Mina Cikara, a sociologist at Carnegie Mellon University, recently attended a New York Yankee game with her husband. They are Red Sox fans and wore a hat to prove it. They were, of course, jeered, and they left the ball field stunned and wondering: Why did they torment us?

In a new research paper led by Cikara, a team of researchers have discovered a new insight into mob mentality — the propensity for groups of people to shed the inhibitions of societal and moral standards. Isolated individuals seldom heckle or riot. But throngs of sports fans torch cars, protesters storm government offices, and gangs go to war over intangible slights. Cikara and her colleagues may have discovered a culprit we can't control: our brains.

"Although humans exhibit strong preferences for equity and moral prohibitions against harm in many contexts, people's priorities change when there is an 'us' and a 'them,'" co-author Rebecca Saxe, an associate professor of cognitive neuroscience at MIT, told MIT NewsThe groups's paper appeared recently in the journal Neuroimage. "A group of people will often engage in actions that are contrary to the private moral standards of each individual in that group, sweeping otherwise decent individuals into 'mobs' that commit looting, vandalism, even physical brutality."

Some of the apparent causes of mob mentality are well understood. Anonymity is one: Yankee fans can disappear into a blur of blue pinstripes. "I have never been called names like that in my entire life," says Cikara of her New York visit. (She has completed post-doctoral research at the Massachusetts Institute of Technology, in Cambridge.) When people feel they will not be recognized or called to answer for their actions, they are more likely to behave wantonly. Another factor is responsibility. Guilt, shared collectively and spread thin, is an easier thing to stomach.

Yet, there may be something else going on: something less subjective and more quantifiable. Cikara and her colleagues wanted to find out whether an individual's sense of self — and therefore the individual's moral compass — is diminished during times of collective endeavor. But how do you measure sense of self? A functional magnetic resonance imaging brain scan, of course.

Hooking test subjects up to these machines, the scientists could monitor activity in different parts of the brain. A brain section called the medial prefrontal cortex lights up when people think about themselves and is more dormant when people act in groups, like playing on a team or rallying for a cause. During the testing, study participants were asked a series of questions about moral judgment as individuals and while competing in groups.

For some people in groups, the medial prefrontal cortex was more inactive than others. Those same people were also the least kind. After the series of questions, subjects were asked to choose pictures of teammates and of members of the opposing team to appear with the published study. The people with less self-reflection chose the most flattering pictures of themselves and their teammates and the ugliest pictures of their opponents. For shame.

"It's been hard to get a direct handle on the extent to which people within a group are tapping into their own understanding of things versus the group's understanding," David Rand, a Yale University psychologist who didn't participate in the study, told MIT News. "This is a nice way of using neuroimaging to try to get insight into something that behaviorally has been really hard to explore."

Incidentally, the people with low self-reflection had trouble remembering the questions that appeared during the study. Cikara says that's because the questions were about their own morality — something they weren't paying much attention to at the time. Which is convenient: You can't feel guilty about something if you can't remember it.

Source: Cikara M, Jenkins A, Dufour N, Sax R. Reduced self-referential neural response during intergroup competition predicts competitor harm. Neuroimage. 2014.