Before COVID-19 halted public life as we know it, professional soccer matches drew in enormous crowds. In stadiums filled with tens of thousands of people, a chorus of cheers, jeers, boos and moans would follow the ball, zigzagging from one player to another, as both teams tried to score. But in pandemic times, these games stay eerily quiet. The roaring masses are gone, and in their place, ghostly rows of empty seats remain.

This unprecedented circumstance has provided researchers with a unique opportunity to study the effect of spectators on the so-called home advantage—a well-documented phenomenon in which sports teams stand a greater chance of winning when playing in their own stadium. Support from home fans, who make up a large proportion of the crowd, has long been thought to contribute to this effect by motivating players and influencing referees’ decisions.

The benefit that fans bring is not the only explanation for the boon that accrues to home teams. It could also come from the effects of travel on the visiting opponents, the local teams’ familiarity with the venue or territoriality—a defensive response to the invasion of one’s home. Territoriality is commonly seen in animals, but researchers have found that soccer players’ testosterone levels rise more before home games than away ones, suggesting the phenomenon may also play a role in sports.

Determining the degree of these factors’ influence persists as a challenge because it is difficult to test their effects experimentally, says Daniel Memmert, a sports scientist at the German Sport University Cologne. He and his colleagues decided to take advantage of the opportunity to undertake a natural social science experiment—a large stretch of audience-free matches afforded by the pandemic—to assess how much the presence of fans influenced the outcomes of games.

The researchers examined matches played by 10 professional soccer leagues across six countries: Spain, England, Italy, Germany, Portugal and Turkey. They compared the outcomes of more than 36,000 games played under normal circumstances—from the 2010–2011 season to the start of the pandemic in the 2019–2020 season—with those from 1,006 games played without onlookers in 2020.

The analysis, published today in PLOS ONE, reveals a small but statistically insignificant drop in the home advantage. Prior to the pandemic, on average, the proportion of wins, draws and losses was 45, 27 and 28 percent, respectively. During the pandemic, without fans to fill the stadium, home teams won 43 percent of the time, drew 25 percent of the time and lost 32 percent of the time. When the researchers looked more closely at the data, they found that while there appeared to only be a minute effect on outcome, there were significant impacts on other aspects of the game. In the absence of spectators, referees were less likely to penalize the visiting teams—for example, by giving out yellow or red cards—and the home teams had less match dominance (as measured by the number of direct attempts to score a goal). “We found that spectators do not have a direct influence on the outcome of a match, but what is happening on the pitch is different,” Memmert says.

At this point, why spectators’ influences on referee behavior and match dominance did not have a significant impact on the outcomes of games is an open question. Overall, however, the results of this study suggest that factors such as familiarity and territoriality may be playing a bigger role than spectator support, Memmert says.

Dane McCarrick, a sports psychology researcher at the University of Leeds in England and a professional soccer referee, says that while the study is noteworthy, one of its potential limitations is the inclusion of all games since 2010—a lengthy stretch that needs to take into account variables such as changes in players, where games were played and rules. For example, the video assistant referee (VAR), a tool that enables referees to review incidents before making decisions, has only been introduced over the past few years. In a preprint study recently posted on PsyArXiv, McCarrick and his colleagues also examined the effect of the pandemic on games played by 15 European soccer leagues during the 2019–2020 season—and came to a different conclusion. They found that the chances of a home team win decreased during spectatorless matches.

Sandy Wolfson, a sports and exercise psychologist at Northumbria University in England and a co-author of McCarrick’s study, notes that several groups have been looking at the empty stadiums’ effects on the home advantage. “Most of the studies I’ve looked at have suggested that the home advantage has been decreased since COVID, but it’s there,” she says, “which suggests that there are other factors at work that need to be considered.” Being able to conduct these studies has been “the only perk of this horrible pandemic, because prior to COVID, you’d never be able to convince the team to not allow their fans in,” Wolfson says. “It worked really well to our advantage.”