"One idea that we seriously considered was that God, to those who believe, is a supernatural policing agent," says psychologist Azim Shariff...

"We can't compare the relative strengths of religion and civics, or draw tight analogies to real-world situations," says Shariff. "What we can do is identify that both concepts have substantial effects on prosocial behaviour."

David Sloan Wilson has written extensively about theory in this respect in "Darwin's Cathedral," and this research supports his hypothesis.

If you want to get into real detail, there's Henrich and Henrich's recent book, "Why Humans Cooperate" that details how cultural evolution works with groups: individual humans learn by imitation; when one member of a group makes a discovery that enhances survival value, and that discovery spreads through the group, the fitness of the group improves. Again, according to cultural evolution theory, religion is an example of a norm that is internalized by members of the group, thus reducing dramatically the cost of policing, that constrains individual behavior but improves group fitness.