One of the main characteristics of the L3 lexicon is that it manifests influences from all previously learned languages, in contrast to the L2 lexicon, which can only be influenced by the L1. The chapter thus focuses on L1 and L2 lexical crosslinguistic influences in the L3. Questions discussed include the extent to which the L1(s) and L2(s) influence the L3 lexicon; in what ways they do so; and why certain background languages tend to influence the L3 lexicon to a larger extent than others. The chapter looks at both so-called negative transfer, such as code-switches, word construction attempts, and semantic transfer, and positive transfer effects, such as to what extent the L3 lexicon benefits from L1 and L2 cognate knowledge. The research area is complex in many different ways. It will be shown that it encompasses various types of data, different language constellations, L3 settings, L3 (and L2) proficiency levels, and age groups.

This chapter aims to contribute to further understanding of the acquisition of speech from a multilingual perspective by offering a state-of-the-art overview of findings and discussing methodological considerations in research on L3 phonological acquisition. As the discipline grows, the methodologies employed advance, yet certain aspects continue to pose a challenge, including divergent research designs; varied types of L3 learners; comprehensive measures of multilingual production and perception, and phonological proficiency assessment. From a theoretical perspective, the applicability of the established L3 morphosyntactic models to phonological data is critically evaluated and alternative explanatory approaches are discussed. Further, the chapter focuses on new insights into the field by overviewing findings from a longitudinal “Multi-Phon” project on crosslinguistic influence in multilingual phonological acquisition. Finally, some avenues for further research in the area are suggested.

Artificial linguistic systems are useful for investigating aspects of adult language learning. These systems are typically composed of a lexicon and a set of grammatical rules. Such systems provide advantages for the study of third language acquisition. For example, they enable researchers to control the amount and type of language exposure as well as crosslinguistic similarity to learners’ known languages. This chapter discusses the use of artificial linguistic systems as a methodological tool for investigating topics in adult third language learning, including the effects of first- and second-language transfer on third language learning and potential bilingual advantages in grammar learning. The chapter considers three subtypes of artificial linguistic systems: artificial languages, semi-artificial languages, and mini-languages. In the chapter, I discuss research that has used these artificial systems to examine third language learning/processing, advantages of the systems, and their validity in adult language learning research.

This chapter reviews a small but growing body of research that examines neuroplasticity stemming from multilingualism, specifically discussing some similarities and differences in brain structure and function stemming from in processing three or more languages, as a departure from bilingualism. The evidence comes from studies using magnetic resonance imaging to examine patterns of grey matter structure across cortical and subcortical regions, and aspects of white matter microstructure. This chapter also highlights the functional activation and temporal information of various processes that occur during language processing in multilinguals, while accounting for the influence of language background factors. It then discusses conflicting and agreeing evidence in the literature and attempts to consolidate the findings with suggestions based on contemporary frameworks such as the Dynamic Restructuring Model (Pliatsikas, 2020). In closing, the chapter highlights gaps and pose questions for future research directions in the field of multilingualism and neuroplasticity.

This chapter discusses L3/Ln acquisition as a step-by-step acquisition process, where crosslinguistic influence is considered to be the result of co-activation of lexical items and syntactic structures of the previously acquired languages in processing. That is, as argued by the Linguistic Proximity Model and the Scalpel Model, L3/Ln acquisition is learning by parsing/processing. The main focus of the chapter is on theoretical arguments for crosslinguistic influence taking place incrementally from either or both previously acquired language(s). The chapter also discusses methodological issues for studies that aim to identify hybrid influence, emphasizing the importance of a subtracted language group design, where the influence of individual languages can be isolated by comparing L3 learners to L2 learners of the same target language, but where one of the previously acquired languages is missing (subtracted). Furthermore, the chapter provides a brief overview of empirical L3 studies, finding support for property-by-property acquisition.

This chapter discusses the development of the language competence of returnee bilingual speakers, (i.e., heritage speakers who return to their (parents’) country of origin and, as a consequence of the return, are affected by changes of their language dominance). We discuss three scenarios in which an environmental shift may affect the status of a L3: (1) L3 attrition, when the L3 acquired in a foreign setting undergoes attrition due to reduction in input after returning to the homeland; (2) theoretical discussions of whether the relearning of an attrited L2 can be considered L3 acquisition; (3) the source of crosslinguistic influence on the L3 and how studying the returnee population can open an exciting opportunity to tease apart internal and external factors that are otherwise confounded in the traditional bilingual population. We combine the interpretation of the limited existing literature on language development in returnees with theoretical considerations, and propose research hypotheses to be tested in future research.

In this chapter we review current research on how the experiences that multilingual learners bring to language learning may shape the trajectory and outcome of learning an L3/Ln. While there is abundant evidence that the particular languages that individuals speak contribute to the process of new language learning, and evidence that multilingualism may enhance new learning, there is now emerging research demonstrating that the cognitive and neural mechanisms that enable bilinguals and multilinguals to use two or more languages effectively may also contribute to the course and consequences of new language learning. The dynamic activation of all of the languages that a speaker uses and interactions across languages shape not only language processing but also new learning. We identify sources of variation among learners and across the contexts in which learning occurs to consider how language learning might reflect multilingual experience.

