Mostly, they are either too poor to qualify for even the lowest tax bracket (but still pay federal payroll tax, and state or local sales taxes, gas taxes, and excise taxes), or they benefit from tax credits for the working poor, the elderly, or students, asthese charts from the Tax Policy Center show. Only 7 percent of the country is non-elderly and has no federal tax liability, and most of them make less than $20,000.
In 2011, payroll tax receipts totaled $818 billion, only $200 billion less than was brought in by the federal income tax. Those Americans who paid payroll taxes, but had no income tax liability, still pay about 15 percent of their income, higher than the 13 percent Romney pays.
As ThinkProgress has explained, more and more income has become concentrated at the top of the income scale in the last few decades, so the wealthy have paid a larger share of federal taxes, because they have a larger share of the income.