From Harvard economics professor Greg Mankiw:
The Progressivity of the Tax System
With all the rhetoric floating around regarding the “Buffett rule,” it might be worth trying extra hard to keep an eye on the facts. Here is the progressivity of the current tax system, according to the Tax Policy Center. If you can remember only one fact, make it this one: The middle class (middle quintile) pays 14.1 percent of its income in federal taxes, while the rich (top tenth of one percent of the population) pay 30.4 percent.