 But positive economics is quite relevantly interested in studying the impact of individual feelings on behavior. Homo œconomicus may be rational without being narrowly focused on his own consumption. The analysis of labor relations, strategic interactions, transfers within the family, generous gifts require a more complex picture of human relations (Fehr and Fischbacher 2002). Reciprocity, in particular, seems to be a powerful source of motivation, leading individuals to incur substantial costs in order to reward nice partners and punish faulty partners (Fehr and Gachter 2000). For an extensive survey of this branch of the economic literature, see Gérard-Varet, Kolm and Mercier-Ythier (2004).