ANALYSIS: Fires in the streets. Smashed windows. Flipped cars. Light poles toppled by alcohol-fuelled crowds. Philadelphia awoke this morning after the triumph of Super Bowl Sunday to a city in disarray and this vexing question: What is it about sports that makes fans riot?

Why do fans care so intensely about their teams? What is going on in their brains after a win or loss? What circumstances make a riot more likely?

For years, psychologist and sociologist have studied this phenomenon of sports fan violence and have found some interesting answers.

Researchers attribute violent behaviour to a heady mixture of factors: Intense fan identification with a team, how behaviour changes when people become part of a mob, and strong psychological and physiological responses when your team wins or loses.

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When do fans most often tend to riot?

GETTY IMAGES MINNEAPOLIS, MN - FEBRUARY 04: Nick Foles #9 of the Philadelphia Eagles raises the Vince Lombardi Trophy after defeating the New England Patriots 41-33 in Super Bowl LII at U.S. Bank Stadium on February 4, 2018 in Minneapolis, Minnesota. (Photo by Patrick Smith/Getty Images)

Sports fan violence occurs all over the world, but the American fan is unusual in a few ways.

Unlike European soccer hooliganism, in which fans of opposing teams often hurt each other, fan rioting in America is usually limited to vandalism, or violence directed at inanimate objects, notes Jeffrey Lewis, a Kent State University sociologist who has spent decades studying fan violence.

American fan riots typically occur after championship or high-stakes playoff games, said Lewis, who has collected statistics from sports riots in the 1960s and 1970s.

And when American sports fans riot, it is almost always in celebration of a victory rather than a defeat.

Why do fans feel so strongly about their teams?

"We're social creatures. We have a need to belong," said Daniel Wann, a psychology professor at Murray State University.

People often split themselves into different categories based on occupation, ethnicity, gender or other factors.

Unlike race or gender, over which people have no choice, sports fandom is like a religion: It's self-selecting but also strongly influenced by the environment, including family and other people you grow up with.

That sense of belong it bestows can often be beneficial. In a series of studies on college students, Wann has found that fans who identify strongly with a team often are less likely to feel lonely or alienated, and they have higher self-esteem.

"Just think about how we interact with each other at games - for guys especially - we hug and touch each other in ways that we often never do otherwise," said social psychologist Edward Hirt. "We all want to be part of something that transcends ourselves."

What happens in fans' brains after a win or loss?

Because of that strong identification for ardent fans, the team becomes an extension of the fan. And that can have profound effects on people's psychology and even physiology.

Trying to measure that effect, Hirt conducted a study in the 1990s on college basketball fans.

Fans who saw their team win believed they could do much better on seemingly unrelated tasks - like solving anagrams or shooting darts. Those who saw their team lose thought they would do worse.

That even applied to how attractive fans saw themselves to be.

When shown a picture of a good-looking member of the opposite sex and asked to rate their chances of scoring a date with them, fans whose team lost were much more pessimistic about their odds.

The effect manifests itself physically. Fans' testosterone levels often increase after their team wins and decrease when they lose, according to some studies. And a 2013 study found that fans of losing teams experience an urge to eat more saturated fats and sugars the day after, while winning teams' fans choose healthier foods.

So where does the urge for violence come from?

Most agree mob mentality has a lot to do with it. In study after study, psychologist have shown people often behave differently in big crowds.

"There's the contagion theory. We know people do things in crowds they would not do alone. They think they're anonymous," said Jason Lanter, a psychology professor at Kutztown University who has studied celebratory fan violence for more than a decade.

"People make poor decisions in crowds."

In crowds, people often lose their self-awareness and feel a sense of safety in numbers. "But there's also the alcohol, which adds fuel to the action," Wann said.

When people act in groups, neuroscientists have also found that their medial prefrontal cortex, responsible in part for self-reflection, is more dormant.

That lack of self-reflection seems to allow people in groups to act in ways they ordinarily would not, said Mina Cikara, a neuroscientist at Harvard, who hastened to add that being in groups also can bring out the best in us, encouraging us to donate or do collective acts of kindness.

Lewis, the sociologist who wrote a 2007 book on fan violence, believes fans riot as a way to identify with and join in the victory of their teams. The rioters, he notes, are almost always young white males, with few women or older fans involved.

"They can't throw a football 60 yards like the quarterback can, but they can throw a rock through the window or pull down a light pole," Lewis said. "To them, it becomes their feat of strength and skill."

Others believe the very nature of sports contributes to the tendency for violence.

"Viewing a sporting event serves to both build up and relieve the 'destructive energy,' " argue the authors of a 2012 academic book on the subject titled Violence and Aggression in Sporting Contests.

"For a certain subset of sports fans, witnessing violent sports is not enough to reduce such energy to tolerable levels, and only personally experienced 'aggressive acts' serve to relieve the tension built up before, during, and after an exciting sports event," conclude the authors before adding this caveat: "Or, maybe some sports fans just enjoy hurting people and breaking stuff."