There will be 378 subjects. Each session will consist of 18 subjects. Subjects are initially divided between Active Participants (AP) and Passive Participants (PP). The AP earnings depend on their decisions, while the PP earnings depend on the decisions of the AP. However, the PP do not know that they are PP until the end. They therefore make hypothetical decisions throughout the experiment.
The game: Subjects are placed in groups of one, two, or three AP and one PP. Within each group, the AP start with 10€ and the PP starts with 0€. The AP then play the following game: they can choose to Pay 3€ or to Not Pay. To Pay 3€ means to get 7€ instead of 10€. If at least one AP in the group chooses to Pay, the PP gets 5€. If none of the AP chooses to Pay, then the PP gets 0€.
The experiment has three stages. In the first stage, subjects play this game in groups of one, two, and three AP (in random order). The decision that subjects make when they are in the group of one AP is called "Decision 1".
In the second stage, subjects play the same game but this time they get information about what other AP in their group chose in Decision 1. More concretely, they are told that the other AP in their group have been randomly selected from a "pool" of 6 AP. They are then told how many of these 6 AP of the pool chose to Pay in Decision 1.
The third stage uses the strategy method to elicit subjects' decision whether to Pay depending on how many of the other AP in their pool chose to Pay in Decision 1.