A new study finds that automated officiating in the Korea Baseball Organization altered outcomes for the league's most recognized players.
The KBO became the first major professional baseball league to introduce a fully Automated Ball-Strike (ABS) system at its highest level in 2024. The system transmits ball-or-strike decisions to the home-plate umpire via an earpiece.
Researchers Jimin Song, Ji Hyuk Kang and Richard J. Paulsen from the University of Michigan School of Kinesiology treated the ABS introduction as a natural experiment to compare player performances before and after its implementation. The study examined 148 batters and 112 pitchers. Players were classified by status using their history of recognition in the KBO's Golden Glove awards, which honor excellence across hitting, fielding and base running.
After the ABS system was adopted, high-status batters saw a significant decline relative to lower-status batters. Their on-base percentage fell by approximately 7 percent compared to the control group. These prominent batters also saw their walk percentage drop while their strikeout percentage rose, producing a substantial decline in their walk-to-strikeout ratio. The disadvantages appeared most clearly in outcomes dependent on marginal ball-and-strike calls.
Broader measures reflecting a batter's ability to hit the ball did not decline significantly. Star batters did not lose their overall hitting skill — they lost the favorable calls they may have received on close pitches.
Star pitchers did not experience a similar loss of advantage. The researchers said the impact of human judgment bias was more pronounced for batters.
The findings provide concrete evidence for long-held suspicions about star treatment in sports. NBA fans have long argued that marquee players — from Michael Jordan to Shai Gilgeous-Alexander — receive more favorable foul calls. NFL supporters contend franchise quarterbacks are protected more carefully than less prominent players. Soccer fans believe the sport's most powerful clubs benefit from officiating decisions in crucial moments. Baseball long fostered the perception that accomplished pitchers such as Greg Maddux and Tom Glavine had earned the edges of the strike zone.
Paulsen said fans have seen calls influence game outcomes. He said some objective decisions — like ball-strike calls or out-of-bounds calls — could be automated easily.
The study also carries implications beyond baseball. Organizations routinely ask managers and scouts to objectively evaluate individuals, yet those decisions are rarely made without knowledge of reputation or existing status. Automated officiating offers a model for using objective data in critical moments.
