Many current psychological models of reasoning minimize the role of deductive processes in human thought. In the present paper, we argue that deduction is an important part of ordinary cognition and we propose that a dual systems Competence [image omitted] Procedural processing model conceptualized within relational developmental systems theory offers the most coherent and productive framework for integrating and explaining the sometimes conflicting findings on the development of deductive reasoning across the lifespan. This model invokes a distinction that is quite similar to, though not identical with, the system 2-system 1 dichotomy employed in other dual systems models. In addition, the Competence [image omitted] Procedural processing model maintains the more specific distinction between algorithmic and reflective subsystems of system 2. In this account, the algorithmic system is represented as a kind of mental logic while the reflective system is the seat of practical and epistemic self-regulation, including emergent epistemic and metalogical norms. While the proposed systems of mind often appear as split-off component features in other dual systems models, relational developmental systems theory conceives of them as the highly complex and relationally integrated outcome of a self-organizing and self-regulating adaptive developmental process. (Contains 1 table and 2 figures.)