Metronomes, cells, neurons, fireflies, and human beings all fall into synchrony with each other, given the opportunity. Synchrony between people appears to generate social cohesion by increasing liking and feelings of togetherness. But the function of dancing, chanting and singing is not just to produce affiliative feelings, anthropologists have speculated, but also to improve group action. The group that chants and dances together hunts well together. Direct evidence for this is sparse, as research has mainly focused on studies of pairs, the effects of bodily movement, and measured cooperation and affiliative decisions. In contrast, in our experiment, large groups of people were studied, the synchrony of their verbal behaviour alone was manipulated, and in addition to affiliation, we measured their performance on a memory task and on a group action task. Our evidence suggests that the effects of synchrony are stable across modalities, and can be generalized to larger groups.