The disparities may be even greater for another reason. The Internal Revenue Service estimates that it is able to accurately tax 99 percent of wage income but that it captures only about 70 percent of business and investment income, most of which flows to upper-income individuals, because not everybody accurately reports such figures.

The Bush administration argued that its tax policies, despite cuts that benefited those at the top more than others, had not added to the widening gap but “made the tax code more progressive, not less.” Brookly McLaughlin, the chief Treasury Department spokeswoman, said that this year “the share of income taxes paid by lower-income taxpayers will be lower than it would have been without the tax relief, while the share of income taxes for higher-income taxpayers will be higher.”

Treasury Secretary Henry M. Paulson Jr., she noted, has acknowledged that income disparities have increased, but, along with a “solid consensus” of experts, attributed that shift largely to “the rapid pace of technological change has been a major driver in the decades-long widening of the income gap in the United States."

Others argued that public policies had played a role in the shift. Robert Greenstein, executive director of the Center on Budget and Policy Priorities, an advocacy group for the poor, said that the data understates the widening disparity between the top 1 percent and the rest of the country.

He said that in addition to rising incomes and reduced taxes, the equation should take into account cuts in fringe benefits to workers and in government services that middle-class and poor Americans rely on more than the affluent. These include health care, child care and education spending.

“The nation faces some very tough choices in coming years,” he said. “That such a large share of the income gains are going to the very top, at a minimum, raises serious questions about continuing to provide tax cuts averaging over $150,000 a year to people making more than a million dollars a year, while saying we do not have enough money” to provide health insurance to 47 million Americans and cutting education benefits.

A major issue likely to be debated in Congress in the year ahead is whether reversing the Bush tax cuts would slow investment and, if so, how much that would cost the economy.