An individual covered by a collective bargaining agreement but who is not a union member is estimated to earn about 13% lower wages than a union member. Sectors with relatively few covered nonmembers are associated with a large coverage differential, while sectors with high proportions of covered nonmembers are associated with small differentials. This suggests freeriders either weaken the bargaining position of the union or weak bargaining positions increase the incentive to freeride. Only a modest amount of this differential is accounted for by unmeasured ability, the probationary period associated with newly hired union workers, or union status misclassification.
Schumacher, E.J. (1999). What Explains Wage Differences between Union Members and Covered Nonmembers? Southern Economic Journal, 65(3), 493-512.
Southern Economic Journal