This article tests a prediction made by Processability Theory (Pienemann, 1998; 2005) that morphological acquisition is the driving force in English as a second language (ESL) development. It first outlines the model of psycholinguistic processing assumed by Processability Theory and shows how stages fall out from it. It then presents the hypothesis that morphological information propels development before sentence-level processing at stage 5 and describes what this should predict for ESL learners. A study is then presented that tested these predictions on oral data collected from two Mandarin speaking, adolescent, ESL learners over one academic year. The study found the acquisition of structures both predicted and not predicted by Processability Theory. While the results afford some evidence consistent with the claims about stages of development, they also provide counter-evidence to the hypothesis that the acquisition of morphology drives development up to stage 5: one learner acquired the predicted syntax for stages 3 and 4 without the morphology, and both learners acquired syntactic structures before associated morphology. Indeed, the findings suggest that the acquisition of morphology, and syntax, varies with learner orientation. To explain these findings, the article presents a proposal that draws on both Processability Theory and generative approaches to second language acquisition (SLA), and concludes by considering the implications of the study.

Dynamic Variation in Second Language Acquisition makes a cutting-edge contribution to knowledge about how second language learners develop their second language. Drawing comprehensively on Processability Theory’s theoretical understanding that individual variation dynamically interacts with ordered stages of language acquisition, the book provides an informative, critical analysis of historical and contemporary debates about the role of variation in linguistic variation, particularly second language variation. Richly illustrated with a forensic year-long study of how eight adolescent learners of English vary in their acquisition of syntax and morphology, this monograph shows that learners vary in their timing of development between two distinct learner types along a continuum and without skipping stages. The book uncovers how learner variation is dynamic and quite (although not entirely) systematic and how this variation contributes to change in the second language. It will be essential reading for researchers, students, and practitioners.

This book aims to help researchers and teachers interested in language processing and Processability Theory (PT) to understand this theory and its applications. PT is an influential account of second language processing which hypothesizes that, due to the architecture of language processing, learners acquire second languages in developmental stages. This book lays out PT’s predictions and research on the development of diverse target languages – particularly English and Scandinavian languages – by learners of various categories. It discusses the typological issues facing PT and its contribution to an understanding of variation and cognitive constraints on pedagogy. However, the book also raises a critical eye to the literature which, after almost twenty years of evolution, requires explanation, clarification and, in some cases, extension. Why do some phenomena belong to different stages in different languages? Why are important types of variation under-represented? Is teaching as constrained as proposed in PT?

This chapter aims to provide an up‐to‐date overview of research findings on ESL/EFL developmental sequences and stages, and the pedagogical implications of these findings. It opens by defining and exemplifying key terms and surveying previous approaches, focusing on the morpheme order studies and frequency analysis. This section highlights the advances made by these approaches, particularly in the identification of developmental sequences, as well as some of their major limitations, specifically the lack of a clear explanation for sequenced development and a precise definition of acquisition. The next section presents the major, contemporary theory positing developmental stages: Processability Theory (PT) (Pienemann, 1998, 2005). This section shows how, by addressing the shortcomings of previous work, PT offers a coherent explanation of developmental stages and variation within them, as well as a principled criterion of acquisition. This section also presents the developmental stages for ESL morphology and syntax, including the well‐known question stages. The final section explores the rich literature on the practical applications of the ESL developmental stages. By outlining PT's pedagogical suggestions and discussing major criticisms of them, it offers ESL/EFL teachers a time‐tested way of matching pedagogy with the developmental readiness of students.

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