Decisions to donate time or money for charitable purposes are typically seen as make-or-buy decisions, implying that there should be a clear distinction between individuals engaging in one of these two forms of giving and that this distinction should be somehow linked to opportunity costs. But...

In public good games, voluntary contributions tend to start off 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 self-select into groups committed to charitable...