Setting the Context General IntroductionThis book is about the acquisition of a third language (or more additional languages) in adulthood; that is, when a bilinguala child who is a simultaneous (2L1) bilingual, a child who has sequentially acquired a second language (L2) or an adult who is a sequential L2 bilingualacquires yet another language later in life. Is learning a third (L3) or more (Ln) language different from learning an L2 or just more of the same? If the process is different or similar, what are the implications for important questions related to linguistics, psychology, cognitive science and other fields? Addressing and providing some answers to the aforementioned is the overarching goal of this book.For a very long time, it was taken for granted that all instances of nonnative, sequential language acquisition were fundamentally equivalent. Such a claim was never stated explicitly; however, standard empirical practice in the study of adult L2 acquisition across virtually all paradigms suggested that few people were preoccupied with the heterogeneous groups in so-called L2 studies before the turn of the millennium. In fact, it was not until the mid-2000s that researchers, at least those studying the acquisition of morphosyntax, began to contemplate in earnest the effect of knowing more than one previous language and thus to differentiate true L2 from multilingual learners systematically, at least with regard to L3 learners. Consequently, new questions began to emerge organically, such as the role that having more than one previously acquired system has on subsequent acquisition/processing or how this influence is selected among choices. At the time of writing this book in 2017-2018, gone are the days in which no one questioned linguistic-experience inclusion criteria in L2 acquisition. We now know that whether or not a target nonnative language is chronologically a second or later language matters a great deal for morphosyntax in the L3 initial stages and throughout L3 development. While we do not yet 1

The present systematic review examines what factors determine when, how and to what extent previous linguistic experience (from the first language, second language or both languages) affects the initial stages and beyond of adult third language (L3) acquisition. In doing so, we address what a bird’s eye view of the data tells us regarding competing theoretical accounts of L3 morphosyntactic transfer. Data couple together to suggest that some factors are more influential than others. As discussed, the systematic review transcends the field of adult multilingualism precisely because of what it reveals, as a prima facie example in behavioral research, in terms of how different types of methodological considerations impact the way data are interpreted to support or not particular claims.

Aims: Over the past decade in particular, formal linguistic work within L3 acquisition has concentrated on hypothesizing and empirically determining the source of transfer from previous languages-L1, L2 or both-in L3 grammatical representations. In view of the progressive concern with more advanced stages, we aim to show that focusing on L3 initial stages should be one continued priority of the field, even-or especially-if the field is ready to shift towards modeling L3 development and ultimate attainment. Approach: We argue that L3 learnability is significantly impacted by initial stages transfer, as such forms the basis of the initial L3 interlanguage. To illustrate our point, the insights from studies using initial and intermediary stages L3 data are discussed in light of developmental predictions that derive from the initial stages models. Conclusions: Despite a shared desire to understand the process of L3 acquisition in whole, inclusive of offering developmental L3 theories, we argue that the field does not yet havealthough is ever closer to-the data basis needed to effectively do so. Originality: This article seeks to convince the readership of the need for conservatism in L3 acquisition theory building, whereby offering a framework on how and why we can most effectively build on the accumulated knowledge of the L3 initial stages in order to make significant, steady progress. Significance: The arguments exposed here are meant to provide an epistemological base for a tenable framework of formal approaches to L3 interlanguage development and, eventually, ultimate attainment.