ooperation is a joint effort with costs and benefits. In the peer-to-peer file sharing system, Gnutella, the individual cost of cooperation is the contribution of computer power involved in making files available to others [1]. The benefit is being able to download files that others have contributed. For Amazon.com users, the individual cost of having book reviews to read is the effort involved in writing one. In a cost-benefit analysis, cooperation may be advantageous, but this analysis is subject to a critical "technical" problem, namely, that individuals can often benefit from the effort invested by others without paying the cost: a Gnutella user can download others' files without contributing his or her own; an Amazon buyer can leave review writing to the rest of the community. Such selfish behavior (henceforth, "free-riding") may appear beneficial to the individual and spread in the population at the expense of cooperation or prevent cooperation from emerging in the first place. Hence, we have the paradox that, although each actor strives for maximum advantage, all end up suffering (on the Internet: The Tragedy of the Digital Commons [1,2]).
This is an issue of human behavior rather than technology and has been divorced from engineering, although it has been extensively studied in various fields, including economics and evolutionary biology. In evolutionary biology, for example, it fueled debates about altruism [3] and illuminated the major transitions in evolutionary history from lower to higher levels of organization [4]. Observation, experimentation, and simulation studies have yielded valuable insights into how different factors influence the emergence and stability of cooperation in a group of selfinterested individuals. It was found that repetitive interaction [5] between individuals-as opposed to a single interaction with no future consequence-makes it advantageous to invest in fostering trust in order to enjoy its fruits in the future and