Mother Jones has published a video of Mitt Romney at a private fund-raiser making incendiary remarks about Obama voters – and, well, about half of the electorate.

“There are 47 percent of the people who will vote for the president no matter what,” Mr. Romney said. “There are 47 percent who are with him, who are dependent upon government, who believe that they are victims, who believe the government has a responsibility to care for them, who believe that they are entitled to health care, to food, to housing, to you-name-it, that that’s an entitlement. And the government should give it to them. And they will vote for this president no matter what.”

“These are people who pay no income tax,” he added.

I’ll address just that last part in this post.

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.

Indeed, the recession and its aftermath have left tens of millions of workers out of a job or underemployed, removing more households from payment of federal income taxes. Moreover, the Bush tax cuts – the signature Republican economic policy of the 2000s, which doubled the child tax credit, increased a number of other deductions and exemptions, and lowered marginal tax rates – erased millions of families’ federal income tax liabilities.

It is also worth noting that though tens of millions of families do not pay federal income taxes, there are virtually no families that do not pay any taxes – between payroll taxes, sales taxes, state and local taxes, and on and on.

For much more detail on the 46 (or 47) percent, read my colleague David Leonhardt’s 2010 column or my 2011 piece for Slate.