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Purpose: There is mounting evidence that the agrammatism that defines Broca's aphasia can be explained in processing terms. However, the extant approach simply describes agrammatism as disparate deficits in a static, mature system. This tutorial aims to motivate and outline a developmental alternative. This alternative is processability theory (PT), a root-to-apex theory of language development, with its origins in the field of second language acquisition, which can connect the findings of aphasia research. Method: This tutorial critically reviews research on agrammatism as a language deficit, a representational deficit, and a processing phenomenon. Given evidence from research applying PT to language disorders, this tutorial outlines PT's multidimensional architecture of language processing. Using an emergence (onset) criterion, PT predicts fixed developmental stages in word order (syntax) and inflection (morphology) and individual differences in the timing of syntax and morphology. To link PT to agrammatism, this theory's applications to diagnosis and teaching are overviewed, and a case study of five individuals with moderate agrammatism is presented. Results: Analysis showed that all individuals were positioned in the early PT stages and differed in their timing of syntax and morphology consistent with theoretical predictions. Conclusions: Evidence from the case study suggests that, although agrammatism results from neural damage and associated language loss, the processing procedures necessary for relearning remain and can be exploited for recovery. A program of diagnosis and intervention is proposed, and future research directions are discussed. Supplemental Material: https://doi.org/10.23641/asha.19416488

This article enters the debate about the complex and dynamical nature of second language acquisition (SLA) by discussing and commenting on Pallotti’s critique of Complex Dynamic Systems Theory (CDST). Pallotti’s critique brings to the fore the argument that, due to its anti-reductionist stance, CDST research fails to observe three fundamental research criteria, namely that research should (1) construct models, (2) make generalizations and falsifiable predictions and (3) adopt clear empirical approaches. While Pallotti does not explore research already moving SLA beyond anti-reductionist approaches to complexity and dynamism, the debate can be advanced by examining research doing exactly this. This article aims to evaluate the extent to which Processability Theory (PT) – a theory of second language (L2) processing that advocates reductionism for the sake of theory construction – meets Pallotti’s criteria for researching L2 complexity and dynamics. By discussing Pallotti’s – and PT’s – criticisms of CDST, the article outlines the implied research criteria and delineates how PT meets these criteria by (1) its bidimensional design, (2) its generalizations and falsifiable predictions for development and variation, and (3) its operationalization of constructs. Findings from a longitudinal study of L2 English are presented. These findings elucidate that, by meeting the criteria, PT can reveal the predictable, implicational development of processing complexity in eight learners and the patterned, dynamical interaction between development and variation in the trajectories of two of these learners. The article concludes that PT fulfils Pallotti’s (2022) criteria and, by a reductionist approach which models SLA as multidimensional, it can generate what CDST cannot; dynamical and complex L2 systems which progress in predictable stages, with well-defined constraints on variation, including DST-defined constraints. The article closes by discussing the implications of PT’s approach for future research on L2 complexity and dynamics.