"About 46 percent of American households will pay no federal individual income tax in 2011, roughly half of them because of structural features of the income tax that provide basic exemptions for subsistence level income and for dependents. The other half are nontaxable because tax expenditures— special provisions of the tax code that benefit selected taxpayers or activities—wipe out tax liabilities and, in the case of refundable credits, result in net payments from the government.
Most important of those tax expenditures are provisions that benefit senior citizens and low-income working families with children. While those factors particularly affect lower-income households, different provisions eliminate taxes for other households. Itemized deductions and credits for children and education are more important for middle-income households, while the relatively few high-income nontaxable households benefit most from above-the-line and itemized deductions and reduced tax rates on capital gains and dividends."
~ 'Why Some Units Pay No Income Tax," abstract, Tax Policy Center, Urban Institute and Brookings Institution, available at http://www.urban.org/