Attendance and Home Team Bias in Major League Baseball Umpire Decisions

We study crowd?induced home?favoring umpire bias in Major League Baseball (MLB). Using game?level measures constructed from called?pitch data and run?expectancy?based evaluation metrics for 22,738 regular?season and postseason games from 2015 to 2024, we find that higher ballpark occupancy is associated with a positive and statistically significant increase in home?favoring umpire bias. In contrast, when attendance is negligible or absent (such as in the 2020 regular season) we find no systematic evidence of home bias in umpires? calls. These results are robust across a wide range of ordinary least squares (OLS) and two?stage least squares (2SLS) specifications that account for differences in pitch?tracking technology, team quality, as well as umpire and game characteristics.