Chapters 9 and 10 try to find an explanation for persistent home-field advantage. If one were to only read two chapters, I recommend these two. First, they deal with a well-known principle with an elusive answer. Second, they form an excellent example of proper scientific process. Formulate a testable hypothesis, isolate the hypothesized mechanism as much as possible, adjust for other factors that might influence the mechanism, test your hypothesis with lots of quantitative data, then think of other possible, alternative explanations and test them too.
Both chapters use an actual game as an illustrative example, but by themselves these examples do nothing to prove the theory because they form a sample size of two. However, they provide a source for funny quotes, like this:
“The Blazers were introduced in a lifeless and staccato monotone that recalled the no-purchase-necessary-void-where-prohibited-consult-your-doctor-if-your-erection-lasts-more-than-four-hours-nobody-is-listening-to-me diclaimers at the end of commercials.”
“Then it was time to introduce YOURRRRRR SAN ANTONIOOOOOO SPURRRRRRSS!!! … as the players took the floor to thunderous applause, voluptuous dancers with black-and-silver skirts aerosoled onto their impossibly sculpted bodies did elaborate pirouettes. Charles Lindbergh was barely treated to this kind of fanfare when his plane touched down in Paris.”
That home-field advantage exists is not in dispute. Chapter 9 begins with a table listing the home winning percentage for various sports and leagues. For the NBA, it is a little over 60%. The NFL and NHL are in the upper 50s, and MLB is 54%. Soccer leagues worldwide vary from 60-70%, with an average of around 63%.
It should be acknowledged that baseball has the lowest home winning percentage even though it is the only sport on the list that has a built-in advantage for home teams – they bat last which means they know exactly how many runs are needed to win or tie before they bat in the 9th inning or extra innings. The NL I think has another advantage for home teams – a pitcher has the opportunity to complete the top of an inning before possibly being removed for a pinch hitter in the bottom of the inning. No AL vs NL data are given, so I’m not sure if that is empirically true.)
What’s remarkable is that while there is variation among the sports, the percentages are very consistent within a given sport when one looks at different historical periods, and also for different leagues of the same sport. For example, the NBA and WNBA have the same home winning percentage, as do MLB and Japanese professional baseball. The only exceptions are soccer, which has some variation, and NCAA basketball and football, which are higher than the corresponding professional leagues. My first instinct when seeing the college/pro discrepancy is that the college numbers are biased because dominant programs schedule a lot of patsy opponents on their non-conference home schedule. (The host school will pay the no-namers to come, so the no-name schools get money and their players get some exposure. The host school gets a virtually guaranteed win in front of their boosters.) In fact, when they adjust for strength of schedule, the college home winning percentages revert almost exactly to that of the corresponding pro leagues.
Here are some conventional explanations for home-field advantage that are refuted: (1) positive crowd energy gives the home team a boost, (2) faraway travel makes road teams weary, and (3) familiar surroundings or quirks of the home team’s field give them an advantage. A fourth explanation, schedule bias, has some effect, but accounts for only about 20% of home-court advantage in the NBA, the sport in which its effect is largest. Of course, in pro basketball, schedules are made by the league and are supposed to be balanced; good teams cannot choose to play inferior teams exclusively at home like in college. However, it turns out that the NBA and NHL like their home teams to win; it translates into happier fans who are more likely to buy extra merchandise on their way out or come back for another game. They favor home teams by scheduling road teams to play on back-to-back nights more often, giving rested home teams a chance to play against tired road teams. This manipulation is more difficult given the nature of MLB and NFL schedules, although the NFL is going in the direction of having a game every night of the week, especially in December after the college season finishes, leaving free nights on the television schedule in which deprived Americans would otherwise not be able to watch football.
The first explanation, positive crowd energy, is tested by isolating the effect of the crowd on a player’s performance as much as possible. In basketball, this means looking at free throw percentages; the shooter isn’t influenced by coaches, teammates, referees, or opponents, only (maybe) the crowd. But the data show that home and away free-throw percentages are exactly the same (76% in the NBA), despite beating thundersticks, waving noodles, and the “questioning of the chastity of the shooter’s sister” that routinely attempt to distract visiting players.
Finding an isolated situation in which the performance of away players is not influenced by the context of the game is more difficult in other sports, but here are a few situations that come close. In football, kickers’ and punters’ statistics are the same home and away. In hockey, home-ice advantage disappears during shootouts, although it’s still present in overtime. In baseball, pitchers’ velocity and accuracy (measured by the computerized f/x pitch data; analysts must have been salivating to get pitch-by-pitch data years before it became available) are the same home and away. (44% of pitches are in the strike zone according to the computerized pitch-tracking software. This I found to be surprisingly low, since most in-game stats I see show 60-70% strikes, but that counts balls swung at and balls put into play as strikes. I don’t have all the data I need to make this calculation, but combining the numbers above with the umpire accuracy percentages from Chapter 1, it appears that on the order of 20-25% of all major league pitches involve batters swinging at balls outside the strike zone.)
