The following essay explains how religion may evolve to support cooperation among anonymous partners. It first reviews honest signalling theory, and reveals a limitation in the model’s capacity to explain large-scale cooperation. It then suggests that much cooperation is threatened by uncertainty, rather than by cheating. Finally, it explains how signalling theory can be extended to address the problem of cooperation threatened by uncertainty, ‘fragile cooperation’. The resulting extension of signalling theory—called ‘charismatic signalling’—directs attention to potential cooperative benefits from religion’s fascinating and diverse effects on the body. The charismatic signalling model is presented as a ‘how-possibly model’, not as a ‘just-so story’. The model’s interest comes from its ability to organise seemingly unrelated puzzles under a common solution, and to motivate the study of cooperative strategies harboured in shared ecologies.