Referees favour home teams in judgment calls, particularly those that happen at a crucial stage in a game. If a batter chooses not to swing at a baseball pitch, the pitch is more likely to be called a strike if the home team is pitching. This tendency is most extreme in close games. In soccer, referees are more likely to award penalties to the home team, hand out fewer punishments for offences to home players.

Are referees deliberately biased? The authors (of Scorecasting) think not. Instead, they blame the fact that referees, like the rest of us, tend subconsciously to rely on crowdsourcing, picking up on the mood of the crowd when making their decision. “Anchoring” is the name economists give to people’s tendency to be unduly influenced by outside suggestion. Take away the crowd and the home bias shrinks, as it did a few years back when 21 Italian soccer matches were played without supporters following incidents of crowd violence. In these games the home bias declined by 23% on fouls called, by 26% for yellow cards and by a remarkable 70% for red cards, which remove a player from the game and have a particularly big impact on the result.

From The Referee's an Anchor in The Economist