A new study says that the decline in union power and membership since the 1970s is responsible for a full third of wage inequality since then, the New York Times reports.

The study, published in the American Sociological Review, says that the decline of the U.S. labor movement “has added as much to men’s wage inequality as has the relative increase in pay for college graduates.”

When unions represented more than 30 percent of the private sector workforce, even nonunion employers would raise wages in an attempt to stop their workers from organizing. The study also says that “when one in three male workers were organized, unions were often prominent voices for equity, not just for their members, but for all workers.”

Wage inequality in the private sector increased by 40 percent since 1973, the study says.

But try telling that to Gov. Scott Walker.