The Optimum Quantity of Money: Theory and Evidence

In this paper we propose a simple and general model for computing the Ramsey optimal inflation tax, which includes several models from the previous literature as special cases. We show that it cannot be claimed that the Friedman rule is always optimal (or always non-optimal) on theoretical grounds. The Friedman rule is optimal or not, depending on conditions related to the shape of various relevant functions. One contribution of this paper is to relate these conditions to measurable variables such as the interest rate or the consumption elasticity of money demand. We find that it tends to be optimal to tax money when there are economies of scale in the demand for money (the scale elasticity is smaller than one) and/or when money is required for the payment of consumption or wage taxes. We find that it tends to be optimal to tax money more heavily when the interest elasticity of money demand is small. We present empirical evidence on the parameters that determine the optimal inflation tax. Calibrating the model to a variety of empirical studies yields a optimal nominal interest rate of less than 1%/year, although that finding is sensitive to the calibration. Our model for computing the Ramsey optimal inflation tax includes several models from the previous literature as special cases. The model highlights the various assumptions in that literature which have led to such different results, assumptions which relate to the interest and scale elasticities of money demand and how they vary with the interest rate, whether money is required to pay taxes, and the nature of transactions when interest rates are very low. Calibrating the model to a variety of empirical studies yields an optimal nominal interest rate of less than 1%/year, although that finding is sensitive to the calibration