LONDON—It's official: referees are biased.

An academic report in the U.K. has revealed what many sport aficionados have always suspected: referees are influenced by their nationality when it comes to making decisions on the pitch.

After analyzing two major rugby competitions of opposing clubs from different countries—rugby union's Super 14 and rugby league's Super League—the researchers discovered that when the referee shared the same nationality as one of the teams playing, they were more prone to giving their nation's team a significant advantage.

Referees were less likely to penalize the team of their own country and would punish the opposite team, said Dr. Lionel Page, of the Judge Business School at Cambridge University.

"The situations in which we found referees to be less biased were when there was more monitoring," Dr. Page said. The researcher also said biased decisions were more likely to take place at the most important moments of a particular match when the scoreline was tight.

The study also revealed the extent of which these biased decisions impact the final outcome of a game.

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