While it is clear that the social marginal product of knowledge is
greater than the private marginal product in the no-intervention
competitive equilibrium, this does not necessarily imply that interest
rates in the socially optimal competitive equilibrium with taxes will be
higher than in the suboptimal equilibrium. In each case, the real
interest rate on loans made in units of output goods can be written as
r(t) = - (p), where p(t) = e - btDU(c(t)) is the present value price for
consumption goods at date t. When utility takes the constant elasticity
form U(c) = [c - I/( - 0), this reduces to r(t) = 8 + O(Qc). In
the linear utility case in which 0 = 0, r will equal 8 regardless of the
path for consumption and in particular will be the same in the two
equilibria. This can occur even though the marginal productivity of
knowledge differs because the price of knowledge in terms of consumption
goods (equal to the marginal rate of transformation beINCREASING
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tween knowledge and consumption goods) can vary. Holders of
knowledge earn capital gains and losses as well as a direct return equal
to the private marginal productivity of knowledge. In the case of
linear utility, these capital gains and losses adjust so that interest rates
stay the same.
This logical point notwithstanding, it is likely that interest rates will
be higher in the social optimum. On average, Uc will be higher in the
social optimum; higher initial rates of investment with lower initial
consumption must ultimately lead to higher levels of consumption. If
there is any curvature in the utility function U, so that 0 is positive,
interest rates in the optimum will be greater than in the nointervention
equilibrium. In contrast to the usual presumption, costbenefit
calculations in a suboptimal equilibrium should use a social
rate of discount that is higher than the market rate of interest.