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)

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Centre for Climate Change Economics and Policy Working Paper No. 195 Grantham Research Institute on Climate Change and the Environment Working Paper No. 172 The Centre for Climate Change Economics and Policy (CCCEP) was established by the University of Leeds and the London School of Economics and Political Science in 2008 to advance public and private action on climate change through innovative, rigorous research. The Centre is funded by the UK Economic and Social Research Council. Its second phase started in 2013 and there are five integrated research themes: 1. Understanding green growth and climate-compatible development 2. Advancing climate finance and investment 3. Evaluating the performance of climate policies 4. Managing climate risks and uncertainties and strengthening climate services 5. Enabling rapid transitions in mitigation and adaptation More information about the Centre for Climate Change Economics and Policy can be found at: http://www.cccep.ac.uk. The Grantham Research Institute on Climate Change and the Environment was established by the London School of Economics and Political Science in 2008 to bring together international expertise on economics, finance, geography, the environment, international development and political economy to create a worldleading centre for policy-relevant research and training. The Institute is funded by the Grantham Foundation for the Protection of the Environment and the Global Green Growth Institute. It has nine research programmes: 1. Adaptation and development 2. Carbon trading and finance 3. Ecosystems, resources and the natural environment 4. Energy, technology and trade 5. Future generations and social justice 6. Growth and the economy 7. International environmental negotiations 8. Modelling and decision making 9. Private sector adaptation, risk and insurance More information about the Grantham Research Institute on Climate Change and the Environment can be found at: http://www.lse.ac.uk/grantham.This working paper is intended to stimulate discussion within the research community and among users of research, and its content may have been submitted for publication in academic journals. It has been reviewed by at least one internal referee before publication. The views expressed in this paper represent those of the author(s) and do not necessarily represent those of the host institutions or funders.

Governments across the world are coming under increasing pressure to invest heavily in projects that have maturities of decades or even centuries. Key areas of concern include climate change mitigation, environmental and biodiversity protection, nuclear decommissioning, enhancing infrastructure and coastal defenses, and long-term health care management. Whether such projects are evaluated as being economically justifiable depends on the social discount rate (SDR) that the government deploys. This variable converts the future costs and benefits of public policy into their value today, thereby facilitating the comparison of social investments with different maturities. Critically, the result of such analysis is extremely sensitive to small changes in the choice of the SDR, yet policy guidelines differ widely across countries and international institutions. In this article, we provide a review of the academic literature on long-term SDRs, with particular emphasis on how these insights have been integrated into governmental guidance. Expected final online publication date for the Annual Review of Resource Economics, Volume 14 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.