I take a friend out to a fancy restaurant for dinner; after a discreet interval, my friend does the same. As anthropologists have long been in the habit of pointing out, the very existence of such customs—especially, the feeling that one really ought to return the favor—can’t be explained by standard economic theory, which assumes that any human interaction is ultimately a business deal and that we are all self-interested individuals trying to get the most for ourselves for the least cost or least amount of effort.
