We Are Not Alone: The Impact of Externalities on Public Good Provision

Public good provision is often local and also affects bystanders. Is provision harder if contributions harm bystanders, and is provision easier if outsiders gain a windfall profit? In an experiment we observe that both positive and negative externalities reduce provision levels whenever actors risk falling back behind bystanders. The mere presence of unaffected bystanders already dampens contributions. This behavior seems to result from the interplay of two motives: the desire to realize opportunities for joint gains, and concerns for comparative performance. Individual payoff comparisons to the other actors as well as to individual bystanders drive contributions down.