WASHINGTON » When people heard that Mitt Romney’s federal income tax rate was about 15 percent, the immediate reaction of many was to assume that their own tax rate was higher. The top marginal rate is 35 percent, after all, and the marginal rate on a couple with $70,000 in taxable income is 25 percent.

But the truth is that most households probably pay a lower rate than Romney. It is impossible to know for sure, given that he has yet to release his tax return. What is clear, though, is that a large majority of U.S. households — about two out of every three — pays less than 15 percent of income to the federal government, through either income taxes or payroll taxes.

This disconnect between what we pay and what we think we pay is nothing less than one of the country’s biggest economic problems.

Many Americans see themselves as struggling under the weight of a heavy tax burden (partly for the understandable reason that wage growth has been so weak). Yet taxes in the United States are quite low today, compared with past years or those in other countries. Most important, U.S. taxes are not sufficient to pay for the programs that many people want, like Medicare, Social Security, road construction and education subsidies.

What does this combination create? An enormous long-term budget deficit.

Together, all federal taxes equaled 14.4 percent of the nation’s economic output last year, the lowest level since 1950. Add state and local taxes, and the share nearly doubles, to about 27 percent, according to the Tax Policy Center in Washington — still lower than at almost any other point in the past 40 years.

As the economy recovers and incomes rise, tax payments will increase somewhat. But they will not keep pace with projected spending, in the form of Medicare, Medicaid and Social Security. And total taxes at current rates would still make up a smaller share of the economy than in virtually any other rich country — not just European nations but also Australia, Canada, Israel and New Zealand.

Obviously, tax increases are not the only way to solve the deficit. Spending cuts can, too. But so far, at least, many voters seem to prefer small, symbolic cuts, like those to foreign aid. Substantial cuts — be they the changes to Medicare that President Barack Obama included in his health care bill or the Medicare overhaul that Republicans prefer — tend to be politically unpopular.

Since the modern tax-cutting push began in the early 1980s, total federal tax rates have fallen for every income group. The payroll tax has risen, but declines in the income tax have more than made up for those increases. Nearly half the population now pays no federal income tax.

Most households pay less than 15 percent because of tax breaks, like the exclusion for health insurance, and because marginal rates apply to only a small part of a taxpayer’s income. On the first $70,000 of a couple’s taxable income, the federal income tax rate is 13.7 percent.

That said, taxes have fallen the most for the very affluent. Romney and his father — George W. Romney, the former automobile executive, Michigan governor and presidential candidate — do a nice job of illustrating the change.

The elder Romney, who died in 1995, paid an average federal tax rate of 37 percent in the 12 years for which he released his tax returns, according to an analysis by Joseph J. Thorndike, a columnist for Tax Notes magazine. Mitt Romney’s tax rate has been far lower, thanks mostly to the decline in taxes on stocks and other investments. The top marginal tax rate on ordinary income has also fallen sharply.

And George Romney paid a lower tax rate than most affluent Americans in the 1950s and ’60s, mainly because of deductions for his large donations to the Mormon Church. Then, a typical household near the very top of the income distribution would have paid almost 50 percent of its income in direct federal taxes, research by the economists Emmanuel Saez and Thomas Piketty has shown.

In recent years, that number has been below 30 percent.

Besides the drop in tax rates, affluent households have benefited disproportionately from tax breaks and deductions. The mortgage interest deduction, widely considered a middle-class benefit, actually saves a typical middle-income household about only $200 a year, because so many families claim the standard deduction, rather than itemized ones. The average family in the top 1 percent saves more than $5,000 from the mortgage deduction.

Such breaks are probably one reason that so many people feel as if their own taxes are such a burden: They have a sense, and not incorrectly, that others are benefiting from tax breaks not available to them.

"If we had a simpler tax code," said Roberton Williams of the Tax Policy Center, "people might be more accepting of what they pay."

The group for which tax rates have fallen the least is the upper middle class: households earning between about $75,000 and $300,000 a year. Their tax rates have declined over the past few decades, but only by a couple of percentage points.

Of course, many of the people who talk publicly about taxes — economists, policy experts, journalists — happen to fall into this group, which may be yet another reason that the public debate does not always match reality.

© 2012 The New York Times Company