From the article by Susan Dynarski, New York Times, July 6, 2018:
"Union workers now earn about 20 percent more than nonunion workers in similar jobs. Remarkably, this union premium has held steady since the 1930s."
"Thanks to the new research [“Unions and Inequality Over the Twentieth Century: New Evidence from Survey Data”], evidence going back nearly a century now shows that unions have formed a critical counterweight to the power of companies. They increase the earnings of the lowest skilled and sharply reduce inequality.
But the Supreme Court’s [Janus] decision will curtail the capacity of unions to organize and represent workers. The court ruled that unions can no longer collect “agency fees” from those government workers whom they represent but who have chosen not to join. These fees have helped pay for contract negotiations as well as prevent the free-rider problem that arises when only some pay for benefits enjoyed by everyone."