Directed Altruism and Enforced Reciprocity in Social Networks
通过在线实地实验，将亲社会捐赠分解为基线利他、定向利他（对朋友多给52%）和未来互动效应（再增24%），发现高基线利他者的朋友也更高。
We conducted online field experiments in large real-world social networks in order to decompose prosocial giving into three components: (1) baseline altruism toward randomly selected strangers, (2) directed altruism that favors friends over random strangers, and (3) giving motivated by the prospect of future interaction. Directed altruism increases giving to friends by 52% relative to random strangers, whereas future interaction effects increase giving by an additional 24% when giving is socially efficient. This finding suggests that future interaction affects giving through a repeated game mechanism where agents can be rewarded for granting efficiency-enhancing favors. We also find that subjects with higher baseline altruism have friends with higher baseline altruism. (c) 2009 by the President and Fellows of Harvard College and the Massachusetts Institute of Technology..