Batter up, bias down: Robot umpires curb favoritism for star hitters

A University of Michigan study examined the Korea Baseball Organization’s adoption of its Automated Ball-Strike system in 2024 and found evidence that automated calls reduced an apparent advantage previously held by prominent hitters. Before the change, human umpires judged balls and strikes visually. Under the new system, cameras and pitch-tracking technology determine whether pitches cross the strike zone, while the home-plate umpire announces the result.

Researchers compared player performance from 2023, the final season of human ball-and-strike calls, with 2024, the first ABS season. They compared high-status players with lower-status players and focused on measures closely connected to strike-zone decisions. After ABS began, famous batters walked less often, struck out more often and reached base less often relative to lesser-known hitters. Across 100 at-bats over a season under ABS, a famous batter had nearly three additional strikeouts and nearly two fewer walks than a less prominent hitter.

The researchers reported that broader hitting statistics changed less, which they said supports an explanation tied to umpiring rather than a general decline in star batters’ ability. They suggested that human umpires may have been more likely to give well-known hitters favorable calls on borderline pitches. High-status pitchers did not show the same pattern; the researchers noted that pitchers may have had fewer opportunities to display a measurable change or that their performance may be more variable.

The study, published in European Sport Management Quarterly, also discusses implications for other evaluations involving people. The authors said blind review procedures and carefully designed technologies could help limit status-based favoritism in areas such as hiring, promotions, auditions and performance reviews. Major League Baseball has implemented an Automated Ball-Strike Challenge System, but has not set a timetable for fully automated ball-and-strike calls. The researchers do not expect automation to eliminate human officiating, particularly for subjective judgments.