Contemporary political theory often assumes that individuals cannot make credible commitments where substantial temptations exist to break them unless such commitments are enforced by an external agent. One such situation may occur in relation to common pool resources, which are natural or man-made resources whose yield is subtractable and whose exclusion is nontrivial (but not necessarily impossible). Examples include fisheries, forests, grazing ranges, irrigation systems, and groundwater basins. Empirical evidence, however, suggests that appropriators in common pool resources develop credible commitments in many cases without relying on external authorities. We present findings from a series of experiments exploring (1) covenants alone (both one-shot and repeated communication opportunities); (2) swords alone (repeated opportunities to sanction each other); and (3) covenants combined with an internal sword (one-shot communication followed by repeated opportunities to sanction each other).

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HE PREVALENCE OF EXTREMEobesity in the United States is increasing at a rate greater than moderate obesity. 1 , 2 Unfortunately, lifestyle therapy is generally insufficient as a weight management intervention for patients who are extremely obese. To date, effective long-term weight loss through pharmacological therapy has been marginal, leaving bariatric For editorial comment see p 1160.

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A growing number of field and experimental studies focus on the institutional arrangements by which individuals are able to solve collective action problems. Important in this research is the role of reciprocity and institutions that facilitate cooperation via opportunities for monitoring, sanctioning, and rewarding others. Sanctions represent a cost to both the participant imposing the sanction and the individual receiving the sanction. Rewards represent a zero sum transfer from participants giving to those receiving rewards. We contrast reward and sanction institutions in regard to their impact on cooperation and efficiency in the context of a public goods experiment.