March Madness is upon us. Baseball is just around the corner. The NFL Draft is next month. The NHL and NBA will be thinking playoffs soon. So let’s talk sports.

In particular, let’s think about home field advantage. Every sports fan knows how important it is to play your games at home. That’s why NBA and NHL players make some effort to care about the regular season–because the regular season determines home field advantage and home field (or ice or court) is a big advantage. Just look at these numbers laying out the percentage of games won by the home team in major American sports:

NCAA (basketball)        69.1%       1947-2009
NBA                                  62.7%       1946-2009
NHL                                  59.0%       1917-2009
NCAA (football)             64.1%        1869-2009
NFL                                   57.6%        1966-2009
MLB                                  54.1%        1903-2009

In every major sport playing your chances of winning are greater at home than on the road. And the percentages are even higher for the elite football (soccer) leagues around the world.

So what’s the big deal about playing at home?

In the fascinating book Scorecasting, Tobias Moskowitz and L. Jon Wertheim make a convincing case that home field advantage is real (see the numbers above, found on p. 112 in the book), but not for the reasons you imagine.

Teams win at home because of crowd support, right?

Wrong (or at least not in the way you think). In order to determine the effect of the crowd on the opposing players, we need to find moments in the game where other factors (e.g., referees, teammates, defenders) are isolated. If supportive crowds (and antagonistic crowds) got into the players psyche, we’d expect there to be a significant difference in, say, free throw shooting. But in over 23,000 NBA games played over the last two decades, Moskowitz and Wertheim found that the visiting team made 75.9% of their free throws and the home team made. . . 75.9% of their free throws (118). Absolutely identical percentages. Similarly, in the 624 games decided by shootouts in the NHL from 2005 to 2009, the home team won 49.4 percent of the games and the visiting team won 50.6 percent. At the moment when you would expect the crowd to play the biggest factor in cheering on their team, home field advantage counts for nothing. Likewise, Moskowitz and Wertheim found that punters kick for the same yards at home as they do on the road and field kickers hit with the same accuracy. In other words, when other factors are turned off and you have just a player with or against the crowd, home field seems to have no effect (122).

Well, then teams win at home because the travel is so brutal on the visiting team.

Wrong again. If this were true we would find a different win percentage for visiting teams playing “same city” games (e.g., Clippers v. Lakers, Cubs v. White Sox, Jets v. Giants). But even when the rigors of travel are eliminated, the home team wins at exactly the same rate at which they normally do (124). Moreover, there is no different in home ice advantage for Canadian hockey teams traveling to the U.S. or vice versa. Moskowitz and Wertheim also note that home field advantage in soccer is the same in small countries like Costa Rica (where the travel is minimal) as it is in large countries like Brazil or the United States where the travel is longer.

I know, home teams win more often because they benefit from a kinder schedule.

That’s partially true. The home court advantage is more pronounced in the NBA in part because when teams leave home they often play three games in four nights and back-to-back games in different cities. Moskowitz and Wertheim estimate that 21% of the home court advantage in the NBA can be attributed to the league’s scheduling (125). This same phenomenon is present in the NHL, but is not a factor in football and baseball. The fact that home ice advantage is not more pronounced in hockey suggests that the gentler home schedule isn’t the reason teams are more likely to win at home. (Note: scheduling plays a huge role in college athletics because powerhouse conferences schedule cupcakes at home at the beginning of the season. Once you take those games out of the equation, the home field advantage looks very similar to their professional counterparts.)

Teams win at home because they are well suited for their stadium and their weather, that’s it right?

Nope. Despite all the hubbub about Florida teams playing in the “frozen tundra,” Moskowitz and Wertheim found that climate is largely irrelevant (131). This doesn’t mean the weather never matters in a particular game–or that travel never matters or fans jeering the free throw shooter never matter. It means that when you compile the date from hundreds and thousands of games, these factors are simply non-factors. Even in baseball, there is little evidence that teams are effectively stacking their rosters with just the right players to take advantage of their stadium’s idiosyncrasies (134).

So how, then, do you explain home field advantage? If it’s not the weather, the stadium, the supportive fans, or the travel–and only a little bit the schedule–what is it? Why do home teams in every major sport win more often than the visiting team?

Are you ready for the answer?

Here it is: “Official bias is the most significant contributor to home field advantage” (138).

It sounds simple, but it’s true. Vocal fans make a difference, but not by getting into the head of their opponents–by getting into the head of the referees. Consider a few of the findings from Moskowitz and Wertheim:

  • In examining 750 matches from Spain’s premier league, they found that in close matches with the home team ahead, referees shortened the game and when the home team was behind they abnormally lengthened the game. There was significant official bias in the allotment of discretionary time. And strikingly, in games that were not close, there was no bias at all (140).
  • Referees also award more penalties in favor of the home team (141).
  • Baseball umpires are generally pretty fair in calling balls and strikes, except as the game gets into the later innings. Then the advantage goes to the home team. The visitors were also shown to be at a disadvantage on tough calls, like pitches on the corners or at a full count. To bolster their claim of official bias, Moskowitz and Wertheim analyzed the called balls and strikes in the stadiums that used QuesTec computer technology to monitor umpire calls from 2002 until the system was discontinued in 2008. In venues where the umpires knew their calls were being monitored, not only did home field advantage disappear, it swung over to the visiting team (146).
  • In the NFL, home teams receive fewer penalties per game and are charged with fewer penalty yards. With the advent of instant replay challenge in 1999, the home field advantage in the NFL has dropped from 58.5 percent to 56 percent.
  • The home team in hockey gets 20 percent fewer penalties called and receives fewer minutes in the box per game (156).
  • In the NBA, home teams shoot more free throws than do visiting teams and are less likely to be charged with traveling. Loose ball falls and offensive fouls–two of the hardest and most ambiguous calls to make–go for the home team twice the rate of other fouls (153).

To be clear, Moskowitz and Wertheim are not alleging any conspiracy against the visiting team. No one is instructing the referees to favor the home team. In fact, they are most likely unaware they are doing so. But it seems that the human inclination to please others and the propensity to conform, takes a toll on fallible officials. In fact, the evidence demonstrates that official bias increases with the size of the home crowd. The larger and more intense the crowd, the more advantage there is for the home team, especially in sports and in settings which allow for a lot of anxiety and a lot of discretion in decision making (e.g., extra time, yellow cards, personal fouls, end of game situations).

The bottom line: fans matter in sports. Not because the athletes find them so inspiring or so annoying. But because, unbeknownst to themselves, the refs do.