studyIn at least one sense, the shift represents economic progress: While the share of U.S. adults living in both upper- and lower-income households rose alongside the declining share in the middle from 1971 to 2015, the share in the upper-income tier grew more.Over the same period, however, the nation’s aggregate household income has substantially shifted from middle-income to upper-income households, driven by the growing size of the upper-income tier and more rapid gains in income at the top. Fully 49% of U.S. aggregate income went to upper-income households in 2014, up from 29% in 1970. The share accruing to middle-income households was 43% in 2014, down substantially from 62% in 1970."
