The Derivation of Social Time Preference Rates

As a measure of society's marginal rate of substitution between consumption in consecutive years, the STP rate is basic to evaluating public investment alternatives. This paper derives the STP rate as a function of two ‘observable parameters’ (the growth rates of population and consumption) and three ‘policy parameters’ (the ‘pure’ time-preference rate, the elasticity of the utility-consumption function, and a population parameter). The equality of the rate of growth and the STP rate is shown to be only a very special case. The STP rate is unaffected by the relative shares of different income groups as long as the distribution remains unchanged.