Today, Brazil will face off against Germany in the World Cup semi-finals, and they'll do it without their superstar Neymar, who was taken down in Brazil's game against Colombia. FIFA has decided that they won't punish Juan Camilo ZĂșñiga, whose knee-to-the-back tackle fractured Neymar's vertebrae. But some are arguing that it's the referee, not ZĂșñiga, who should be under investigation for letting the game get out of control.

Referee Carlos Velasco Carballo let 40 fouls and 64 minutes go by before handing out a yellow card. As the line goes, by keeping the cards in his pocket, Carballo fostered an environment where players didn't fear being carded and were more willing to go in hard. While no one will ever know what was going on in Carballo's head during the game, we do know a lot about how home field advantage and crowd noise—two things Carballo certainly dealt with—impact a referee's decisions. And in fact, when you look at what one might expect to happen in a World Cup game with Brazil, Carballo behaved exactly as would be predicted, calling fewer fouls on the home team and letting them play on more often than he might have otherwise.

Home field advantage is a well-documented phenomenon caused by a whole host of factors. Home teams know their stadium better, sleep in their own beds before the games, and feel more pressure to perform in front of their home town fans. Many of these factors don't apply in the World Cup, where the stadiums are new and all the teams have to travel. But one big part of home field advantage—the impact it has on how referees call games—is certainly at play here. When it comes to crowd noise, the Brazil fans have done an excellent job reminding the players, and more importantly the referees, exactly where these games are taking place.

This kind of crowd noise has been proven to impact referee decisions. In one study, researchers showed 40 referees from the North Staffordshire Referees Club in England tape of 47 different challenges. They told these referees what colors the home and away teams were wearing, and asked them to make decisions about whether a foul should have been called. During this process, half of them watched the videos in silence, while others watched with crowd noise. The referees who heard the crowd roaring were significantly less keen on calling fouls against the home team—awarding 15.5 percent fewer fouls to home players than the refs who watched the very same tackles in silence.

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In other words, it's not that a roaring crowd increases the number of fouls a referee calls on the away team, but rather it decreases their willingness to call fouls on the home team. Alan Nevill, a researcher at the University of Wolverhampton and lead author on that study, says he thinks that's exactly what we saw happen to Carballo. "If you were to analyze the game in retrospect I suspect that the number of fouls would go up—probably for both teams but definitely more for the home team," he says. "The crowd appeared to be protecting the home team from being penalized."

Another study looked at how referees award penalty kicks and determine extra time. The authors found that referees give significantly more extra time when the home team is lagging behind by a goal than they do when the home team is leading by a goal. When they looked at how many penalties they were awarded, they found that referees were far more likely to give the home team a shot than the away team—something we may have seen at work when Brazil was awarded a penalty against Croatia in the very first game.

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Studying this kind of bias in the World Cup is hard, Nevill points out, because the tournament isn't at all like a regular season league, where teams play an equal number of home and away games. On the other hand, home field advantage at the World Cup may be amplified: the home team is always the home team, the crowds are louder, and the pressure on each game is higher than any regular season game. Matthias Sutter, a professor of economics at the European University Institute Florence, who conducted the study on extra time and penalty kicks, thinks that this might increase the effects of crowd noise and referee bias.

"In particular in the elimination rounds the pressure is extremely high," he says, "and in these situations the home team might get the little extra from favorable referee decisions that is needed to win close games."

Referees are, of course, trained to be impartial. They're well aware of how a home crowd might impact their decisions. And while home field advantage has been shrinking—likely due to better referee training and sports psychology—there is no way to remove the fact that refs are human, swayed by the conditions around them. Aside from replacing referees with robots, there's no way to completely eliminate the ways in which a home crowd can bias a referee. Sutter proposes "a very unpopular alternative solution," which is to move the fans further from the pitch. But he doesn't expect any club to intentionally build a home stadium that cuts out their home field advantage any time soon.

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We may see more of Carballo. According to ESPN he's the favorite to referee the final game, which may involve Brazil once again. And like all referees in the World Cup, Carballo surely knows that every decision he makes will be picked apart and analyzed. Nevill thinks that perhaps this is part of what kept him from pulling cards out earlier in the game against Columbia. "He's aware that his decisions will be replayed, so rather than making the wrong decision he hesitated to make any decision," Nevill says. Of course, "sometimes the wrong decision is not making a decision."

When Brazil faces Germany, it will be Marco Rodriguez calling—or not calling—the fouls. Rodriguez has faced criticism of his own for missing the Suarez bite, but only time will tell how he'll handle the roar of the Brazillians.


Rose Eveleth is a writer, producer, and designer based in Brooklyn, New York. She's dabbled in everything from research on krill to animations about beer to podcasts about fake tumbleweed farms. In her spare time she makes weird paper automata and daydreams about hanging out with a pack of foxes. You can find her on Twitter @roseveleth.

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