Interindustry wage differentials in wage regressions estimated for individuals have been interpreted as evidence consistent with efficiency wage models. A principal competing explanation is that these differentials are generated by differences across workers in unobserved ability. This paper tests the unobserved ability hypothesis .by incorporating test scores into standard wage regressions as error-ridden indicators of unobserved ability. The results indicate that differences in unobserved ability explain relatively little of interindustry or interoccupation wage differentials.

We provide evidence that US workers face a wage-effort offer curve with the high-wage high-effort jobs occurring in the capital intensive sectors. We find that real wage offers rose at every level of effort during the 1960's, a shift which is consistent with a decline in the rental cost of capital. During the 1970's, when relative prices of labor-intensive goods declined, the wage-effort offer curve twisted, offering lower pay for the low-paid jobs in the labor-intensive sectors but higher pay for the high-paid jobs in the capital-intensive sectors. In the 1980's, workers at every wage level began to work more hours for the same weekly wage. This we loosely attribute either to the increasing cost of non-wage benefits, especially health care, or to the introduction of new equipment. In studying the wage-effort offer curve rate of unionization, education, and rent sharing.