Aging is a multidimensional process that introduces biological, social, and cognitive changes. The cognitive changes individuals experience during healthy aging are impacted by one’s lifestyle and patterns of behavior. In this chapter, we explore how multilingual language use uniquely contributes to cognitive aging and may stave off cognitive and neurological declines that commonly occur as we age. To this end, we first examine the range of executive functions that are impacted by multilingualism across the lifespan. Next, we explore the neurological implications of cognitive aging by outlining three neurocognitive mechanisms involved in aging, namely cognitive reserve, maintenance, and compensation. Integrating research from the available literature on healthy and pathological aging, we consider whether multilingualism strengthens and alters the neural bases of cognitive aging. As research on multilingual aging is nascent, our chapter ends with an agenda for future research that considers factors unique to multilingualism.

The aim of the chapter is to first discuss the theoretical motivations for Full Transfer as well as some methodological considerations one needs to adopt when testing whether there is Full or property-by-property transfer in L3/Ln acquisition. The chapter then provides an overview of two clusters of studies providing some empirical evidence towards Full Transfer at the initial stages of L3/Ln acquisition. The first cluster of studies examined is that of the acquisition of Brazilian Portuguese by bilingual English–Spanish speakers, illustrating how bringing together these 16 studies can be helpful when evaluating the tenets of the theoretical proposals that predict Full Transfer. The second is that of Catalan–Spanish bilinguals acquiring L3 English. By reviewing these seven different studies, it is shown how Full Transfer applies in cases where structural similarity between the Lx/Ly and L3 is not straightforward. The chapter ends by suggesting some directions for future research.

This chapter provides an overview of research into L3/Ln that is based on corpus data, and provide the reader with information on publicly available L3/Ln corpora. As both L3/Ln corpus-based research and corpora are quite scarce to date, I devote the better part of this chapter to describing the outcome of a survey I conducted to gather colleagues’ needs and preferences regarding future L3/Ln corpus compilations.

This chapter examines a population of third language (L3) learners that has received little attention, namely heritage speaker (HS) L3 learners, who grow up speaking two languages in early childhood. Unlike traditionally studied L3 learners, who acquired their L2 as older children, HSs have two naturalistically acquired languages to draw from, which has been suggested to have implications for L3 acquisition. By examining existing linguistically oriented research with HS L3ers, we discuss (1) whether patterns of acquisition differ in HS L3ers as compared to traditionally studied populations, and (2) whether this differs for L3 morphosyntax versus L3 phonetics-phonology. After a brief discussion of methodological limitations, we conclude that transfer patterns in early bilinguals in many respects mirror those for late bilinguals. However, differences across the domains of L3 morphosyntax versus L3 phonology are evident, with phonetic–phonological transfer more likely to stem from the dominant language. Some explanations and suggestions for further research are offered.

This chapter discusses usage-based approach perspectives on a number of topics related to learning and development in multilinguals. The chapter is organized around a basic construction: “The perspective of the usage-based approach on X ,” where X is a relevant issue in the acquisition of non-native languages. The chapter starts with an introduction to the approach that outlines its cognitive and social underpinnings, with an emphasis on the role of categorizations, exemplars, constructions, adaptive systems, analogy, and entrenchment. This section is followed by a review of how these concepts contribute to our understanding of language, learning, development, multilingualism, and crosslinguistic influence.

This chapter provides an introduction to formal linguistic approaches to third language acquisition. A substantial body of work on sequential multilingualism has been produced in the last two decades under assumptions and theoretical constructs that followed from the generative tradition in second language acquisition. My aim in this chapter is to briefly survey the main theories, models, and proposals originated in this subfield, and to highlight which of the assumptions in the tradition of generative grammar bring them together as a coherent approach to the acquisition of third or further languages. Some of these are, essentially, questions the field inherited from second language acquisition; others are core theoretical concepts from the generative enterprise. The recognition of these common bases and goals can help future work on these models focus on moving the field forward as a whole.

Taking the notion of L1 plasticity as a point of departure, this chapter explores the logical prediction that an L3 influence can influence an L1 and L2. After an overview of relevant L1 attrition research and the predictions it yields for L3 influence on existing systems, I pivot to two primary research questions that are central to L3 effects on an L1 and/or L2: (1) Does an L3 affect an L2 (quantitatively and qualitatively) differently than it affects an L1? and (2) Can an L3 have facilitative effects on an L2? I review the foundational research that underlies these lines of inquiry and follow with outstanding questions stemming from this research and discussion of how we might model attrition in multilingualism. I conclude with key methodological considerations, highlighting the need to draw further from experimental approaches used in L1 attrition as a complement to L3-specific methods.