Usually people don’t agree with one another as much as they should. Aumann’s Agreement Theorem (AAT) finds:
…two people 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 Bayesian rationalists with common priors, and if they each have common knowledge of their individual posteriors, then their posteriors must be equal.
The surprising part of the theorem isn’t that people should agree once they have heard the rationale for each of their positions and deliberated on who is right. The amazing thing is that their positions should converge, even without knowing how the other reached their conclusion.