Reciprocity is a system of obligatory gift giving: I will do you a favor or give you a gift, but you are then obligated to do an unspecified favor or give me a gift at some (possibly unspecified) point in the future. Reciprocity requires a sense of community. Kinship ties or membership in the community is needed so that the obligation of returning a favor is enforced by social forces. If a friend helped you move apartments one weekend and then helped you fix your car the next weekend, your refusal to help that person would have social repercussions. Your common friends might come to regard you as a freeloader. Social pressure may induce you to return the favor.
