"What everyone around the adversaries is doing during an escalation is extremely important, from egging them on to frowning on what they are doing," says Mr. Williams. "Imagine if 60 people stood strong behind someone who was being wronged by a bully. That would stop it dead in its tracks. The problem is we are not taught this anywhere." The Gatto bill, he says, may help to create a greater culture of watchfulness among fans at sporting events, because of the reward potential, and a disincentive to take part in violence because potential perpetrators would know they are being watched.
Some major league sports teams have instituted new safety precautions. The NFL's Cincinnati Bengals set up a system for fans to text security officials about altercations, to remain uninvolved. MLB's Oakland Athletics cut off alcohol sales after the seventh inning, while security personnel and concessionaires monitor crowds for disorderly behavior.
Though the two California incidents are receiving much media attention, violence at sports stadiums is not on the upswing, experts say.
"This [San Francisco] episode and the one at Dodger Stadium should not be viewed as sign of a new trend. Crowd violence is hardly a new phenomenon," says criminologist James Fox, at Northeastern University in Boston. "These are just two extreme cases that happened to occur coincidently in the same state."
"We would echo that this is nothing new and needs to be looked at in the context of masculinity in our culture," says Mr. Chin. The bigger trend is that American men are gravitating toward more violence within the games – as seen by a desire to watch harder hits in football and by increased attendance to mixed martial arts. "It has become an issue of hypermasculinity and the question needs to be asked, 'Why are men wrapping themselves up in their teams so much that they are losing sight of having a good time?' It's no longer a family environment at some of these arenas."
Others say fan violence at sports events is an outgrowth of American society itself.
"We see it daily in popular culture, advertising, film, and sports: The big car crash, blood, sex, and gore get people's attention. We have seen so much of this in American culture that people have become desensitized to violence," says Mark Tatge, professor of journalism at DePauw University's Center for Contemporary Media.
"The John-Wayne tough-guy never-back down mentality has taken over America again," says Drexel's Mr. Williams. "Americans have learned from the age of infancy that that's who we are and what we look up to. Look what just happened in Washington with the debt-ceiling debate. Neither party wanted to back down, so the US lost its triple-A bond rating for the first time in history."