Welfare Aspects of Current U.S. Corporate Taxation
分析美国公司税在战后联邦收入中占比下降的趋势，指出平均税率无法衡量税收扭曲，并引入有效税率概念，通过比较税前和税后内部收益率来评估税收对资本配置的扭曲效应。
The corporate tax has provided a steadily declining fraction of federal revenues during the postwar era, accounting for 23.0 percent during fiscal year 1966, and 14.2, 13.9, and 10.2 percent in 1971, 1976, and 1981, respectively. However, for several reasons, aggregate tax collections are of limited use in measuring the distortionary impact of the corporate tax. First, one must allow for cyclical and, potentially, long-run changes in the of profit to measure changes in average tax rates. More fundamental problems lie in attempts to relate such an average tax to the desired measure, the marginal tax rate. Assets of different age cohorts historically have faced different tax schedules because of a succession of nonretroactive tax law changes. Average tax rates on current represent an amalgam of such tax rules. Furthermore, even with a constant tax schedule, the taxable income which serves as the base of the corporate tax diverges systematically from economic income, because of the investment tax credit and the acceleration of depreciation allowances relative to actual economic depreciation. An asset typically receives deductions and credits that are more than sufficient to shelter all associated with it in the years immediately after purchase, but faces a tax base exceeding economic in more distant years, after all depreciation allowances have been used up. This implies that taxes will vary as a fraction of depending on the age structure of assets. Fast growing firms will face low and potentially negative average tax rates on current income, though facing the same tax schedule as stagnant firms with positive current tax payments. (This ignores the further complications that arise from the asymmetric treatment accorded gains and losses, which I discuss below.) Finally, there may be serious distortions in the allocation of capital within the corporate sector that aggregate tax rates, average or marginal, cannot measure. These difficulties have led to the general use and acceptance of the tax rate to measure the distortionary effects of the tax system. This is based on a comparison of beforeand after-tax internal rates of return on investment projects; letting rg and rn be these two measures, we define the effective tax to be