Crowd Social Pressure and Baseball Umpires’ Home Team Bias

We examine whether in-person crowds cause Major League Baseball (MLB) umpires to become more likely to respond to match-events with decisions that favor the home team. Using MLB’s Statcast data, we examine umpires’ pitch calls made when the home team is pitching, and compare them to similarly located pitches thrown by visiting team pitchers. Furthermore, we examine whether this differential was affected by the absence of crowds at MLB games during the 2020 season. During matches played in front of in-person crowds, umpires are approximately 0.28 percentage points more likely to call a pitch a strike if it is thrown by a home team pitcher. However, we find no such bias during the 2020 season, and this decline in home bias is statistically significant at the 5% significance level.