This paper addresses the interplay between the disciplines of psychology and sociolinguistics in second language acquisition. Psychology explains the process by which learners create their second language system, or interlanguage. Sociolinguistics examines the effects of social factors on learners' interlanguage. This paper focuses on an interactionist perspective to second language instructed development, highlighting the marriage between external (social) and internal (cognitive) factors. The first section introduces the cognitivist approach to learning and examines Vygotsky's socioconstructivism, contending that this is an interactional model. It also establishes parallels between the Zone of Proximal Development (ZPD) and interlanguage. The second section focuses interlanguage as a variable phenomenon due to the interplay between social and cognitive variables. The third section discusses Ellis' Variable Competence Model, looking at its shared mechanisms with Vygotsky's ZPD. The fourth section analyzes implications of this interactionist perspective to second language learning, arguing for the adoption of an interactionist perspective to second language teaching as well. (Contains 36 references.) (SM)