Another hypothesis I’ve heard (the authors don’t discuss this one) is that people are descended from humans who passionately protected their home territory, and thus harbor an evolutionary instinct to defend one’s home turf in competition.
So, what is driving home field advantage? This question is the title of chapter 10, which is spent by the authors making a thorough case that referee bias is the true cause. Officials apparently respond to group pressure and an innate psychological desire to please people (i.e. fans), or at least not get mugged on the way out. Here’s the key supporting evidence:
1. Across multiple sports, discrepancies in calls, favoring the home team, are more prevalent for more ambiguous decisions made by the official, like loose ball fouls in basketball, hooking and holding penalties in hockey, and close ball and strike decisions and stolen base calls in baseball. There are no discrepancies for non-judgment calls, like shot clock violations or delay of game for shooting the puck into the stands, suggesting that general sloppy play on the part of the visitors is not causing refs to make more calls against them.
2. Bias increases in situations that have a high impact on the outcome of the game, like in the 9th inning or 4th quarter of a tied game.
3. In parts of the game in which a referee has essentially no role, like the act of free throw shooting, penalty kicks in soccer, or shootouts in hockey, home and away success rates are virtually identical.
4. In soccer, the referee makes one decision every game that has nothing to do with any particular play on the field: deciding how much “injury time” to add on to the end of regulation time (the clock runs continuously after it starts, even during stoppages in play, but the referee is supposed to estimate the amount of time lost to injury and other fracases, and add it back on at the end). When the home team is trailing by a goal, referees add on average twice as much injury time (4 min vs 2) than if the home team is winning.
5. Referee bias is stronger in games with higher attendance and in stadiums in which fans are closer to the field. In a natural experiment in Italian soccer, some teams were forced to play without fans due to hooligan violence, and discrepancies in fouls between home and away teams disappeared.
6. As mentioned before, home field advantage has been remarkably consistent across decades, just as the role of officials has stayed largely the same. However, in 1999, the NFL did change one important aspect of officiating – it allowed instant replay challenges. Before 1999, home teams would fumble the ball as frequently as visitors, but somehow they managed to recover those fumbles at a higher rate. After instant replay was instituted, the advantage disappeared. However, discrepancies in penalties, which cannot be reviewed, remained exactly what they were before.
I don’t think this conclusion is really damning to referees; it just shows they are as human as the rest of us. The bias is subtle enough that it’s rarely recognized by the naked eye. (A lot of partisan fans probably smell home cooking when their team is on the road, but shrug it off at home.) Instead, officials appear to be deferring to the crowd on a few ambiguous calls each game when the crowd is most vocal, which usually corresponds to the most important times in the match.
Still, in retrospect this seems so obvious that I feel I should have suspected it before. The key is that ranking sports by the strength of their home field advantage matches up almost perfectly to ranking sports by the amount of officials’ influence on the game. Basketball refs make subjective calls or non-calls on almost every play*, as players constantly bump into each other. The question of whether a foul has been committed is usually one of degree. By contrast, in baseball there are not more than a couple bang-bang plays each game. Most umpire judgments involve pitches on the edge of the strike zone.
* Pat Riley once devised a strategy that called for his players to foul on almost every play. He reasoned that the officials weren’t psychologically willing to call that many fouls, and gambled that his team could make up for a few extra fouls by gaining an advantage on plays on which a foul wasn’t called.
However, I didn’t realize the extent of referees’ influence on soccer. Soccer is the one sport I have refereed so part of this is probably wishful thinking. (I reffed youth soccer, which was basically neutral because I got an earful from both sets of spectators.) It turns out that in addition to injury time decisions, awarding a penalty kick has a huge impact on the match. Soccer matches are naturally low scoring and the penalty conversion rate is 75%. For this reason, referees call or do not call fouls in the penalty area differently from the rest of the field, even though the rulebook doesn’t advise this, and the higher threshold for penalty fouls is widely accepted (see chapter 1, swallowing the whistle). Referees can also unduly influence outcomes with their decisions to give out yellow and red cards. Americans usually label these reprimands as “warnings” and “ejections”, respectively, but soccer aficionados prefer the British terms “caution” and “send-off”. Bon voyage!
As told in the third paragraph above, soccer home-field advantage varies throughout the world. Interestingly, Africa has the lowest, 60%, although Africa and Asia are grouped together and data only goes back to 2005. It is also widely believed that African referees call fewer fouls than Europeans, leading to rougher matches with less referee influence, possibly explaining the lower home field advantage there.
So, in a way, it is fans who make the difference. But their influence is indirect, channeled through umpires, referees, and officials.