2The decline in the middle represents both economic progress and polarization. The shift shows progress in the sense that a larger share of Americans now live in upper-income households. Fully 21% of American adults in 2015 were upper income, compared with 14% in 1971, a 7-percentage-point increase. The increase in the share of upper-income adults was greater than the change in the opposite direction. Some 29% of U.S. adults were low income in 2015, compared with 25% in 1971.
But the data also show increasing economic polarization: As the distribution of adults thins in the middle, it is bulking up most at the extreme ends of the income distribution, the lowest and highest tiers.