Military leaders have long known that marching in unison makes for a tight-knit platoon. Past research by psychologist Scott Wiltermuth of the University of Southern California Marshall School of Business suggests that this cooperation emerges when the group members’ emotions are aligned. Now he finds such synchrony can also encourage aggression, according to a study published in January in the Journal of Experimental Social Psychology.

Wiltermuth and his colleagues assigned subjects to groups. The researchers gave each group a set of cups and taught them a choreographed cup-moving routine that they would perform later to music. To create an atmosphere of competition, the researchers tasked them with memorizing a list of cities—they would be tested later, and the highest-scoring groups could win $50. Then all participants put on headphones and performed the cup routine in time to the music they heard. In some groups, participants ended up moving the cups in sync with one another; in other groups, each subject heard music with varying beats and could not coordinate with other participants. After completing the cup activity, the researchers told each group they could select the music a different group would hear during its cup-moving routine. One of the options was a loud, aggravating blast of static. Teams that had moved in sync were more likely to choose the noxious noise than those that had been out of sync. A more tightly knit team, it seems, is a fiercer foe.

In a companion study, to be published in Social Influence, Wiltermuth found that members of an in-sync group were also more destructive. The groups were given live pill bugs and told to shoo them into boxes described as “exterminators” (in reality, the boxes held the bugs unharmed). When prompted by a leader, those that had moved in sync earlier drove 54 percent more insects into the extermination boxes than did out-of-sync control subjects.

Wiltermuth explains that these findings underscore the importance of questioning our actions and those of our leaders. “We are doing things we wouldn’t otherwise do, because we feel an emotional connection to our team,” he says.

This article was published in print as "Emotions in Lockstep."