Scorecasting: The Hidden Influences Behind How Sports Are Played and Games Are Won

by L. Jon Wertheim

  What about the NHL? By now you can probably guess what we found. Home teams in hockey get 20 percent fewer penalties called on them and receive fewer minutes in the box per penalty. (In other words, home teams are not just penalized less often but penalized for less severe violations.) The net result is that on average per game, home teams get two and a half more minutes of power play opportunities—a one-man advantage during which goals often are scored—than away teams. That is a huge advantage. To provide some perspective, the average NHL team succeeds in scoring a goal during a two-minute power play about 20 percent of the time. So if you take the power play advantage and multiply it by the 20 percent success rate (per two minutes), this gives the home team a 0.25-goal advantage per game. The average point differential between home and away teams in the NHL is 0.30 goals per game, so this alone accounts for more than 80 percent of the home ice advantage in hockey.

  But is the penalty difference driven by refereeing bias? Repeating the same exercise we conducted for the NBA, we looked at more ambiguous calls—holding, hooking, cross-checking, boarding, tripping—and found that these penalties in particular went the home team’s way. Less ambiguous calls such as too many men on the ice, illegal equipment, delay of game from sending the puck into the stands, and fighting had much less home team bias. Again, this is consistent with officiating bias—and not with tired or sloppy play from visiting teams.

  Also, don’t forget the shootout results we discussed earlier. Remember, in a shootout we found no home ice advantage. Not coincidentally, this is the only part of the game in which the referee essentially plays no role.

  The fact that we can identify an officiating bias toward the home team is unsettling—that this may be the chief reason home field advantage exists in every sport is very unsettling. But why are officials biased toward the home team?

  WHY DO OFFICIALS FAVOR THE HOME TEAM?

  First let’s be clear: Is there a conspiracy afoot in which officials are somehow instructed to rule in favor of the home team, especially since the league has an economic incentive to boost home team wins? Almost unquestionably no. We’re convinced that the vast majority of, if not all, officials are upstanding professionals, uncorrupted and incorruptible, consciously doing their best to ensure fairness. All things considered, they do a remarkable job.

  They are not, however, immune to human psychology, and that’s where we think the explanation for home team bias resides. Despite fans’ claims to the contrary, referees are, finally, human. Psychology finds that social influence is a powerful force that can affect human behavior and decisions without the subjects even being aware of it. Psychologists call this influence conformity because it causes the subject’s opinion to conform to a group’s opinion. This influence can come from social pressure or from an ambiguous situation in which someone seeks information from a group.

  In 1935, the psychologist Muzafer Sherif conducted a study about conformity, using a small point of light in an otherwise dark or featureless environment. Because of the way the human eye works, the light appears to move, but the amplitude of the movements is undefined—individual observers set their own frames of reference to judge amplitude and direction. Therefore, each individual saw the “movement” differently and to differing degrees.

  When participants were asked individually to estimate how far the light had moved, as one would expect, they gave widely varying answers. Then they were retested in groups of three. The composition of the group was manipulated; testers put together two people whose estimate of the light movement when alone was very similar and one person whose estimate was very different. Each person in the group had to say aloud how far he or she thought the light had moved. Sherif found that over numerous trials, the group converged on a common estimate. The subject whose estimate of movement had been vastly different from that of the other two in the group came to conform to the majority view.

  More important, when interviewed afterward, the subject whose initial estimate had been very different now believed his or her initial estimate was wrong. That is, that subject did not succumb to social pressure and state something he or she didn’t believe; his or her actual perception of the light’s movement had changed. The experiment demonstrated that when placed in an ambiguous situation, a person will look to others for guidance or additional information to help make the “right” decision.

  After the Sherif study, Solomon Asch, a pioneer of social psychology, conducted an experiment in which he asked participants to look at two cards and decide which line (A, B, or C) on the card on the right in the following illustration was most like the line on the card on the left.

  The answer, you probably guessed, is C. The participants, though, were asked to make this assessment in a group setting. Asch had put one unwitting subject in a room with seven confederates, or actors. The actors were told in advance how to respond. Each person in the room gave his or her answer, and the “real” participant offered his or her answer second to last. In most of the cases, the subject yielded to the majority at least once, even though he or she suspected it was wrong.

  Asked why they readily conformed to the group even though they felt the answer was wrong, most participants said that they did not really believe their answer; rather, they went along with the others for fear of being ridiculed or thought “peculiar.” A few, however, said that they really did believe the group’s answers were correct. Asch also found that subjects felt enormous stress when making these decisions; giving a response that was at odds with the majority caused anxiety, even though they knew they were right.

  The takeaway here is that human beings conform for two reasons: (1) because they want to fit in with the group and (2) because they believe the group is better informed than they are. Makes sense, right? If you are asked to make a decision and are unsure of your answer, wouldn’t you look for other cues and signals to improve that answer? And don’t you accord weight to people’s answers by the confidence with which they provide them? After a difficult test in school, who hasn’t polled other classmates for the answer to a question, paying particular attention to the responses of the known “A” students?

