Sustaining cooperation through self-sorting: The good, the bad, and the conditional.

In four public-good game experiments, we study self-sorting as a means to facilitate cooperation in groups. When individuals can choose to join groups precommitted to charity, such groups sustain cooperation toward the group's local public good. By eliciting subjects' conditional contribution profiles, we find that subjects who prefer the charity groups have higher average conditional contribution levels but do not differ with respect to the slope of their profiles. The majority of subjects in both group types are conditional cooperators whose willingness to contribute is stimulated by generous group members but undermined by free-riders. Charity groups thus seem better able to sustain cooperation because they attract a greater number of more generous individuals, triggering generous responses by conditional cooperators.