When economists studied it, they found that the referees (using football match data) [slightly favored the home team](https://freakonomics.com/2011/12/football-freakonomics-how-advantageous-is-home-field-advantage-and-why/) likely due to the large crowd’s reactions to their calls, with meaningful differences that depended on how physically close or distant the crowd was from the playing field.
Also, in US sports, time zone differences seem to be a component (east coast teams going to the west coast perform noticeably less well than visitors whose games are in the same time zone or western teams heading east).
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