Rising inequality caused by weaker unions

A new study by the International Monetary Fund (IMF) has tracked three decades of income (1980 to 2010) and found that as unionisation declined, the wealth of the richest 10% in advanced countries showed a continuous increase. The decline in unionisation explained about half of the rise in incomes for the richest 10% and half of the increase in the Gini coefficient (a measure of income inequality).
http://www.imf.org/external/pubs/ft/fandd/2015/03/jaumotte.htm