WASHINGTON – The rich got richer in the past three decades – and the very rich got much richer – according to a new government study.

The top 1 percent of households saw their after-tax incomes grow by 275 percent from 1979 to 2007, said the study by the nonpartisan Congressional Budget Office. That was more than quadruple the growth of the rest of the top 20 percent of the population during that period.

Meanwhile, income for the 60 percent of households that make up the middle of the income scale increased by slightly less than 40 percent, the study found. The poor – the 20 percent of the population with the lowest incomes – saw just an 18 percent increase.

“As a result of that uneven income growth, the distribution of after-tax household income in the United States was substantially more unequal in 2007 than in 1979,” said the report released Wednesday. Indeed, the share of all U.S. after-tax income that went to the top 1 percent doubled during that period to more than 17 percent from about 8 percent.

The findings came as protesters have occupied parks near Wall Street, Los Angeles City Hall and elsewhere around the country, decrying the growing wealth and influence of what they call “the 1 percent.” The protesters have declared themselves “the 99 percent.”

Overall, inflation-adjusted, after-tax income for the entire population rose 62 percent from 1979 to 2007.

The report said the exact cause of the rapid income growth for the richest Americans is unclear, but researchers have some theories: soaring salaries of superstar actors, athletes and musicians; more liberal executive compensation; and the growth of the financial sector.

At the same time, “the equalizing effect of federal taxes was smaller,” the report said. The overall average federal tax rate fell slightly during the period because of income tax cuts, and the tax system did less to balance out incomes because of rate cuts for high earners.

Rep. Sander M. Levin, D-Mich., said Wednesday the report was “just the latest evidence of the alarming rise in income inequality in this country.”

“This report confirms what the American people already know: The rules have been changed by the unfair tax policies of the last decade, and our tax code is doing less to level the playing field than it was in the past,” he said.

President Obama has proposed that the tax code be changed so people earning more than $1 million a year pay at least the same tax rate as middle-class earners. He called the principle the “Buffett Rule,” after billionaire Warren Buffett, who has complained that he pays a lower tax rate than anyone in his office at Berkshire Hathaway Inc.

Republicans, including House Budget Committee Chairman Paul D. Ryan, R-Wis., have decried the measure, saying it could incite “class warfare.”