Bayesian Instinct

Abstract

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