From Wiltermuth and Heath, slightly edited:
The decline of the bayonet and the advent of the machine gun have made marching in step a terrible, if not suicidal, combat tactic; yet armies still train by marching in step. Similarly, religions around the world incorporate synchronous singing and chanting into their rituals. Why? We suggest that acting in synchrony with others can foster cooperation within groups by strengthening group cohesion. The widespread presence of cultural rituals involving synchrony may have evolved as partial solutions to the free-rider problem, the tendency for some individuals to shoulder less than their share of the burden of producing public goods and participating in collective action.Their abstract:
Armies, churches, organizations, and communities often engage in activities—for example, marching, singing, and dancing—that lead group members to act in synchrony with each other. Anthropologists and sociologists have speculated that rituals involving synchronous activity may produce positive emotions that weaken the psychological boundaries between the self and the group. This article explores whether synchronous activity may serve as a partial solution to the free-rider problem facing groups that need to motivate their members to contribute toward the collective good. Across three experiments, people acting in synchrony with others cooperated more in subsequent group economic exercises, even in situations requiring personal sacrifice. Our results also showed that positive emotions need not be generated for synchrony to foster cooperation. In total, the results suggest that acting in synchrony with others can increase cooperation by strengthening social attachment among group members.
Some description of the experiments:
In the first experiment an experimenter led 30 participants (60% female; mean age = 20, SD= 2.0) in groups of 3 on walks around campus. In the synchronous condition, participants walked in step. In the control condition, they walked normally. After their walk, participants completed a questionnaire designed to convince participants that they had finished the experiment....In an ostensibly separate experiment, a second experimenter conducted the Weak Link Coordination Exercise, which models situations in which group productivity is a function of the lowest level of input...the game measures expectations of cooperation.
In a second experiment groups were randomly assigned to one of four conditions: In the control condition (i.e., the no-singing, no-moving condition), participants (American students) listened to "O Canada," held a plastic cup above the table, and silently read the lyrics to the anthem. In the synchronous-singing condition, participants listened to the anthem, held the cup, and sang the words "O Canada" at the appropriate times. In the synchronous-singing-and-moving condition, participants listened to the anthem, sang the words "O Canada," and moved cups from side to side in time with the music. In the asynchronous condition, participants sang and moved cups, but participants each listened to the anthem at a different tempo, causing them to move their cups at different rates and sing "O Canada" at different times. Participants in all conditions were told that they might hear the same or different versions of "O Canada," but only participants in the asynchronous condition actually heard different versions. Participants in the two synchrony conditions cooperated more in the subsequent Weak Link Coordination Exercise described in Study 1 than did participants in the control or asynchronous conditions.
The third experiment used the same synchrony, asynchrony, and control groups as the second to show that after behaving in synchrony with others, people contribued more to a public account in a commons dilemma known as a public-goods game. Moving in synchrony boosted cooperation even when behaving cooperatively conflicted with personal self-interest.