Three central facts describe inter-firm worker mobility in modern labor markets: 1) long-term employment relationships are common, 2) most new jobs end early, and 3) the probability of a job ending declines with tenure. Models based on firm-specific capital provide a parsimonious explanation for these facts, but it also appears that worker heterogeneity in mobility rates can account for much of what we observe in these data. I investigate tests of the specific capital model and consider whether these tests are successful in distinguishing the specific capital model from a model based on heterogeneity. One approach uses longitudinal data with detailed mobility histories of workers. These analyses suggest that both heterogeneity and specific capital (implying true duration dependence in the hazard of job ending) appear to be significant factors in accounting for mobility patterns. A second approach is through estimation of the return to tenure in earnings functions. This is found to have several weaknesses including endogeneity of tenure and the lack of tight theoretical links between tenure and accumulated specific capital and between productivity and wages. A third approach is to use of data on the earnings experience of displaced workers. Several tests are derived based on these data, but there is generally an alternative heterogeneity-based explanation that makes interpretation difficult. Nonetheless, firms appear willing to pay to encourage long-term employment relationships, and they may do so because it is efficient to invest in their workforce. On this basis, I conclude that, while deriving convincing direct evidence for the specific capital model of mobility is difficult, it appears that specific capital is a useful construct for understanding worker mobility and wage dynamics.
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