Bayesian Instinct

23 Pages Posted: 15 Feb 2017 Last revised: 29 Jul 2021

See all articles by Etan Green

Etan Green

Wharton - Operations, Information and Decisions

David P. Daniels

NUS Business School

Date Written: July 1, 2021


Do experts make split-second decisions on the basis of rational beliefs? This paper studies belief formation by professional umpires in Major League Baseball. We show that umpires’ decisions reflect an accurate, probabilistic, and state-specific understanding of their rational expectations—as well as an ability to integrate those prior beliefs with signals in a manner that approximates Bayes rule. Given that umpires have at most a second to decide, we conclude that the instincts of professional umpires mimic a sophisticated level of rationality remarkably well.

Keywords: beliefs, rational expectations, Bayes rule, field study, experts

JEL Classification: D81, D83, D84

Suggested Citation

Green, Etan and Daniels, David P., Bayesian Instinct (July 1, 2021). Available at SSRN: or

Etan Green (Contact Author)

Wharton - Operations, Information and Decisions ( email )

Philadelphia, PA 19104
United States

David P. Daniels

NUS Business School ( email )

15 Kent Ridge Drive


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