Score contribution per author:
α: calibrated so average coauthorship-adjusted count equals average raw count
The nature of union endogeneity is examined in the light of recentl y proposed estimators for union wage differentials. The instrumental variable approach adopted by G. Duncan and D. Leigh is shown to yield little information on the precise nature of the endogenous process. In particular, it cannot be used to sign the direction of selection into the union and nonunion sectors since, by construction, the instrumental variable estimates impose opposite signs on the selection terms. Copyright 1989 by University of Chicago Press.