In yet another round of the controversy over discounting in the Stern Report, Megan McArdle refers to Stern’s use of “a zero or very-near-zero discount rate”. Similarly Bjorn Lomborg refers to the discount rate as “extremely low” and Arnold Kling complains says that it’s a below-market rate.
So what is the discount rate we are talking about? Stern doesn’t pick a fixed rate but rather picks parameters that determine the discount rate in a given projection. The relevant parameters are the pure rate of time preference (delta) which Stern sets equal to 0.1 and the intertemporal elasticity of substitution (eta) which Stern sets equal to 1. The important parameter is eta, which reflects the fact that since people in the future will mostly be richer than us, additional consumption in the future is worth less than additional consumption now.
Given eta = 1, the discount rate is equal to the rate of growth of consumption per person, plus 0.1. A reasonable estimate for the growth rate is 2 per cent, so Stern would have a real discount rate of 2.1 per cent. Allowing for 2.5 per cent inflation, that’s equal to a nominal rate of 4.6 per cent. The US 10-year bond rate, probably the most directly comparable market rate, is currently 4.44 per cent; a bit above its long-run average in real terms. So, Stern’s approach produces a discount rate a little above the real bond rate.
Arguments about discounting are unlikely to be settled any time soon. There’s a strong case for using bond rates as the basis for discounting the future. There are also strong arguments against, largely depending on how you adjust for risk. But to refer to the US bond rate as “near-zero” of “extremely low” seems implausible, and to say it’s below-market is a contradiction in terms. It seems as if these writers have confused the discount rate with the rate of pure time preferences.