tax that imposes a larger burden (relative to resources) on those who are richer; its opposite, a regressive tax, imposes a lesser burden on the wealthy. Tax progressivity is motivated by a belief that the urgency of spending needs declines as the level of spending increases (economists call this the declining marginal utility of consumption), so that wealthy people can afford to pay a higher fraction of their resources in taxes.

Measurement of the degree of tax progressivity is conceptually problematic. The first difficulty is in deciding the appropriate unit for measuring resources. Compare, for example, a system in which individual people's wages are taxed at a progressive rate (an “individual tax base”) to a system in which total wages earned by members of a household are combined and then taxed at a progressive rate (a “household tax base”). It is simple to construct examples in which each system can be made to look more progressive than the other, depending on the distribution of incomes within and across households and on whether progressivity is calculated by comparing individuals or comparing households. Comparison becomes even more difficult when attempting to judge progressivity across different household structures: is a household with a single earner and an income of $100,000 better or worse off than a household with two earners who have a combined income of $130,000? To measure progressivity, questions like this must be given a precise quantitative answer.

Another problem involves defining the time frame over which progressivity should be calculated; a given program may be regressive when examined using annual data but progressive when considered over a lifetime. For example, Social Security taxes in the United States are levied only up to an inflation-adjusted wage cap, meaning that wages beyond the cap are free of this particular tax. Considered on its own, then, the Social Security tax appears regressive because low-wage earners pay proportionately more of their income in Social Security taxes. However, payment of Social Security taxes entitles the taxpayer to future benefits that are strongly progressive, and, over the course of an entire lifetime, low-wage workers obtain a better return on their Social Security contributions than do high-wage workers. From a lifetime perspective, therefore, the U.S. Social Security tax is progressive, even though at a point in time it looks regressive. (See Social Security Act.)

There is a generally acknowledged trade-off between the degree of progressivity and economic efficiency. At the extreme end of progressivity is communism, in which salaries are nearly equalized. Equalization of salaries, however, reduces the incentive to work and can lead to stagnation and inefficiency. How to draw the right balance between equity and efficiency is a matter of perpetual debate in democratic societies. Tax codes in all developed countries promote a substantial degree of progressivity. Across a wide range of alternative measurements, the tax code in the United States is considered less progressive than those in most other developed countries, while tax codes in the Scandinavian countries tend to be among the most progressive.