Why would hockey fans greet the disappointing end of the season with violence? This is hardly the first time that riots have been sparked by a sporting event.

There are at least four factors that influence these riots. Not all of them are necessary, but their combination helps.

1) Diffusion of Responsibility Nothing promotes bad behavior like a crowd. Going back to John Darley and Bibb Latane's classic work in 1968, we know that as the size of a group gets bigger, people are less likely to take individual responsibility. In their studies, participants heard a fellow participant undergoing an epileptic seizure. When participants thought they were the only other person in the study, they were quite likely to alert the experimenter, but if they thought there were 5 other participants, then few people elected to help.

Similarly, you are unlikely to smash a car window when you are alone, but if there are many people around you smashing and burning things, it feels safer to join in. The responsibility for the actions is diffused across the group.

. I have written often in this blog about the idea of goal contagion. Research by Henk Aarts, Peter Gollwitzer, and Ran Hassin demonstrates that people adopt the taken on by people around them. There are two key influences of goal contagion here. First, sports like hockey, basketball, and football have an aggressive element to them. Watching players engage in aggressive behaviors for a few hours increases the activation of the goal to be aggressive. Second, if a member of a crowd then does something aggressive like punching another person or breaking a window, that behavior also influences the goals of people around them.

Putting this together, the presence of some aggressive and violent behavior following a sporting event can create the conditions that support a riot.

3) . Adding alcohol to the mix can also increase the tendency toward violence. Alcohol influences people's ability to inhibit themselves from performing behaviors. That is why people who have been drinking are more likely to engage in risky behaviors than people who have not been drinking.

4) History. We use cultural schemas and social norms to decide how to act in specific circumstances. One reason we hold the door for people behind us is that we have learned that holding the door is an appropriate behavior. We stand in an orderly line when ordering food at a restaurant, because that is the social norm.

Sadly, the riots that have broken out after past sporting events suggest that rioting is an appropriate response following a big sporting event.

To be clear, I don't think that anyone explicitly endorses the idea that rioting is the appropriate response. Instead, there is a cultural norm to release publicly after a big sporting event. That norm leads some people to pound on cars, strike out at other people, or destroy property. Once that behavior starts, the other three factors described here kick in to allow that behavior to spread to a group.

Ultimately, it is important to recognize that riots after sporting events have no single cause. So, there won't be a single factor that can minimize the chances of riots either.

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