Errors of commission, home advantage and the 2017 Lions series

New-Zealand-v-British-and-Irish-Lions-Third-Test-Eden-Park

Listen to the noise of the crowd at two pivotal moments of the 2017 Lions series, and the impression is striking.

Kyle Sinckler leaps into the tackle of Charlie Faumuina in the 77th minute of the second test, and without delay the left arm of Jérôme Garcès shoots into the Wellington sky. A roar of approval rises from the thousands of Lions fans on their quadrennial pilgrimage, matched and emulated only by that which follows Owen Farrell’s successful conversion of the resulting penalty kick.

Likewise, as Beauden Barrett kicks off in the final minutes of the deciding test and Kieran Read times his run and jump to perfection, it is their inimitable rallying cry that resounds at Eden Park. After Ken Owens’ catch in an offside position, Romain Poite reacts as swiftly as his compatriot at the Westpac Stadium – and it is the turn of the locals in the crowd to spring to their feet. After confirming his decision with TMO George Ayoub and relaying to the captains that Read’s leap has been adjudged a fair challenge in the air, Poite walks calmly towards the spot of the penalty until an interjection from Garcès himself – the assistant referee on the far side of the field, and the only official not involved in the previous consultation. “Oui, Jérôme” – this is all that is heard on the referee’s microphone for a period of around thirty seconds, at which point Poite informs Warburton and Read that in fact Owens’ actions did not constitute a deliberate attempt to play the ball. Kiwi boos and British & Irish cheers cancel each other out in equal measure, and the rest is history.


“In the NBA, there’s an unwritten directive: ‘When the game steps up, you step down.’”

In 2011, Sports Illustrated writer Jon Wertheim and behavioural economist Tobias Moskowitz set out to challenge conventional sporting wisdom. Of the many phenomena – common across all disciplines – which they address in their book, Scorecasting, the intertwined concepts of the behaviour of officials and home-field advantage are among the most compelling.

Their study of referees begins at the climax of Super Bowl XLII, and the decision of Mike Carey not to whistle for a sack of Giants quarterback Eli Manning as he attempts to evade the grasp of the Patriots’ defensive line. This non-call allowed one of the most famous plays in modern NFL history –  David Tyree’s helmet catch –  to take place. It is from this moment that they draw their premise:

“People view acts of omission – the absence of an act – as far less intrusive or harmful than acts of commission – the committing of an act – even if the outcomes are the same or worse.”

Omission bias is evident in a number of real-world scenarios (to use one of the authors’ examples, the approach of parents to the relative risks and rewards of vaccination and non-vaccination), and can be seen fairly clearly in the sporting world. The strike-zone calls of Major League Baseball umpires are contingent on the count, the identity of the hitter and whether or not the previous call was correct; NBA referees call 40% fewer offensive fouls per minute in overtime than in the rest of the game.

This is not limited to North American sports. As Wertheim and Moskowitz note, “European officials are no better at overcoming omission bias than their American counterparts. Fouls, offsides, and free kicks [in European football matches] diminish significantly as close matches draw to a close.” Moreover, there are few better examples of this concept than the controversial performance of Craig Joubert – in front of a home crowd – in the 2011 Rugby World Cup Final. When the game is on the line, officials – for better or for worse – take themselves out of the game.


Home-field advantage is a clear factor in results across every professional sport. (Per Wertheim and Moskowitz, “For nearly every rugby match in more than 125 countries dating back to as early as 1871, the home field advantage is 58 percent.”) Many potential causes are traditionally put forward – travel, crowd pressure diminishing opposition performance, crowd support augmenting home-team performance, crowd pressure impairing officials’ objectivity – but Wertheim and Moskowitz’s studies show compelling evidence for only one of these.

They begin by examining the case for home-team performance improvement – and away-team performance reduction – directly resulting from crowd support or opposition:

“How do we isolate the crowd effect from all these other potential influences on the player? We need to look at an area of the game divorced from all these factors, such as free throws. Free throws are an isolated interaction between one player—the shooter—and the crowd that is trying to distract and heckle him…Over the last two decades in the NBA, including more than 23,000 games, the free throw percentage of visiting teams is 75.9 percent and that of home teams is … 75.9 percent—identical even to the right of the decimal point.

Little in the way of persuasive evidence appears. Travel?

“[I]f you look at all these “same city” games [i.e. fixtures where teams from the same city play each other – e.g. the NBA’s Los Angeles Lakers and Los Angeles Clippers], you find that home teams have the exact same advantage they do in all the other games they host. Likewise, road teams don’t lose more often when they travel greater distances. Controlling for the quality of the opponent, the San Antonio Spurs, for example, fare no better when they take puddle-jumpers to play the Dallas Mavericks and Houston Rockets than when they make longer trips to Boston, Toronto, and Miami…As with the other sports, when nearby teams play—Oakland Raiders versus San Francisco 49ers, New York Giants versus New York Jets, Baltimore Ravens versus Washington Redskins—the home field advantage holds firm at its normal level.”

The same results are found in examination of the fortunes of NFL teams playing in unfamiliar weather conditions across 25 seasons and approximately 6,000 games: home-field advantage remains consistent regardless of this context.

