Building on recommendations from several of the articles in the special section on conscientiousness in the June 2014 issue of "Developmental Psychology," the present study tested predictions from the interactionist model (IM) of socioeconomic influences on individual development. In an approach consistent with the idea of cumulative advantage, the model proposed that adolescent and child conscientiousness would be fostered by higher family socioeconomic status (SES) and the parenting and material investments that SES promotes. The IM also predicted a transactional process in which adolescent conscientiousness would promote future socioeconomic success which, in turn, would foster greater adult conscientiousness. Analyses with a cohort of 347 adolescents followed for over 20 years were largely consistent with these predictions, although the findings suggested some modifications to the IM, including the addition of a stronger direct role for family processes in eventual social and economic outcomes. Moreover, additional analyses with 282 of the children of these cohort members demonstrated that this same process was partially replicated in the next generation of children. The findings suggest reciprocal or transactional influences that promote conscientiousness and accumulating personal, economic, and social advantages over time and generations.