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This study examines the effect of trade unionism on the dispersion of wages among male wage and salary workers in the private sector in the United States. It finds that the application of union wage policies designed to standardize rates within and across establishments significantly reduces wage dispersion among workers covered by union contracts and that unions further reduce wage dispersion by narrowing the white-collar/blue-collar differential within establishments. These effects dominate the more widely studied impact of unionism on the dispersion of average wages across industries, so that on net unionism appears to reduce rather than increase wage dispersion or inequality in the United States.
Richard B. Freeman (1980) studied this question.