Groups often accomplish things that an individual couldn’t achieve alone. But there can be a darker side to such alliances: belonging to a group makes people more likely to harm others outside the group.

“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,’” says Rebecca Saxe, PhD ’03, an associate professor of cognitive neuroscience at MIT. “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.”

Several factors play into this transformation. When people are in a group, they feel more anonymous and less likely to be caught doing something wrong. They may also feel a diminished sense of personal responsibility for collective actions.

Saxe and colleagues recently studied a third factor that cognitive scientists believe may be involved in this dynamic: the hypothesis that when people are in groups, they “lose touch” with their own morals and beliefs, so they are more likely to do things that they would normally believe are wrong. Mina Cikara, a former MIT postdoc and lead author of the paper, got the idea for the study after being heckled for wearing a Red Sox cap in Yankee Stadium.

In a study published recently in the journal NeuroImage, the researchers measured activity in a part of the brain involved in thinking about the self as subjects took part in a game that required them to press a button if they saw a phrase relating to social media. The researchers found that in some people, this brain activity was reduced when the subjects participated in the competition as part of a group rather than as individuals. Those people were more likely to harm their competitors than people who did not exhibit this decreased brain activity. (When asked to select photos that would appear with the published study, they chose the least flattering photos of the opposing team members, but not of their own teammates.)

“This process alone does not account for intergroup conflict: groups also promote anonymity, diminish personal responsibility, and encourage reframing harmful actions as necessary for the greater good,” says Cikara. “Still, these results suggest that at least in some cases, explicitly reflecting on one’s own personal moral standards may help to attenuate the influence of mob mentality.”