Causal Effects of an Absent Crowd on Performances and Refereeing Decisions During Covid-19

The Covid-19 pandemic has induced worldwide natural experiments on the effects of crowds.  We exploit one of these experiments that took place over several countries in almost identical settings: professional football matches played behind closed doors within the 2019/20 league seasons.  We find large and statistically significant effects on the number of yellow cards issued by referees.  Without a crowd, fewer cards were awarded to the away teams, reducing home advantage.  These results have implications for the influence of social pressure and crowds on the neutrality of decisions.