Eliminating Supportive Crowds Reduces Referee Bias

We use a series of historical natural experiments in association football (soccer) to test whether social pressure affected behaviour and outcomes. We observe how the normal advantage for the home team of playing in their own stadium was eroded behind closed doors, with no supporters. After designing a three-step sample selection and regression strategy, to get as close as possible to a causal interpretation, the standout effect of an empty stadium was that referees cautioned visiting players significantly less often, by over a third of a yellow card per match or once for every twenty-two fouls. Closed doors matches were different because referees favoured the home team less in their decision making. These results add to the literature describing how home advantage in sports decreased during the Covid-19 pandemic, though many other factors changed at that time besides the emptying stadiums.