From a recent CBO report, here are effective tax rates (total taxes divided by total income) for 2005, the most recent year available:
Lowest quintile: 4.3 percent
Second quintile: 9.9 percent
Middle quintile: 14.2 percent
Fourth quintile: 17.4 percent
Percentiles 81-90: 20.3 percent
Percentiles 91-95: 22.4 percent
Percentiles 96-99: 25.7 percent
Percentiles 99.0-99.5: 29.7 percent
Percentiles 99.5-99.9: 31.2 percent
Percentiles 99.9-99.99: 32.1 percent
Top 0.01 Percentile: 31.5 percent
N.B.: These figures include all federal taxes, not just income taxes.
That is, even before the Obama tax hikes, the rich face average tax rates more than twice those of the middle class, and about seven times those of the lowest quintile. These data do not tell you the optimal degree of tax progressivity, but they do describe the starting point from which policy is working.
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Update: Here are the CBO data graphed for David Leonhardt's article on the Obama policy changes:
