Despite the substantial and prolonged sociological interest in health disparities, much remains unknown about the processes that initiate them. To investigate this topic, we focus on the case study of cocaine use, for which a socioeconomic disparity emerged across all age groups in a short period of time around 1990. We examine whether the newly-formed disparity represents a selective remnant of previous users or, instead, a selective recruitment of new users. To evaluate these two potential processes we use latent class regression on a nationally representative cohort with repeated measures of past-year cocaine use before and after 1990. Results support the "remnant" hypothesis and show that the newly-formed disparity resulted primarily because people in the lower social strata were less likely to have a trajectory of cocaine use with a sharp drop in use after 1990. These results point to the "remnant" concept as a way to bring together disparity analysis of very different and diverse health outcomes.