# Aumann's agreement theorem

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Revision as of 03:36, 8 September 2009 by Vladimir Nesov (talk | contribs) (there is the theorem, and then there are informal inferences, more like rationalization than conclusions; one should keep track of the distinction)

**Aumann's agreement theorem**, roughly speaking, says that two agents acting rationally (in a certain precise sense) and with common knowledge of each other's beliefs cannot agree to disagree. More specifically, if two people are genuine Bayesians, share common priors, and have common knowledge of each other's current probability assignments, then they must have equal probability assignments.

Outside of well-functioning prediction markets, Aumann's conditions can probably only be reached by careful deliberative discourse: whether deliberation tends to resolve disagreements in practice is an empirical question.

## See also

## Blog posts

- Agreeing to Agree by Hal Finney
- The Proper Use of Humility by Eliezer Yudkowsky
- The Coin Guessing Game by Hal Finney, a toy problem illustrating the mechanics of Aumann agreement

## References

- Robert J. Aumann (1976). "Agreeing to Disagree".
*The Annals of Statistics***4**(6): 1236-1239. ISSN 00905364. (PDF) - Tyler Cowen and Robin Hanson (2004).
*Are Disagreements Honest?*. (PDF, Talk video)