  Now, back to referees. When humans are faced with enormous pressure—say, making a crucial call with a rabid crowd yelling, taunting, and chanting a few feet away—it is natural to want to alleviate that pressure. By making snap-judgment calls in favor of the home team, referees, whether they consciously appreciate it or not, are relieving some of that stress. They may also be taking a cue from the crowd when trying to make the right call, especially in an uncertain situation. They’re not sure whether that tailing 95-mph fastball crossed the strike zone, but again, even if it’s subconsciously, the crowd’s reaction may provide a useful signal that changes their perception.

  If beliefs are being changed by the environment, as psychology shows, referees aren’t necessarily consciously favoring the home team but are doing what they believe is right. It’s just that their perceptions have been altered. In trying to make the right call, they are conforming to a larger group’s opinion, swayed by tens of thousands of people witnessing the exact same play they did. As the saying goes in psychology, “I’ll see it when I believe it.” Referees, it’s safe to assume, do not intend this favoritism. They’re probably not even aware of it. But it is a natural human response.

  Remember, too, that on top of the anxiety caused by passionate and sometimes angry fans, the refs receive stress from their supervisors and superiors. In a variety of ways—some subtle, some not—officials must take in cues that the league has an economic incentive for home teams to do well. If your boss sent a subtle but unmistakable message that Outcome A was preferable to Outcome B, when you were forced to make a difficult, uncertain, and quick decision, how would you be inclined to act?

  Let’s look at our previous results on referees through the lens of psychology and our understanding of the human propensity to conform. The extra injury time in soccer? It is probably a response to social pressure,
that is, the desire to please the crowd—and in some cases preserve personal safety. The strike-ball discrepancy in baseball and similar disparities in fouls and turnovers in basketball, along with penalties and turnovers in football and hockey, may also be the result of “informational conformity” in the face of social pressure—using the crowd as a cue to resolve an uncertain or ambiguous situation.

  If this is true, psychology suggests that both the crowd size and the uncertainty or ambiguity of the situation should make a difference. Home team favoritism therefore should be greater the larger and more relevant the crowd and the more ambiguous the situation. We’ve already shown in a variety of ways how the more ambiguous the call—whether it is a 90-mph pitch on the corner of the strike zone in baseball, a fumbled football, a two- or three-step move without dribbling in basketball, or a questionable check in hockey—the more severe the home advantage.

  What about the size of the crowd? Recall the original study of the Spanish La Liga. The authors found that the bias in regard to extra time was even more evident when the crowd was larger. Similarly, the studies in the English Premier League, Italian Serie A, German Bundesliga, and MLS also found that referee favoritism was more apparent when attendance was higher. Maybe most interesting was the study conducted in Germany, where many of the soccer stadiums also house a running track that acts as a moat, separating the stands from the field of play. In those stadiums, the referees are more removed from the fans. Guess what? The bias referees usually exhibit for the home team gets cut in half in those stadiums but is the same as it is in other leagues for German stadiums that do not contain a track. In the three European soccer leagues we examined, attendance also had a marked effect on the number of red and yellow cards the visiting team received relative to the home team. Other studies have also linked attendance to penalties and fouls, showing that the bias in favor of the home team grows with the crowd.

  What about the extra walks awarded to home teams and the extra strikeouts imposed on away teams by the home plate umpire? These, too, occur predominantly in high-attendance games and are not present in the games with the lowest attendance. The chart below shows the net strikeout and walk advantage to home teams from bad umpire calls, reported separately for the games with the lowest and highest attendance (bottom and top fifth of attended games). Although there is virtually no home team strikeout or walk advantage in the least-attended games, the highest fifth of attended games account for more than half of the entire strikeout and walk advantage given to home teams each season. In the highest-attended games, home teams are given 263 fewer strikeouts than their opponents. In the lowest-attended games, that falls to 33 fewer strikeouts. Similarly, the home team receives 93 more walks than the visitors from bad umpire calls in the most-attended games relative to the least-attended ones.

  HOME TEAM ADVANTAGE IN STRIKEOUTS AND WALKS FROM UMPIRE INCORRECT CALLS IN LOW AND HIGH ATTENDANCE GAMES

  In the NBA, crowd size also affects the home-away differences, particularly for the more ambiguous calls. Recall how traveling is called 15 percent less often against home players. Looking at NBA games in the bottom fifth of attendance, this discrepancy goes down to 6 percent. But if we look at the most-attended games, the home team is 28 percent less likely to be called for traveling.

  In the NHL, the bigger the crowd, the more penalties, fouls, and close calls that go against the visiting team, and once again, the effects are greatest for more ambiguous calls. Even in the NFL, in which most games are sold out, the home-away discrepancies in penalties and turnovers increase with crowd size. With virtually every discretionary official’s call—in virtually every sport—the home advantage is significantly larger when the crowd is bigger.

