Etiquette, the customary code of polite behavior among members of a group, provides a means of conveying respect for others, but what is the basis for etiquette's unwritten rules? Here we show that one form of etiquette, holding a door open for another person, reflects the door holder's expectation that the person for whom he or she holds the door shares the belief that the total effort expended by the two of them will be less than the summed efforts of the two individuals acting on their own. Our observations extend recent work on effort reduction in motor control to the management of social interactions.