If you stack up a persons
earned, income-taxable earnings in the order of each dollar earned, under a graduated ,i.e., progressive, income tax, everyone who pays any income tax at all pays exactly the same tax rate on the same taxable dollar earned. That is to say, you and I pay exactly the same rate on the first taxable dollar we earn, and though we pay a different rate on the millionth dollar we each earn, we each pay exactly the same rate on that dollar. Seen in this light, a progressive tax structure on earned income is fair.
This simple observation, however, leaves out the major component of many wealthy person's income, i.e., unearned income. How unearned income is treated may be having a major affect on income inequality, in as much as the poor and middles classes have very little to no unearned income on average, whereas top earners may derive most of their income as unearned.
I would point out that their was a brief time during the Reagan Presidency when nearly all of the progressiveness in earned income tax was taken out of the U.S. tax structure. The marginal rate in the lower brackets, including the lowest bracket, was raised slightly and the rate in the upper brackets drastically compressed and drastically lowered. This has been partly undone by subsequent tax legislation, but the structure for earned income today is still significantly less progressive than it was pre-Reagan.
I, and some others, have attributed the non-linear drift toward greater income inequality since Reagan partly to compression in marginal tax rates.
Marginal rate compression may be only a minor contributor however. One also has to also consider the gap between rates on earned versus unearned income, and the compounding over time of the effect of this gap. This would seem to be the more important factor with regard to income inequality, and the data seems to support this.
Warren Buffett proposed some time ago that the tax structure be altered so upper bracket earners not pay any less percent tax on their income than the middle class. The effect, directly or indirectly, of putting this so-called Buffett rule into action would have been to raise the tax rate on unearned income. See
http://en.wikipedia.org/wiki/Buffett_Rule
Currently, of all the industrialized nations, the U.S. compares itself to, the U.S. has by far the greatest income inequality. In terms of income inequality, the U.S. is on par with Cameroon, Madagascar, Rwanda, Uganda, and Ecuador.