The Multiple-Volunteers Principle

We consider mechanisms for assigning an unpleasant task among a group of agents with heterogenous abilities. We emphasize threshold rules: every agent decides whether or not to ``volunteer''; if the number of volunteers exceeds a threshold number, the task is assigned to a random volunteer; if the number is below the threshold, the task is assigned to a random non-volunteer. We show that any non-extreme threshold rule allows for a symmetric equilibrium in which every ability type is strictly better off than in a random assignment. This holds for arbitrarily high costs of performing the task. Within the class of binary-action mechanisms, some threshold rule is utilitarian optimal. The first-best can be approximated arbitrarily closely with a threshold rule as the group size tends to infinity; that is, there exist threshold numbers such that with probability arbitrarily close to 1 the task is performed by an agent with an ability arbitrarily close to the highest possible ability. The optimal threshold number goes to infinity as the group size tends to infinity.