A well-known fact from the theory of public goods is that voluntary provision will lead to an inefficient undersupply (Samuelson 1954). The reason is the famous free rider problem: Since, by definition of a public good, an agent can benefit from it, even if he or she has not contributed to the public good, everyone has an incentive to hope that others provide the public good. More specifically, a rational and selfish agent will only equate his or her private marginal benefits and costs of the public good, whereas efficiency requires that the sum of marginal benefits should equal the marginal costs. Thus, there exists a tension between individual and collective rationality, which is prototypical for many cooperation problems. This tension lies at the heart of the matter in such diverse areas like warfare, environmental protection, management of commons, tax compliance, corruption, voting, the participation in collective actions like demonstrations and strikes, donations to charities, teamwork, collusion between firms, embargos and consumer boycotts, and so on.