DeMartin, Luke. “Let The Boys Play”, Boston College, 2023. http://hdl.handle.net/2345/bc-ir:109722.
This paper investigates the existence of omission bias in Major League Baseball’s home plate umpires. Omission bias describes the human tendency to prefer harm caused by inaction, or acts of omission, over equal harm caused by action, or acts of commission. For umpires, I define an act of commission as a call made by the umpire that ends the at-bat and an act of omission as a call that does not end the bat. By analyzing over 1.5 million pitches thrown between the years 2018 and 2022, I find that MLB umpires display omission bias by systematically increasing the size of the enforced strike zone on three-ball counts and shrinking the size of the enforced strike zone on two-strike counts. Further, I find that omission bias exists separately from and is not impacted by other biases present in MLB umpiring, such as the biases favoring home batters and star batters.