I came across many articles with summaries of studies that indicate that referee bias does in fact exist across many sports.
In "Basketball: Bias Refs (full article)," the blurb states:
The study found that the probability of a foul call being against a
visiting team was seven percent higher than calls against the home
team. They also noted when the home team is ahead in the game, the
likelihood of a foul being called against them is 6.3 percentage
points higher than when they were behind. Finally, economists observed
the larger the foul difference between the two teams, the more likely
it is the next call will be made against the team with fewer fouls.
Gail Imber notes in his article "Referee Bias: Quantifying the Homer Effect and Officiating Home-Field Advantage":
Specifically, Boyko postulates that "While subconscious referee bias
does not necessarily make home advantage unfair, our finding of
significant variation in home advantage by referee is hard to accept
as fair."
and
All verifiable studies decidedly concluded the "homey effect" is due
to subconscious variations in play calling—not due to a conscious
attempt to manipulate or "throw" the game.
There are links to several studies and articles in Imber's article. Some interesting excerpts:
- "Nevill asked qualified soccer referees to analyze various challenges which had been recorded on videotape, either with or
without sound. Nevill found that when the variable of crowd noise was
introduced, the referees called 15.5 percent fewer fouls against the
home team.";
- "The Los Angeles Times' Douglas Farmer summed it up as
'subconscious submission to peer pressure.'"
In the blog post titled "More evidence for referee bias in soccer," Phil Birnbaum points to two studies related to European football - in the Spanish and German leagues. He notes:
Looking at games in the Primera Division in Spain over two specific
seasons (1994-95 and 1998-99), they found that, in games where the
score difference was exactly one goal, referees awarded almost twice
as much extra time when the home team was trailing as when it was
leading. More time, of course, benefits whichever team is behind, as
it gives them a better chance to tie the game.
and
Also, the authors note that a German magazine, "Kicker Sportmagazin,"
reviews all games and posts an opinion on which penalty calls were
correct and which were incorrect (both actual and missed calls). It
turns out that for penalties called in favor of the home team, 5 out
of 55 were illegitimate. But for visiting teams, it was only 1 out of
21. So referees favored the home team by about twice as many false positives.
False negatives also favored the home team. There were 12 cases where
home team should have been awarded a penalty, but wasn't; there were
19 such cases for the visiting team.
In an article about NBA referees, James Downie notes:
They found "evidence of three biases: favoritism of home teams, teams
losing during games, and teams that are behind in a multi-game playoff
series. All three biases are plausibly profit-enhancing for the
league." The authors calculate that, during the regular season, the
turnover biases "equates to win probability changing by approximately
2.2% when a team switches from away to home status," and a further 2.5% if fouls are included. In the playoffs, the biases do not appear to affect fouls, but the effect on turnovers becomes nearly doubles,
keeping the probability change close to 5%. Maybe beating that 5% is
why coaches ask players to give 110%...
So in summary, it appears that referees across all sports do show some subconscious favoritism towards the home team, even while striving to remain impartial.
EDIT: Added some excerpts mentioned in one of my comments below.
