The hypothesis is that the exchanges of goods structure the society, and especially the non-merchant exchanges. Their aim is to express unselfishness, but also to generate a moral debt that builds strong reciprocal links. A multi-agent system was built with 100 agents and a resource, the interactions being only gifts. For the simulations, the agents have different rules to perform gifts and to organize their memory. The results are analyzed by the agent as well as by the modeller, with different points of view: either local or global and with different time length.