Cross-Disciplinary Evidence for Hyperbolic Social Discount Rates

Researchers in different disciplines have independently found evidence to suggest that the social discount rate may be hyperbolic, rather than exponential, in nature. Both behaviorist research and some empirical economics studies support the hyperbolic discounting hypothesis. U.S. and U.K. government discounting dispensations for some long-run public development projects are explicable in terms of an underlying hyperbolic social discount rate. A statistical aggregation argument demonstrates that even if the behaviorist, economics and public choice evidence for hyperbolic discounting is not conclusive, the social discount rate of a set of people who individually express time preferences best modeled by exponential discounting functions is nonetheless hyperbolic. Further empirical economics studies of discounting preferences are required. Comparisons of different national discounting practices would also be useful to see how widespread is the use of unusually low exponential rates of discount for long-run projects. The application of hyperbolic discounting would have most impact in long-run benefit-cost analyses. However, government agencies would require overwhelming evidence before switching from exponential to hyperbolic discounting.