The middle class is receiving less of America's total income as median wages stagnate and wealth concentrates at the top, according to a new study by the Pew Research Center released on Wednesday. Since 2000, the middle class has shrunk in size and fallen backward in income and wealth. Pew's key findings:

• In 2011, 51 percent of adult Americans were in the middle class, down from 61 percent in 1971.
• From 2001-2010, the median income of the middle-class fell 5 percent, but median wealth declined by 28 percent, to $93,150 from $129,582.
• The median wealth of upper-income tier Americans was essentially unchanged-it rose by 1 percent to $574,788 from $569,905. Meantime, the wealth of lower-class Americans plunged by 45 percent, to $10,151 from $18,421.

Sen. Bernie Sanders recently has noted that the Walton family, the heirs to the Wal-Mart fortune, now own more wealth than the bottom 40 percent of Americans. But the growing gap in wealth and income in American is something he's been warning about for years.

Read the Pew report

Watch Sanders' May, 14, 2008 Senate floor speech