In social dilemmas, there is tension between cooperation that promotes the common good and the pursuit of individual interests. International climate change negotiations provide one example: although abatement costs are borne by individual countries, the benefits are shared globally. We study a...

In public good games, voluntary contributions tend to start of high and decline as the game is repeated. If high contributors are matched, however, contributions tend to stay high. We propose a formalization predicting that high contributors will selfselect into groups committed to charitable...