It is only when they begin to look into studies that examine referee behaviour that progress is made. Referencing a study by LSE professor Natxo Palacios-Huerta, they explain:

“Examining 750 matches from Spain’s premier league, La Liga, they determined that in close matches with the home team ahead, the referees ritually shortened the game by reducing the extra time significantly. In close games in which the home team was behind, the referees lengthened the game with extra injury time. If the home team was ahead by a goal at the end of regulation, the average injury time given was barely two minutes, but if the home team was behind by a goal, the average injury time awarded was four minutes—twice as much time…They found that the exact same injury time bias in favor of the home team exists in the English Premier League, the Italian Serie A league, the German Bundesliga, the Scottish league, and even MLS in the United States.”

Wertheim and Moskowitz’s own study on the frequency of cards relative to fouls in European football displayed similar conclusions:

“Looking at more than 15,000 European soccer matches in the English Premier League, Spanish La Liga, and Italian Serie A, we found that home teams receive many fewer red and yellow cards even after controlling for the number of penalties or fouls on both teams. The dispensing of red and yellow cards has a large impact on a game’s outcome. A red card, which sends the offending player off the field, reduces a team’s chances of winning by more than 7 percent. A yellow card, which precedes a red card as a stern warning for a foul and may therefore cause its recipient to play more cautiously, reduces the chances of winning by more than 2 percent. These are large effects. When a single yellow card, followed by a red card, is given to a visiting player, it means the home team’s chance of winning, absent any other effects, jumps to 59 percent. Add the injury time, fouls, free kicks … and it suddenly isn’t so surprising that the home team in soccer wins nearly 63 percent of its games.”

…and these conclusions are in line both with NFL studies looking at the propensity of referees to award fumble recoveries to the home team, and with NBA studies looking at foul calls which are by nature more subjective:

The chance of a visiting player getting called for traveling is 15 percent higher than it is for a home team player. The fact that ambiguous fouls and turnovers tend to go the home team’s way and unambiguous ones don’t is hard to reconcile with sloppy play on the part of visiting teams. But it’s exactly what you would expect from referee bias.”

Most striking, however, is that the magnitude of this effect on games varies according to the scale 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…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…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…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…even after adjusting for the strength of the team we find similar effects.”

A controlled study of the effects of crowd volume on officials is consistent with this hypothesis:

“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.”

…while two Swedish economists noted similar findings in a study of football matches played in empty stadiums:

When home teams played without spectators, the normal foul rate, yellow card, and red card advantage afforded home teams disappeared entirely…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…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.”

The authors’ supposed reason for this? Conformity with social groups is incredibly important to human beings, and can impact decision-making on both a conscious and a sub-conscious level: referees have clear incentives (in some cases, the preservation of their own physical safety) to make decisions that align with the crowd, while the forthright expression of opinion by a surrounding group also makes those referees more likely to think that the favoured decision is the correct decision. “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.”


This research has a number of interesting implications both for how we review the events of the Lions series and how we view rugby in general. Outliers like Saracens’ rescheduled fixture against Clermont in this year’s Champions Cup become especially interesting: it is plausible given the hypotheses above that moving the fixture from Sunday to Monday – and thus taking away the home side’s 10,000-person sell-out of Allianz Park – eliminated a significant proportion of Saracens’ home advantage in the game. These hypotheses are particularly important to bear in mind entering the 2018 6 Nations: over the past two years of the competition, home teams have won 80% of matches contested between the top five sides (i.e. excluding Italy), and England are the only team in this group to record an away win.

In the context of the second and third tests of last summer’s Lions tour –  while the series result was lauded to an even greater degree for the fact that it was achieved away from home – it is certainly possible given the evidence above that the Lions’ superb support eradicated much of New Zealand’s actual home-field advantage. The actions of Garcès in the closing minutes at Westpac Stadium are certainly not an example of omission bias under the pressure of partisan Kiwi supporters, and his conduct throughout was not consistent with the theory of the nominal ‘home’ crowd exerting significant influence on an official’s decision-making. Prior knowledge of this makes his intervention in the final test even more striking: when the game steps up, Garcès persuades his colleague to step down.


In the aftermath of the drawn series, former England cricketer and current Test Match Special commentator Ed Smith penned an article on the role of the referee:

“What, in that context, constitutes good refereeing? It revolves, surely, around officiating in such a way that the “right” team wins. The rugby referee, unavoidably, now adjudicates as much as he officiates. He must curate the rules in such a way that the crowd enjoys a spectacle, the players loosely follow some kind of code and no one feels that he has reversed natural justice.”

Smith’s thesis on the role of the referee appears to be close to the conclusions of Wertheim and Moskowitz: for what, in the examples of the authors above, are the officials doing but ensuring that “the crowd enjoys a spectacle”? In the context of notably quiet New Zealand crowds, record numbers of away supporters and officials eager to take a back seat and ‘let the players decide’ the games’ pivotal moments, perhaps the hordes of Lions fans that veiled New Zealand in a sea of red deserve more credit for the series outcome than we think.

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