  In fact, in the least-attended games in each sport, the home field advantage all but vanishes. In MLB, if you look at the 20 percent least-attended games, the home field advantage is only 50.7 percent. In other words, home and away teams are about equally likely to win when the crowd is small. In the one-fifth of games with the highest attendance, however, home teams win 55 percent of the time in MLB. In the NBA, the least-attended games are won by the home team only 55 percent of the time, and the most attended games 69 percent of the time. In the NHL, the home team wins only 52 percent of the time in the lowest-attended games but 60 percent of the time in the highest-attended games. And in European soccer, the home team wins 57 percent of the time in the lowest-attended games and an astonishing 78 percent of the time in the highest-attended matches.

  Wait a second, you might say. Doesn’t this stand to reason? After all, crappy teams draw crappy crowds, so the games with the empty seats usually involve the worst teams. Never mind official bias; just look at the standings. You’d expect the Pittsburgh Pirates or the New Jersey Nets—lame teams, lame crowds—to win fewer home games than, say, the Boston Red Sox or the Los Angeles Lakers. True, but even after adjusting for the strength of the team we find similar effects. Also, it doesn’t matter as much as you might think, because when a bad team hosts a good team, attendance often spikes. When LeBron James and the Miami Heat visit Memphis or Milwaukee, the crowds swell. The worst-attended games usually involve two terrible teams, and the most-attended games feature two great teams. So it turns out there isn’t much of a difference in ability between the two teams in either case.

  Still not convinced by the psychological explanation for referee bias? Consider a final study, this one performed in 2001. Researchers recorded videos of soccer matches, focusing on tackles during the game, and showed them to two groups of referees. The first group was shown the tackles with the crowd noise audible. The second group was shown the same tackles with the crowd noise muted. Both sets of referees were asked to make calls on the tackles they saw. The referees who watched the tackles with the crowd noise audible were much more likely to call the tackles with the crowd. That is, tackles made against the home team (where the crowd complained loudly) were more likely to be called fouls and tackles made by the home team were less likely to be called fouls. The referees who viewed the tackles in silence showed no bias.

  You probably guessed correctly which group of referees made calls consistent with the actual calls made on the field. Yes, the ones who could hear the crowd noise. Not only that, but the referees watching with sound also reported more anxiety and uncertainty regarding their calls, consistent with the stress they felt from the crowd. Imagine how much more intense that stress would have been if they had been on the actual field of play.

  But perhaps the most convincing evidence for the effect of crowds on referees occurred when no fans were present. On February 2, 2007, supporters of two soccer clubs in Italy—Calcio Catania and Palermo Calcio—clashed with each other and police. It was a typical hooligan-induced riot, and following the episode the Italian government forced teams with deficient security standards at their stadiums to play their home games without any spectators present. Two economists (and soccer fanatics) from Sweden, Per Pettersson-Lidbom and Michael Priks, collected the data from the 21 soccer matches that were played before empty bleachers.

  What they found was amazing. When home teams played without spectators, the normal foul rate, yellow card, and red card advantage afforded home teams disappeared entirely. Looking at the same team with the same crew of officials, the authors found that when spectators were no longer present, the home bias in favorable calls dropped by 23 to 70 percent, depending on the type of calls (a decline of 23 percent for fouls, 26 percent for yellow cards, and 70 percent for red cards). That is, the same referee overseeing the same two teams in the same stadium behaved dramatically differently when spectators were present versus when no one was watching.

  When the economists also looked at player behavior, they found that, unlike the referees, the players did not seem to play any differently when the crowd was there yelling versus in an empty, silent stadium. Home and away players shot the same percentage of goals on target, passed with the same accuracy, and had the same number of tackles as they normally do. The
absence of the crowd did not seem to have any effect on their performance. This is in keeping with what we saw for NBA foul shooters, hockey penalty shots, and MLB batters and pitchers: Crowds don’t appear to have much effect on athletes.

  So it is that we assert that referee bias from social influence not only is present but is the leading cause of the home field advantage.

  We started with three questions that any explanation of the home field advantage must address: (1) Why does it differ across sports? (2) Why is it the same for a particular sport no matter where the game is played? (3) Why hasn’t it changed much over time?

  To answer the first question, if the refs are responsible for the home advantage, it must be the case that referees are more important or have more influence in some sports (say, soccer, in which home teams have the greatest success) than in others (such as baseball, where the advantage is weakest). As it turns out, this is precisely the case. In soccer, the official has an enormous influence on the outcome of the game. One additional penalty, free kick, or foul can easily decide a game, in which one goal is often all that separates the two teams. In basketball, which has the second highest home team advantage, the official could call a foul on almost every play. By contrast, the umpire’s role in baseball is limited relative to other sports. Most plays and most calls are fairly unambiguous; a home run is a home run—either it cleared the fence or it didn’t. Most force-outs are not close. Sure, the umpire has discretion over called balls and strikes, but more than half the time the batter swings, eliminating umpire judgment.