Income inequality in the United States is high compared to other rich democratic nations, and it has risen sharply since the late 1970s. During that period, household incomes in the middle have grown slowly — much more slowly than the economy (GDP per capita or per household). It looks very likely that top-end income inequality has been a key cause of slow income growth in the middle. The two are arithmetically related, the timing fits, and the key hypothesized causal path, wages, behaves as predicted. The high and rising income share of the top 1 percent appears to have cut income growth since 1979 for the median American household roughly in half.
The full paper is here.