When it comes to making fair decisions, the Korean Baseball Organization (KBO) uses robots. The country’s professional league added a fleet of so-called robot referees to the field during its 2024 season, hoping to level the playing field. Called the Automatic Ball-Strike (ABS) system, it uses pitch-tracking sensors and camera arrays to quickly analyze when a pitch crosses the strike zone — not weird machines wearing catcher’s gear. The ABS data is then presented to the human home plate umpire, who announces his decision based on the additional analysis.

ABS is designed to deliver more accurate decisions while reducing overall bias in games, but did it deliver? Two years later, researchers at the University of Michigan (UM) say the effects are already clear, although not necessarily what some baseball fans (or players) anticipated.

According to a study published in the journal European Sports Management Quarterlyfamous top-ranked hitters fared worse during the 2024 KBO season compared to the previous year in statistics involving strike zone judgment. Meanwhile, high-status players walked less, struck out more, and reached bases less frequently when ABS entered the equation.

“This suggests that there may have been a bias in favor of top hitters before the ABS,” Jimin Song, a UM kinesiologist and co-author of the study, said in a statement. “Before ABS, when a big-name batsman was batting, umpires could have given more favorable decisions on boundary pitches.”

While top hitters saw statistical declines in multiple plate-focused areas after the introduction of ABS, the change in longer-range hitting performance was less noticeable. Song and his colleagues believe this strengthens the theory that it is the referees’ decisions that change between seasons, and not the KBO players themselves. However, these patterns were not evident in big-name pitchers, which the researchers attributed to potentially more varied performances for the position or fewer opportunities to show decline.

“We’ve all seen calls that have influenced outcomes at the end of the game,” said study co-author and UM kinesiologist Richard Paulsen. “Some decisions made by referees, such as ball hitting decisions or off-field decisions, are very objective and could easily be automated.”

Song and Paulsen say their findings go beyond baseball. Higher status bias frequently occurs in professional life, educational institutions, and other situations involving power dynamics. Although the ABS system is unlikely to reach the office building, the team believes its study underscores the importance of working to reduce bias as much as possible through a variety of methods such as blind performance reviews and evaluations.

In the meantime, don’t expect robots to come for real umpires, even though Major League Baseball added its own ABS system this season.

“In my opinion, human judgment is still useful for more subjective decisions, and for that reason I don’t think we’ll see human arbitration disappear completely anytime soon,” Paulsen said.

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