Social Cognitive Theory (SCT) is among the most influential theories of health behavior and has been used as the conceptual basis of interventions for smoking cessation, weight management, and other related health outcomes. SCT and other related theories were developed primarily to explain differences between individuals, but explanatory theories of within-person behavioral variability are increasingly needed to support new technology-driven interventions that can adapt over time for each person. This paper describes a dynamical system model of SCT using a fluid analogy scheme. A series of simulations were performed to explore and better understand SCT. The model incorporates a nonlinear feature called habituation, an impor- tant feature of behavioral response resulting from continuous stimulus. It also illustrates how control systems engineering principles provide a promising approach for advancing health behavior theory development, and for guiding the design of more potent and efficient effective interventions.