The first chapter provides an introduction to the field of multilingual development explaining its scientific and societal relevance. The discussion begins with an overview of current globalization and migration processes where, based on OECD data of the past ten years, international migration streams receive illustration and substantiation. Case studies of prominent urban areas include London, Hamburg, Toronto, Vancouver, New York City, San Francisco, Sydney, Melbourne, Auckland, Singapore, Hong Kong, and Dubai. Multilingualism, as such, is not a new phenomenon and may be considered a defining condition of the human species. However, the speed with which it keeps spreading and the density that it developed in urban areas are relatively recent phenomena without historical precedents. These processes are not universal, though. While the Western World appears to have rediscovered multilingualism after a long period of pervasive monolingualism, China currently appears on its way towards a form of social organization in which monolingualism and homogeneity are of paramount importance. The chapter further addresses the social hierarchy of languages and the special role of English in it.

Chapter 6 extends the discussion of multilingual development to the so-called New Englishes as symbolic systems that developed in the former colonial territories and continued to develop after the collapse of the British Empire in the newly created independent polities. More precisely, the focus here lies on outer circle Englishes in the sense of Kachru (1985). The New Englishes are analyzed from the perspective of their surrounding multilingual ecologies and not, as is more customary, in terms of hermetically delineated national varieties of English. On that account, the chapter focuses on recent – and also more historical – multilingual outcomes of globalization where English plays a prominent role, has been incorporated into the local ecologies, interacts with many other languages, and shows or is beginning to show traces of localization or nativization. Case studies include Singapore, Hong Kong, and Dubai. The chapter thus brings together the key issues discussed in the preceding chapters – globalization, migration, urban areas, multilingual advantages or effects, cross-linguistic influence, language acquisition and learning, language policies, identities, and attitudes – and pivots them on contexts of particular prominence.

Countering traditional monolingual ideologies that associate multilingual development with various deficiencies, more recent research probes into potential advantages of this developmental experience. The discussion about putative multilingual advantages remains highly controversial and also emotional, as it affects far-reaching policy decisions. The chapter attempts to provide an objective state-of-the-art report, discussing research findings on executive function (control), cognitive reserve, cognitive development, educational attainment, and metalinguistic awareness. It furthermore tries to identify the boundary conditions that help to explain why some studies report positive results while others do not. Current research suggests that the characteristics of the speaker groups sampled, especially in terms of their type of bilingualism and multilingualism (balanced, unbalanced, heritage speakers, etc.), offer important clues for a better understanding of this domain. Moreover, the social prestige associated with the languages studied appears to influence the results considerably. Language users are not deterministic machines, but react to social pressure in intelligent ways.

Chapter 3 explores cross-linguistic influence and various theories thereof in multilingual settings in which more than two languages interact with one another. It is specifically interested in the direction and strength of such influence, contingent on parameters like age of onset, proficiency, L1 and L2 status, linguistic and typological proximity, recency of use, language dominance, as well as various others. It identifies cross-linguistic influence as the more encompassing concept in comparison to the traditional notion of transfer widely used in second language acquisition studies. It also discusses the relationship between cross-linguistic influence and language interdependence, as proposed in education studies. While language interdependence builds on the proficiency-enhancing qualities of multilingualism (qua metalinguistic awareness), cross-linguistic influence can be both facilitative and inhibitory, the latter manifesting itself as interference. Crucially, non-facilitative transfer only arises within a normative system. Moreover, the mechanisms underlying cross-linguistic influence show important parallels to those identified in language contact studies with its focus on contact-induced language change.

Chapter 5 is based on the conviction that the problems dealt with in the preceding chapters need to be embedded into the larger multilingual ecologies in which they occur. Since language dominance has proven an important predictor of cross-linguistic influence, which, in turn, determines the acquisition of additional languages, including expectable benefits of previous multilingual experience, one needs to follow up on the factors that are responsible for language dominance. Evidently, these factors are related to or the result of various issues of language policy and planning – both explicit and implicit – that shape the language ecology encountered in a particular region, even down to the nuclear family. It makes a difference whether one studies these issues in traditional European monolingual ecologies where other languages are learnt as classic second or foreign languages, in de jure monolingual ecologies with high numbers of immigrant speakers of other languages, in bilingual territories where the two languages enjoy the same status, coexist peacefully, and where the number of balanced bilinguals is high, in bilingual or multilingual areas with minority languages or stigmatized languages, or in highly multilingual ecologies with a common lingua franca.

Chapter 4 explores potential advantages of multilingual upbringing in relation to further language development. It argues that any such advantages play out more forcefully and are easier to identify in regard to general proficiencies rather than specific grammatical phenomena. Chapter 4 further argues that type of bilingualism, as operationalized in terms of language dominance, and the type of language knowledge investigated play a pivotal role for understanding language learning processes against a multilingual substrate. Cross-linguistic influence, as observable in different grammatical domains and the lexicon, can aid additional language acquisition by means of cumulative enhancement, but certain constellations also produce inhibitory effects. The assumption of universally facilitating effects has turned out to be overly optimistic and needs to be tempered. Much of the discussion in Chapter 4 is based on the acquisition of English as an additional language in bilingual heritage contexts (third language acquisition). