We test a mechanism whereby groups are formed voluntarily, through the use of voting. These groups play a public-goods game, where efficiency increases with group size (up to a limit, in one treatment). It is feasible to exclude group members, to exit one's group, or to form larger groups...

We test a mechanism whereby groups are formed endogenously, through the use of voting. Once formed, groups play a public-goods game, where there are economies of scale: in two treatments the social value of an incremental contribution to the group account increases with the size of the group,...