There is considerable income mobility in the United States. Between 50 and 80 percent of the lowest 20 percent of income earners are still part of that category 10 years later. Within 20 years, 95 percent are no longer classified among the lowest 20 percent of income earners.

Furthermore, many of the lowest income residents make dramatic gains in earnings. Nearly half of U.S. residents who began in poverty in the 1980s, reached the middle-income 20 percent by the 2000s. Almost a fourth reached the top 20 percent of the U.S. income distribution.

Why do you suppose that some economists argue against using the designations “lower class,” “middle class,” and “upper class” when discussing the U.S. income distribution?