The shadow cost of mobile public bads

We price the transboundary externality of mobile public bads such as diseases and invasive species. We focus on the marginal cost to country B of an increase in the stock (i.e. an “outbreak”) in country A. These cross-jurisdiction marginal costs depend not only on economic, ecological, and spatial features of both jurisdictions but also on jurisdictions’ strategic reactions to the outbreak. Using a spatial dynamic game, we calculate the “cross-jurisdiction shadow costs” of an outbreak of mobile public bad under the Markov-perfect Nash equilibrium of control efforts. We find that under reasonable conditions, the source country has private incentives to control the outbreak itself, which can lead to a situation where the cross-jurisdiction shadow cost is, in fact, zero. We also derive conditions where a country optimally fails to control the outbreak (for example, damages in that country are small), in which case cross-jurisdiction shadow costs are positive. Finally, we note that since cooperative control of the mobile public bad delivers substantially higher welfare than non-cooperative control, we derive an externality pricing instrument that perfectly internalizes the externality and induces cooperative control among all countries.