Real wages have stagnated over the last decade partly because of dwindling collective bargaining power, according to an analysis released Wednesday by the Economic Policy Institute.

In his research for the institute’s “State of Working America” report, which will be published Sept. 11, Lawrence Mishel said “deunionization” also can explain some of the growing wage gap between the nation’s highest paid workers and everyone else.

About 13.1 percent of the U.S. workforce was represented by unions last year. The share has decreased steadily since the late 1970s.

“The decline of unions has affected middle-wage men more than any other group and explains about three-fourths of the expanded wage gap between white- and blue-collar men and over a fifth of the expanded wage gap between high school and college-educated men from 1978 to 2011,” Mishel wrote.

The research institute generally advocates for public policies that help the economic conditions of low- and middle-income workers.

It currently receives 29 percent of its funding from labor organizations.

The decline of labor unions “has lowered wages, not only because some workers no longer receive the higher union wage but also because there is less pressure on nonunion employers to raise wages,” Mishel’s analysis states.

Collective bargaining power has been siphoned by the recession, which dampened overall wages, and by competitive pressure from overseas labor, technological changes, the continuing shift to a service economy, and anti-union sentiment, he said.

Mishel calculated the “union premium” — a higher-wage effect — at 17.3 percent for men and 9.1 percent for women who are covered by collective bargaining contracts.

Union members also tend to have richer employee benefits than nonunion workers.

The study found that 83.5 percent of union members have health insurance coverage, compared with 62 percent of nonunion employees. Also, 71.9 percent of union members have pension coverage, compared with 43.8 percent of nonunion workers.