A supervisor and a scab move bread into a Washington, D.C., Safeway store during a union strike in 1974.
The precise effects of the broad deunionization of the American workforce since the 1970s are difficult to quantify, but a recent paper from the American Sociological Review has made an effort anyway. The study found that in addition to raising the income of union laborers, the existence of those organizations increased surrounding nonunion wages as well, decreasing income inequality by as much as one-third, as employers were forced to match union pay in order to attract quality employees. —ARK
Among men, if you account only for the effect of individual membership in unions, it would be about a fifth lower, which agrees pretty well with previous estimates. But if you also account for the effect of unions on surrounding nonunion employers (who often raised wages to compete with union employers and to avert the threat of unionization in their own workplace), the effect is larger: Unionization at 1973 levels would decrease income inequality by a full third.
... deunionization has allowed income inequality to rise partly because unions are negotiating wages for fewer people than they used to, and partly because unions no longer have the power to force the political system to pay attention to the needs of the middle class.