A COMMON PROPERTY EXPERIMENT WITH A RENEWABLE RESOURCE

I. INTRODUCTION This series of experiments demonstrates over-harvesting of a so-called common-property renewable resource. The experiments explore two different solutions to the tragedy of the commons: group communication and the establishment of individual property rights. They can be used in economics courses at all levels, and in courses covering related topics such as environmental studies. The experiments take a total of approximately 50 minutes, depending on class size. Professors have run them in classes as small as seven and large as 75, with all students participating. II. DESCRIPTION OF THE EXPERIMENTS In Experiment I, students take the roles of joint owners of a renewable resource. A plate of M&Ms serves as the resource. At the beginning of the first period the plate holds a number of M&Ms equal to ten times the number of students in the class. This amount of M&Ms constitutes the plate's carrying capacity, so the plate can at no point hold more M&Ms. In each period, each student privately writes down the number of M&Ms he or she desires to harvest. It costs nothing to harvest up to 20 M&Ms, but no one may try to harvest more than 20 in one period. In this first experiment, students may not communicate or try to influence each other's desired harvest choice. Students reveal their desired harvests simultaneously and publicly. If the total desired harvest is less than or equal to the number of M&Ms on the plate, then each person takes his desired harvest. If the total desired harvest exceeds the number of M&Ms on the plate, then each person gets a prorated share of the total harvest, i.e. each gets his desired harvest times the number of M&Ms on the plate, divided by the total desired harvest. The M&Ms left on the plate after the harvest will reproduce, provided there exist enough M&Ms to form a viable population. At least eight M&Ms must remain after harvest to provide enough genetic diversity for reproduction. If there do remain at least eight M&Ms, then each M&M has one offspring, so that the total M&Ms on the plate will double if the carrying capacity permits. After reproduction, another period begins in which students again write down their desired harvest, and so forth. If less than eight M&Ms remain after harvest, then the population crashes, and the experiment ends. Otherwise, the experiment continues for a certain number of periods which the instructor has determined in advance, but which she does not tell the students. Given the way M&Ms reproduce, if the class as a whole harvested half of the initial M&Ms (five per person), then the plate would return to its carrying capacity in the next period, and students could again harvest five per person. A harvest totaling five M&Ms per person (not necessarily divided equally among class members) represents the maximum sustainable yield for this population. However, a student can try to harvest as many as 20 M&Ms and will likely do so if he believes the others intend to try to harvest 20, thereby crashing the population and ending the experiment. Since students cannot communicate in this first experiment, they have no way of reassuring each other that they will each restrict themselves to a low harvest that allows the population to replenish itself. To try to get around the tragedy of the commons result, Experiment II introduces communication. This second experiment differs from the first only in that students may communicate and attempt to influence each other's harvest choice. Of course, communication may not solve the tragedy of the commons. Students might still attempt large harvests in order to free-ride off the conservation efforts of others. Moreover, since the agreements students reach don't bind (students still write down their desired harvests privately) a student might not believe others' promises to restrain their harvests. A student who had no faith in others' promises to restrict their harvests would not necessarily feel bound by his promise to restrict his own. …