Echoes: what happens when football is played behind closed doors?

We use a series of natural experiments in association football (soccer) to test whether the lack of social pressure from spectators affected behaviour and outcomes. We observe that the normal advantage to the home team from playing in their own stadium was on average eroded when they played behind closed doors, with no supporters. Among the various effects from no fans being present, visiting players were cautioned significantly less often by referees. This suggests that closed doors matches are different because referees favour the home team less in their decision making. We discuss these findings in the light of the Covid-19 pandemic that has led to the remainder of the 2019/20 European football season playing out in empty stadiums.