Abstract
We ran an experiment with users of Internet forums. In a dictator game, we find that the level of altruism is positively related to the activity in the forum. In a public good game, there is no relation between cooperation in the game and contribution to the content of the forum. Subjects are not more altruistic with partners from the same forum but do cooperate more with them. These results suggest that the public good provided in Internet forums is mainly provided by a group of unconditional altruistic users, and that the sense of belonging supports the cooperation in that provision.