http://economix.blogs.nytimes.com/20...no-income-tax/



From the article (hotlinks not live):

"Mr. Romney is absolutely correct that about half of American households do not pay federal income tax. (He is also tapping into a now long-running vein of conservative anger at those households.) But he is missing some crucial context on why they do not pay federal income tax.
The nonpartisan and highly respected Tax Policy Center derived the 47 percent number – it is actually 46 percent, as of 2011 – and published an excellent analysis of it last summer.
It found that about half of the households that do not pay federal income tax do not pay it because they are simply too poor. The Tax Policy Center gives as an example a couple with two children earning less than $26,400 a year: The household would pay no federal income tax because its standard deduction and other exemptions would simply erase its liability.
The other half, the Tax Policy Center found, consists of households taking advantage of tax credits and other provisions, mostly support for senior citizens and low-income working families.
Put bluntly, these are not households shirking their tax liabilities. The pool consists mostly of the poor, of relatively low-income working families and of old people. The tax code is specifically designed to reduce the burden on them."