The effects of standardized tax rates, average tax rates, and the distribution of income on tax progressivity

This study examines the changes in US individual income tax progressivity over the 1986–2003 period using the indexes developed by [Kakwani, N.C., 1976. Measurement of tax progressivity: An international comparison. Economic Journal 87(March), 71–80]. Although progressivity over this time frame has generally been studied in the literature, we provide additional insights by decomposing the changes in index values to account for the effects of concurrent changes in the standardized tax rates, average tax rates, and the income distribution. The decomposition should prove to be particularly useful when different summary indexes lead to conflicting conclusions about progressivity changes, as is often the case. From a policy standpoint, we show that it is the standardized tax rates, a derivative of the legislated tax rates, which need to be monitored and managed to offset the negative progressivity effects of increasing before-tax income inequality.