In "Unions and Inequality Over the Twentieth Century" Ilyana Kuziemko develops a new source of micro-data on union membership dating back to 1936 to examine the long-run relationship between unions and inequality. The paper documents that around the middle of the twentieth century, union households were much less educated and more non-white than other households, whereas pre-World-War-II and today they are more similar to non-union households on these dimensions. Despite these changes, the household union premium holds relatively steady between ten and twenty log points. Ilyana and coauthors provide evidence that unions reduce inequality, explaining a significant share of the dramatic fall in inequality between the mid-1930s and late 1940s.
Farber, Henry S., Daniel Herbst, Ilyana Kuziemko, and Suresh Naidu. "Unions and inequality over the twentieth century: New evidence from survey data." The Quarterly Journal of Economics 136, no. 3 (2021): 1325-1385.