Abstract
Previous research has determined that home field advantage (HFA) is positively related to crowd density. Isolating this effect is a substantial empirical challenge as crowd density is endogenous with home win-likelihood via fan interest. We consider a natural-experimental setting that introduces exogenous crowd-density variation into National Football League (NFL) games. COVID-19 safety protocols allow us to disentangle crowd-presence, crowd-density, and built-environment effects upon HFA. We find strong evidence that crowd presence is a significant, substantial source of HFA, but crowd density is not. No-crowd games in 2020 featured no measurable HFA conditional upon team strengths, whereas partially fan-restricted games featured no significant decline in HFA relative to games without crowd restrictions (2016-2019 and 2021). Results suggest HFA is fully attributable to crowd presence, with no evidence of stadium familiarity or travel distance effects. Betting markets efficiently predicted the partial-crowd effect from season's outset but adjusted incrementally (behaviorally) to incorporate the true no-crowd effect only by season's end.
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