The aim of this paper is to present an alternative methodology for discounting far distant future externalities genereted by an investment project: time-declining discount rates. First I present the experimental evidence on individuals’ time-inconsistency. Second I consider the theoretical justification for using hyperbolic discounting in a simple uncertainty framework where marginal social utility is discounted hyperbolically if the investing Government believes that social wealth might increase or decrease over future period with a small probability that wealth will deteriorate below its current level.