On optimal audit mechanisms for environmental taxes

We consider the auditing problem of an environmental enforcement agency with fixed audit resources: How to decide which firms to audit after having observed the firms' taxable emissions reports. The goal of the agency is to implement the socially efficient emissions level. The audit mechanism is the agency's sole choice variable, while other variables such as the tax rate on emissions and the fine for non-compliance are determined by other governmental actors. The fines and budget of the agency are constrained in such a way that the common random audit mechanism fails to implement socially efficient emissions. Assuming perfect information among the firms, we derive an optimal audit mechanism capable of implementing the socially efficient emissions level. The optimal audit mechanism creates a contest exploiting the strategic interdependencies between the firms, where the probability of winning (not being audited) for each firm depends on costly efforts (their taxable emissions reports).