Time-consuming and costly religious rituals pose a puzzle for economists committed to rational choice theories of human behavior. We propose that either through selection or a causal relationship, the performance of religious rituals is associated with higher levels of cooperation. To test this hypothesis we design field experiments to measure the in-group cooperative behavior of members of religious and secular Israeli kibbutzim, communal societies for which mutual cooperation is a matter of survival. Our results show that religious males (the primary practitioners of collective religious ritual in Orthodox Judaism) are more cooperative than religious females, secular males and secular females. Moreover, the frequency with which religious males engage in collective religious rituals predicts well their degree of cooperative behavior.