Stephen Rose of the Urban Institute (not exactly a right-wing or libertarian think tank) compares recent studies measuring changes in inequality and finds that although inequality has increased the Piketty and Saez (2003) results, which generated a tremendous amount of discussion and research, are very likely over-stated.
The results from at least four studies were compared for three measures of income change: change in median incomes, share of growth captured by the top 10 percent, and the changing income share of the top 1 percent. In all cases, Piketty and Saez (2003) were the outlier, showing the most increased inequality. And in all three measures of income change , Piketty, Saez, and Zucman (2018) found much less growth in income inequality than Piketty and Saez (2003).
This brief does a meta-analysis of different findings to estimate a “consensus” level of change…I find that instead of stagnating, real median incomes grew by just over 40 percent (1 percent a year) from 1979 to 2014; the top 10 percent of the income ladder captured 45 percent of income growth from 1979 to 2014; and the share of the top 1 percent grew 3.5 percentage points.
All studies find that income inequality rose after 1979, but common perceptions that all income gain went to the top 10 percent and middle class incomes stagnated (or even declined) are wrong.
Russ Roberts also has several good videos showing how the numbers can be cut in various ways.