This article reports small group experiments in which subjects may choose to contribute a fixed amount of money toward a monetary public good, and in which the good itself is supplied only if a specified number of contributions (or more) are made. Given the opportunity to communicate, our subjects organized themselves by specifying precisely the number of required contributors and who they would be. This organization, which we call designation of a minimal contributing set, always resulted in provision of the public good, and provision in a nearly optimal manner. In contrast, groups presented with the identical problem but not allowed to communicate failed to generate a sufficient number of contributions 35 percent of the time, and in slightly over half of the successful groups, overprovision produced inefficiency.
We present hypotheses about why designating a minimal contributing set works, and data indicating that the mechanism results in reduced normative conflict and felt risk, as well as increased efficiency. The essential property of the minimal contributing set, we hypothesize, is criticalness: the contributions of the members of the minimal contributing set are each critical to obtaining the public good the members desire, and they know it. It is reasonable (albeit not a dominant strategy) to contribute because reasonable behavior can be expected from other minimal contributing set members who are in the same situation. Unreasonableness is a problem that increases with the size of groups, but adaptations exist that, we argue, can reduce its seriousness.