Maya Bodnick, Slowboring
Happy Juneteenth! It’s a federal holiday, so we’re off today, but enjoy our intern Maya’s first Slow Boring post.
There’s little NBA fans seem to enjoy more than complaining about the referees.
These concerns are easy to dismiss as conspiracy theories, especially when they come from the opposing team’s fans, but there is strong evidence that referees are motivated by psychological factors to favor (1) the home team, (2) the team that’s behind in a game or playoff series, and (3) superstar players.
Until recently, the NBA tolerated — or maybe even subtly encouraged — these biased calls because they were good for business. But this all changed with the rise of digital and social media. Today, fans, journalists, and academics can easily access game data and hold the NBA accountable online. The massive NBA Twitter community has become the ultimate check on bad refereeing and has forced the league to crack down on biased officiating.
Favoring the home team
In the NBA and professional sports in general, the home team has traditionally had a huge advantage:
Looking just at the NBA, in the seasons from 1999-2009, home teams won 60% of regular season games and 65% of playoff games. In their 2011 book “Scorecasting,” Yale behavioral economist Tobias Moskowitz and “Sports Illustrated” editor Jon Wertheim identified referee bias toward the home team as the primary factor in generating this home-court advantage, largely due to three distinct disparities:
More free throws. NBA referees awarded the home team one to 1.5 more free throws per game.
More favorable discretionary turnover calls. This was quantified in a 2012 study by BYU Professor Joseph Price, who found that referees granted the home team 11% more discretionary turnovers per game in the same period.
More aggressive play. The expectation of lenient officiating encourages home teams to play more aggressively, and the expectation of more harsh officiating toward visitors encourages them to play more carefully.
Moskowitz and Wertheim found that this combination of disparities gave home teams an average of over 2.5 points per game and accounted for three-quarters of the home-court advantage in the 2000s.
They also show that other commonly cited explanations — weariness from travel, unique arena characteristics that the home team is comfortable with, and unfavorable scheduling for the visitors — fall short. Visiting teams may be tired from travel, but home teams beat visiting teams at the same rate even when they are based in the same city and travel is marginal. And in the NBA, home teams don’t benefit significantly from unique arena characteristics because arenas are mostly standardized.
Moskowitz and Wertheim do agree that scheduling is an important contributing factor to home-court advantage — road teams play more tiring back-to-back games than home teams do. But the authors conclude that this only accounted for one-quarter of the home team advantage in the 2000s.
The data is clear: referee bias is the number one cause of the home-court advantage.
Other referee biases
In addition to favoring the home team, referees are biased toward the team that is losing, both in regular season games and in the playoffs. From 2002-2008, referees called 15% fewer discretionary turnovers on teams trailing by more than 10 points compared to teams that are trailing or winning by less than three. On the flip side, referees called 10% more discretionary turnovers on teams winning by four to 10 points.
This tendency has not escaped the notice of fans, who regularly complain about referees like Scott Foster (nicknamed “The Extender”), and certain games (like game 6 of the 2002 Western Conference Finals) are particularly controversial. And it’s not just about individual games — fan conspiracy theories were encouraged when disgraced former referee Tim Donaghy, who was sentenced to prison time for betting on games, alleged that the NBA fixes games to extend playoff series.
Looking at the data, it’s clear that referees do make calls that favor the team losing the series. Between 2002-2008, for each game a team was down in the series, referees called 3.4% more discretionary turnovers in favor of that team.
But this is only a small advantage. Because of the way playoff series are structured, home-court bias actually plays a much bigger role in extending playoff series, as economist Kevin Hassett has shown.
For most of NBA history, all playoff games were played, as they currently are, in the 2-2-1-1-1 format. If the visiting team is down 2-0, 2-1, or 3-2, the home team ref bias can help them catch up in games 3, 4, and 6.
From 1985-2013, the structure was slightly different and even more amenable to the home-court advantage extending series. All playoff games were played in the 2-2-1-1-1 format except for the finals, which were played in a 2-3-2 format. According to a paper by 1998 paper by FAU Professor Steven Caudill:
“…the HHAAAHH format leads to a longer series. The Second team gets to play three of the first five games at home. With a home court advantage, the Second team is more likely to win those games and the series is likely to continue.”
The old finals format allowed the lower-seeded team to play three out of five of the first games at home, compared to two out of five in the new format. Giving the weaker team the home-court advantage at the beginning of the series made it less likely that they would be eliminated early. As a result, longer series were more likely.
Also worth noting: home-court advantage decreases significantly in game 7. Once a series has reached its final game, referee bias for the home team could decline because the teams are tied and the underdog bias no longer applies.
Referees also appear to give superstar players more favorable calls.
Fans regularly circulate videos that show referees allegedly giving special treatment to the league’s best players, and there’s more than just anecdotal evidence for this. Multiple analyses of games from 2015-2019 found that the NBA’s biggest stars benefit from referee bias in two ways. First, superstars are disproportionately likely to receive incorrect “no-calls” where they commit a defensive foul but do not draw a call, which makes it less likely that they foul out of games. Second, superstars draw more favorable fouls than other players. Referees call a disproportionate amount of fouls against the players defending stars, sending them to the free throw line to shoot more.
Superstar bias becomes even more consequential in playoff games. During the fourth quarters of the 2011 playoff games, referees gave All-Stars an average of 0.32 extra free throw attempts per minute. If a star plays for all 12 minutes of the final quarter, that amounts to almost 4 additional free throw points on average — that could make or break a close game!
The psychology behind biased officiating
Referee psychology is the simplest (and least sinister) explanation for bias. Social pressure matters! Hometown crowds often scream at the referees when they make calls against the home team. Forced to make split-second decisions, referees are (perhaps unconsciously?) impacted by this pressure and make more calls in favor of the home team.
