I use a sample of over fourteen thousand full-time jobs held by workers in the National Longitudinal Survey of Youth (NLSY) to examine mobility patterns and to evaluate theories of inter-firm worker mobility. The roles of both heterogeneity and state dependence in determining mobility rates for young workers are investigated, and both are found to be very important. There are three main findings. First, mobility is strongly positively related to the frequency of job change prior to the start of the job. Second, job change in the most recent year prior to the start of the job is more strongly related than earlier job change to mobility on the current job. Third, the monthly hazard of job ending is not monotonically decreasing in tenure as most earlier work using annual data has found, but it increases to a maximum at three months and declines thereafter. The first two findings suggest that there is important heterogeneity in mobility but that this heterogeneity is not fixed over time (workers might mature). The third finding is consistent with models of heterogeneous match quality that cannot be observed ex ante. I also find that females hold fewer jobs per year in the labor force than males and that this result is driven by a lower exit rate for females from the first job after entry.
