The lower the “countervailing power” of labor unions, the higher the inequality (see the two charts by Colin Gordon, http://www.epi.org/blog/union-decline-rising-inequality-charts).

[youtube=http://www.youtube.com/watch?v=QSYOedwIjMU&feature=player_embedded]

It would be interesting to find out, (i) which causal mechnismus is responsible for this statistical relationship and which direction the causality has, (ii) whether we would find the same relationship for other countries as well, and (iii) if it still exists when one controls for other possible causes of inequality.