The United States and others should consider adopting a different approach to estimating costs and benefits in light of uncertainty.

Evaluating investments with long-term consequences using discount rates that decline with the time horizon, (Declining Discount Rates or DDRs) means that future welfare changes are of greater consequence in present value terms. Recent work in this area has turned towards operationalising the theory and establishing a schedule of DDRs for use in cost benefit analysis. Using US data we make the following points concerning this transition: i) model selection has important implications for operationalising a theory of DDRs that depends upon uncertainty; ii) misspecification testing naturally leads to employing models that account for changes in the interest rate generating mechanism. Lastly, we provide an analysis of the policy implications of DDRs in the context of climate change for the US and show that the use of a state space model can increase valuations by 150% compared to conventional constant discounting.JEL classification: C13, C53, Q2, Q4

The economic values of investing in long-term public projects are highly sensitive to the social discount rate (SDR). We surveyed over 200 experts to disentangle disagreement on the risk-free SDR into its component parts, including pure time preference, the wealth effect, and return to capital. We show that the majority of experts do not follow the simple Ramsey Rule, a widely used theoretical discounting framework, when recommending SDRs. Despite disagreement on discounting procedures and point values, we obtain a surprising degree of consensus among experts, with more than three-quarters finding the median risk-free SDR of 2 percent acceptable. (JEL C83, D61, D82, H43, Q58)

twelve of the authors were asked by the US Environmental Protection Agency (EPA) to provide advice on the principles to be used in discounting the benefits and costs of projects that affect future generations. Maureen L. Cropper chaired the workshop. Much of the discussion in this article is based on the authors' recommendations and advice presented at the workshop.

The last few years have witnessed important advances in our understanding of time preference and social discounting. In particular, several rationales for the use of time-varying social discount rates have emerged. These rationales range from the ad hoc to the formal, with some founded solely in economic theory while others reflect principles of intergenerational equity. While these advances are to be applauded, the practitioner is left with a confusing array of rationales and the sense that almost any discount rate can be justified. This paper draws together these different strands and provides a critical review of past and present contributions to this literature. In addition to this we highlight some of the problems with employing DDRs in the decision-making process, the most pressing of which may be time inconsistency. We clarify their practical implications, and potential pitfalls, of the more credible rationales and argue that some approaches popular in environmental economics literature are ill-conceived. Finally, we illustrate the impact of different approaches by examining global warming and nuclear power investment. This includes an application and extension of Newell and Pizer [‘Discounting the benefits of climate change mitigation : how much do uncertain rates increase valuations?’ Journal of Environmental Economics and Management 46 (2003) 52] to UK interest rate data. Copyright Springer 2005global warming, intergenerational equity, social cost benefit analysis, time inconsistency, uncertainty, time varying discount rates,