43:33], [45:00]).

Memorable Quotes & Moments

• “Why do we avoid saying what we mean—and why do we sometimes need to say exactly what we mean? Common knowledge is the key difference.” — Steven Pinker [10:10]
• “The solution is: if something is public, conspicuous, self-evident, that can generate common knowledge in a stroke.” — Steven Pinker [14:24]
• “The bias in science journalism to favor the paradigm-threatening discovery...is very un-Bayesian. It’s throwing out the priors.” — Steven Pinker [43:33]
• “Not everyone would go along with [speaking openly], but it shows the tension between what everyone knows and what everyone knows that everyone knows.” — Steven Pinker [66:00]
• “We humans just take what we evolved with and repurpose it...eye contact which, in primates, is a threat signal but in humans can become flirtation or intimacy.” — Steven Pinker [60:14]

Key Timestamps

• [03:09] — Boating rules, coordination, and initial introduction to common knowledge
• [07:37] — Pinker explains his motivation for studying common knowledge
• [14:24] — How public events generate common knowledge
• [20:14] — Bank runs, deposit insurance, and the prevention of panics
• [26:43] — The “Three logicians at a bar” puzzle
• [27:47] — The “Muddy children” logic problem and recursive reasoning
• [33:50] — The Aumann Agreement Theorem and rational disagreement
• [43:33] — Common knowledge, consensus, and the pitfalls of scientific iconoclasm
• [49:02] — Coordination vs. cooperation: mutualism and evolutionary biology
• [54:32] — Corals, coordination, and conspicuous public events in nature
• [58:21] — Weasel words, indirect speech, and managing public knowledge
• [62:06] — Karaoke experiment confirming embarrassment rises with common knowledge
• [73:25] — Collective resistance, demonstrations, and dictatorships; the Gandhi example
• [75:39] — Soviet joke on leaflets and the suppression of common knowledge
• [78:02] — Pluralistic ignorance and how common misconceptions persist

Episode in a Nutshell

Sean Carroll’s deep-dive with Steven Pinker is a rich tour of how social reality, reasoning, and human emotions pivot on the subtle but powerful structure of common knowledge. Through puzzles, stories, and psychological evidence, Pinker lays bare why societies succeed or fail to coordinate, why consensus exists, how social rituals work, and how quickly things fall apart when common knowledge is lacking or manipulated. The episode is both a window into human cognitive fragility and a celebration of our capacity for rational collective action—when we are lucky (or wise) enough to truly all be on the same page.