Common Knowledge and Coordination¶

Why It Matters¶

Many coordination failures stem from knowledge that's shared but not common. Everyone privately knows the emperor has no clothes, but without common knowledge, no one speaks up. Mechanisms that create common knowledge (public announcements, visible commitments, shared experiences) enable coordination that private knowledge cannot.

The Key Move¶

Distinguish shared knowledge (everyone privately knows X) from common knowledge (it's public that everyone knows X). When designing mechanisms requiring coordination, ask: "How do I make this information not just known, but commonly known?" Use public channels, shared observations, or credible commitments that create common knowledge. Conversely, when analyzing coordination failures, check: was the information shared but not common?

Classic Application¶

Bank runs. Each depositor knows the bank is shaky. But withdrawal is individually rational only if others withdraw (if everyone stays, the bank is fine). The problem: you don't know what others know about what others know. A public news story creates common knowledge - now everyone knows everyone knows, triggering the run. The mechanism (public information) converted shared knowledge to common knowledge with disastrous coordinating effect.

Surprising Application¶

Status meetings in organizations. Distributed teams often have shared knowledge (everyone knows the project is behind schedule) but not common knowledge (they don't know everyone else knows). The all-hands meeting creates common knowledge - everyone observes everyone else hearing the update. This enables coordination (acknowledging delays, reallocating resources) that private knowledge couldn't. The meeting's value is often less about information transfer than about creating common knowledge for coordination.

Failure Modes¶

Confusing common knowledge with shared knowledge (they have very different coordination properties). Ignoring the costs of creating common knowledge (public coordination can be expensive or embarrassing). Forgetting that too much common knowledge of problems can create self-fulfilling prophecies (bank runs, market panics). Creating common knowledge unintentionally (public discussions of risks can trigger coordination on the wrong equilibrium).

Go Deeper¶

Aumann (1976), "Agreeing to Disagree." Chwe (2001), "Rational Ritual: Culture, Coordination, and Common Knowledge" - excellent accessible treatment. Lewis (1969), "Convention: A Philosophical Study."