 The presence of a certain proportion of conditional co-operators would explain why contributions start relatively high but go down over time. Empirical investigations confirm this explanation as well as the presence of “mixed motives” (Villeval 2012). In a classic paper, James Andreoni has argued that patterns observed in the data about charitable giving from U.S. national surveys are inconsistent with a model of pure altruism so that other motivations must be included to account for these data (Andreoni 1988).