Abstract
This paper uses a lifetime income simulation model to examine the effects on inequality and progressivity of extending the time period over which income is measured. The income tax schedule typically displays increasing marginal rates, and there is a substantial amount of relative income mobility, along with a systematic variation in average incomes over the life cycle of the cohort. Simulations show that progressivity and inequality measures can often move in opposite directions, both over time for annual accounting periods, and as the length of period is gradually increased. The relationship between summary measures is complicated by the role of the aggregate tax ratio, in addition to the re‐ranking that can occur in the larger period framework. Some tax structures are found to increase in progressivity, while others show less progressivity, as the time period increases. Re‐ranking is found to increase as the accounting period increases: it is higher and increases more rapidly as the accounting period is increased for tax structures displaying more steeply rising rate structures.