Are some referees biased towards certain teams?

Are the referees biased against the home team?

I have come across many articles with abstracts of studies suggesting that referee bias indeed exists in many sports.

In "Basketball: Bias Refs (full article)" it says in the blurb:

The study found that the likelihood of fouling a visiting team was seven percent higher than calling the home team. They also found that they were 6.3 percentage points more likely to be fouled than if the home team were behind. Ultimately, the economists found that the greater the difference in fouls between the two teams, the more likely 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 Reigning Home Advantage":

In particular, Boyko posits: "While unconscious referee bias does not necessarily make home advantage unfair, it is difficult to accept as fair our finding of a significant variation in home advantage by the referee."

and

All of the verifiable studies clearly concluded that the "homely effect" was due to unconscious variations in the game prompt - not a conscious attempt to manipulate or "throw" the game.

In Imber's article you will find links to various studies and articles. Some interesting excerpts:

  1. "Nevill asked qualified soccer referees to analyze various challenges that were taped with or without audio. Nevill found that when the audience noise variables were introduced, the referees reported 15.5 percent fewer fouls on the home team. "
  2. "The Los Angeles Times' Douglas Farmer summed it up as' unconscious submission to peer pressure '."

In the blog post entitled "More evidence of referee bias in football", Phil Birnbaum refers to two studies on European football - in the Spanish and German leagues. He notes:

Looking at the games in the Spanish Primera Division in two specific seasons (1994-95 and 1998-99), they found that referees granted almost twice as much extra time for the home team in games where the difference was exactly one goal followed as it led. More time goes to the team behind it, of course, as it gives them a better chance of tying the game.

and

The authors also note that a German magazine, "Kicker Sportmagazin", reviews all games and gives an opinion on which criminal calls were correct and which were incorrect (both actual and missed calls). It turns out that penalties imposed in favor of the home team were 5 out of 55 inadmissible. For visiting teams, however, it was only 1 in 21. The referees preferred the home team by about twice as many false positives.

False negatives also favored the home team. There have been 12 cases when the home team should have received a penalty but it wasn't. There were 19 such cases for the visiting team.

In an article on NBA referees, James Downie notes:

They found "evidence of three prejudices: preference for home teams, teams that lose during games, and teams that are behind in a multi-game playoff series. All three of these prejudices are plausible for the league's profitability." The authors calculate that during the regular season sales biases correspond to "a probability of winning that changes approximately 2.2% when a team goes from away to home" and a further 2.5% when fouls are involved. In the playoffs, the bias does not seem to have an impact on fouls, but the impact on sales almost doubles, keeping the probability change close to 5%. Maybe that's why coaches ask players to give 110% if they beat that 5% ...

In summary, it seems that referees in all sports show an unconscious preference for the home team, even when they try to be impartial.

TO EDIT: Added some excerpts that were mentioned in one of my comments below.

Marcus Swope

This is a great answer to the first question, but did you come across something that explains Why do they have this tendency? I've seen some evidence that it is an unconscious thing, but I'm also curious what is causing this.

JW8

Yes, some excerpts I've seen suggested that crowd noise / peer pressure contributed to this effect: 1. "Nevill asked qualified soccer referees to analyze various challenges that had been videotaped with or without sound. Nevill noted that among the variables, the referee requested 15.5 percent fewer fouls against the home team. "; 2. "The Los Angeles Times Douglas Farmer summed it up as 'unconscious submission to peer pressure'."

Marcus Swope

It's actually very interesting, can you add it to the answer?

JW8

@MarcusSwope, comments added. Thank you for your feedback.

Matthew Read

By "more likely", I suspect that the likelihood is greater than if the margin is smaller, rather than fouls being called out against the team with fewer fouls than against the team with more fouls, as the latter would not make sense if the Margin would have increased. Confusing formulation! On another point, I wonder whether there has been a study of trained referees to identify possible influences on their decisions, and whether such training works.