The chart below shows the percentage of adjusted gross income (AGI) that different income groups of Americans pay in federal income tax. We chose the top categories ($500,000 to $1,000,000, and over $1,000,000) and matched them up with the some middle class numbers ($30,000 to $40,000, and $50,000 to $60,000). The data in these sets comes from IRS Publication 1304.
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As you can see, there are two major points: one, the overall percentage both groups pay in federal income taxes has fallen over the last twenty years or so, and two, when Congress makes a major income tax policy change, it affects the top earners much more than other groups.
Adjusted gross income is defined as income minus adjustments, meaning after deductions. Of course in addition to federal income taxes there are employee contributions to Social Security, which are usually 6.2% of earnings (but are 4.2% in 2011 because of the payroll tax deduction) up to $106,800, which subsequently has a greater effect on the income below that limit than above. There's also Medicare withholding, which is 1.45% of earnings, and since both are set rates, they by nature have greater effect the less you earn.