The decline of union membership has been a key driver of income inequality in recent decades, a new report found.

The drop in unionization accounts for roughly a third of the growth in wage inequality among men and a fifth among women between 1973 and 2007, according to the left-leaning Economic Policy Institute.

The share of the workforce represented by unions declined from 26.7% in 1973 to 13.1% in 2011.