When South Korea's professional baseball league introduced "robot umpire" ball-and-strike calls in 2024, famous batters appeared to lose an edge-but star pitchers did not.
A new University of Michigan study found that after the Korea Baseball Organization adopted its Automated Ball-Strike (ABS) System, high-status hitters performed worse in statistics most closely tied to strike-zone judgment. Compared with lower-status batters, famous hitters walked less, struck out more and reached base less often after automated calls replaced human judgment on balls and strikes.
Specifically, out of 100 at bats over one season in the ABS System, a famous batter had nearly three more strikeouts and nearly two less walks compared to a lesser-known hitter.
"This suggests that there may have been an existing bias in favor of prominent batters before ABS," said Jimin Song, a graduate student at the U-M School of Kinesiology and first author of the study. "Before ABS, when a big-name batter was at bat, umpires may have given more favorable calls on borderline pitches."
Before 2024, KBO umpires called balls and strikes by eye, as they had for decades. With ABS, cameras and pitch-tracking technology determine whether a pitch crosses the strike zone. The decision is then relayed to the home plate umpire, who announces the call.
To evaluate the impact of the change, Song and colleagues compared player performance in 2023, the final season before ABS, with performance in 2024, the first season after adoption. They focused on high-status players and compared their results with those of lower-status players.
The study found that star batters experienced declines in several plate-discipline measures after ABS was introduced. Broader hitting measures were less affected, strengthening the case that the shift was tied specifically to umpiring rather than an overall decline in batting ability.
The same pattern did not appear among high-status pitchers. Song said one possible explanation is that pitchers may not have had enough opportunities to show the same kind of measurable decline, or that pitcher performance is inherently more variable.
The implications extend beyond sports. In hiring, promotions, auditions, performance reviews and other evaluations, people with higher status may receive an unconscious benefit of the doubt.
"There is a theory called the Matthew Effect, and the idea is that once you establish high status, it is very likely to be reproduced," Song said. "This study shows that automated evaluation systems can help reduce favoritism and bias in competitive settings where humans are evaluated, such as job interviews or career promotions."
More objective systems, including blind review processes or carefully designed technologies, may help reduce favoritism and create a more level playing field, the researchers said.
Major League Baseball implemented the Automated Ball-Strike Challenge System this year, though there is no timeline for transitioning to fully automated ball-and-strike calls. Song said ABS has been generally accepted in the KBO, while the rollout in MLB has been more divisive.
Richard Paulsen, senior author and U-M assistant professor of kinesiology, said that while the team did not test whether reputation-based calls changed game outcomes, it is likely that calls by star-struck officials have swayed results. However, he doesn't see human officiating ending altogether.
"We've all seen calls that have influenced outcomes at the end of the game," Paulsen said. "Some decisions made by officials, like ball-strike decisions or out-of-bounds calls, are very objective and could be automated easily.
"Human judgement, in my view, remains useful for more subjective decisions, and for that reason I do not believe we will see human officiating go away completely anytime soon. Some examples of more subjective decisions that require human judgment include foul calls in basketball, pass interference in football, penalty calls in hockey and card decisions in soccer."
