Do unions contract for added employment?

The efficient contract model predicts unions will contract for added employment, that is, a level of employment higher than would be predicted by the demand curve for labor. Using three data sets (on Canadian industries in 1971-81, U.S. industries in 1972, and U.S. construction projects in 1973-74) and several estimation techniques, the author estimates and compares labor demand equations for both union and nonunion firms. Some regressions that control for few variables support the added-employment hypothesis, but those with more complete specifications reject it.