Sports researchers have been studying home field advantage for decades. It’s a universal phenomenon found in virtually every sport, and professional football is no exception. Home teams win 57% of all regular season games in the NFL. Measuring it is easy. The question is, what causes it?
Several studies have tested theories about crowd noise, referee bias, time zone effects, climate, and peculiarties of ballparks. But these effects have not been shown to account for much if any of HFA.
Some recent research looked at when HFA manifests itself in games. In the NBA, HFA (or HCA rather) is strongest in the beginning of the game and then diminishes as it goes on. I found the same phenomenon in the NFL. The first quarter shows the strongest HFA by far. Now, baseball research reveals the same phenomenon. It’s also been shown that HFA in the NFL is strongest between inter-conference games and weaker between intra-divisional games. With the advent of inter-league play, baseball also appears to have the same tendency.
I think these findings all point toward the same theory, namely that a significant portion of HFA comes from environmental familiarity. I’m not talking about the quirks of an outfield or the type of turf in a stadium. I’m talking about the whole picture—the way we all feel comfortable when we’re in familiar surroundings and often feel anxious in strange places.
I think that game theory can help explain why this is the case. I’m not referring to the usual run-pass or fastball-curve game theory we talk about in sports. Instead, I’m talking about natural selection and behavioral evolution. I realize this sounds a little out-there, but bear with me.
The Hawk-Dove Game
Evolutionary biologists have used game theory to help explain animal behavior since the early seventies when theorist John Maynard Smith created the Hawk-Dove game. Imagine a species of bird in which males compete for mates peacefully. Doves try to wait-out their competitor suitors calmly, just hanging around until the other doves give up and fly off. The lone male with the most patience wins the mating opportunity.
Now imagine that a male bird enters the gene pool with a hawk-like approach to competing for a mate. Hawks fight aggressively to win mating opportunities. The hawk strategy will obviously dominate the passive dove strategy because whenever a hawk encounters a dove, the dove flies off and the hawk wins. You’d think that after enough generations, the hawk behavior would completely wipe out the dove behavior from the population, but that’s not what happens.
Once there are enough hawks in the mix, hawks will begin to encounter other hawks, resulting in bloody battles that often maim and wound even the winner. This is a very costly strategy, and the result is actually an equilibrium between hawk-like and dove-like males within the species. This way hawks don't run into other hawks too often. This balance is what theorists call an Evolutionary Stable Strategy (ESS).
Here is what the payoff matrix of the game would look like, where V is the value of winning the competition, and C is the cost of a bloody battle. (An example on how to read the table: The bottom left quadrant's 0 says "the payoff to a dove when it encounters a hawk is zero.")
Hawks would always beat doves and earn value V. When doves encounter other doves they win half the time for a long-run value of V/2. When a Hawk meets another hawk they win half the time, but always have to pay the cost of the fight--C. Victory in a hawk vs. hawk conflict would be a Pyrrhic one if C is greater than V.
The "Bourgeois" Strategy
What happens if there were another strategy? Imagine that along comes a smarter bird, which sometimes acts as a hawk and sometimes as a dove based on whether he's on his own territory. If he's in competition on his home turf, he'll act as a hawk. But when he is intruding outside his territory, he'll act as a dove. The resulting dynamic is a relatively peaceful process for resolving conflicts.
This kind of behavior is exactly what we see so often in nature, from butterflies to lions, and from dogs to humans. It's why your dog spends half its time marking and expanding its territory. In fact, much of human history is a never-ending conflict over territory. Even today, at every level of society, from the urban gang to the nation-state, marking and patrolling territory is vital. Conflicts can be extremely costly, even for the winner, so nature has developed a relatively safe process for resolving them.
Theorists call this territorial strategy the 'bourgeois strategy' because it closely mimics the middle-class entrepreneurial ethic of property rights. Game theory shows us that the bourgeois strategy is dominant over both the hawk and dove strategies. Here are the payoffs when bourgeois is added to the matrix:
|Hawk||(V-C)/2||V||(V-C)/4 + V/2|
Ok, there's a lot of V's and C's and fractions in that table, and it appears complicated. Behind the notation however, it's very straightforward. For example, when bourgeois encounters a hawk (bottom-left cell), half the time it would be on the bourgeois' own territory, and we'd get hawk vs. hawk. And the half the time the bourgeois would be the intruder, and we'd and up with dove vs. hawk. The total expected payoff for bourgeois vs. hawk conflicts would be:
Repeating the same calculation for each possible bourgeois encounter produces the matrix above. Next we can solve for the equilibrium. If you do the analysis, you find that the bourgeois strategy is the dominant and stable strategy. In the long run it has higher payoffs than hawk and dove or any mix of the two. Deferring to the owner of territory is nature's less costly way of resolving conflicts.
The bourgeois strategy reflects how we generally operate as a society. It's a near-universal value to respect the territory of others. Economic or social systems that tried to ignore property ownership were unnatural, and were ultimately doomed to failure. Unfortunately, it's when both competitors believe they are fighting over their own territory that the real bloody battles occur. That's when lions will maul each other, (or when Hitler and Stalin laid waste to an entire continent). Natural selection helps us avoid costly conflicts because those who lack the instinct to respect territory will eventually cost themselves dearly.
Explaining Home Field Advantage
Hopefully, now you are beginning to see the connection to HFA. If humans have evolved with an instinct to behave differently based on territory, remnants of it would still be with us and would manifest itself even in the artificial conflicts of sports. My suspicion is that environmental familiarity triggers some kind of physiological response that heightens aggression, focus, or strength. Unfamiliar surroundings might trigger an anxiety response that softens our edge. When we are in a very familiar environment, our instincts say "be a hawk." And when we're in an unfamiliar environment, they whisper "be a dove."
Now, when I watch Ray Lewis play in Cincinnati, Cleveland and Pittsburgh each year, I don't exactly get the impression he's a dove. He'll take your head off if he can. The cognitive part of our brains would be able to overcome most of these "dovish" instincts and tell ourselves to try just as hard, but maybe not completely. Even if the bourgeois effect dulls the visitors' edge an by an imperceptibly small 0.2%, leaving the visiting team playing at 99.8%, that difference occurs in play after play. The cumulative effect is what becomes HFA, where home teams win a very noticeable 57% of the time.
A 2003 study (discussed here) found that prior to home games Canadian hockey players had highly elevated levels of testosterone, a hormone known to increase aggressiveness. So it's very plausible that physiological changes can occur based on territory, and we have reason to believe that it's familiarity that causes or enhances home advantage. Further, game theory tells us to expect a physiologically-based territorial advantage. Putting it all together, the bourgeois effect explains a lot.
I suppose the question now is, how do we design research to test the theory?