In this paper it is argued that there are fundamental differences among labor markets in the mechanisms that create the observed associations between socioeconomic attainment and the characteristics of individuals, and that these differences result from the nature of the employment relationship in different labor markets. In open-employment relationships, which predominate in secondary labor markets, employees are exposed to competition from other workers, and marginal productivity/human capital theory applies. In closed-employment relationships, which characterize primary labor markets, employees are protected from competition, and change in attainment takes place when employees obtain access to vacant jobs better than their current job. Evidence for different types of employment relationships in different labor markets is obtained from an analysis of job shifts using life history data on white men aged 30-39 in 1969. This analysis focuses on the transition rates to better and worse jobs as a function of personal resources, current job rewards, and the type of employment relationship. (Author/GC)