WASHINGTON — The richest 1 percent of Americans have been getting far richer over the last three decades while the middle class and poor have seen their after-tax household income only crawl up in comparison, according to a government study.

Average after-tax income for the top 1 percent of U.S. households almost quadrupled, up 275 percent, from 1979 to 2007, the Congressional Budget Office found. For people in the middle of the economic scale, after-tax income grew by just 40 percent. Those at the bottom experienced an 18 percent increase.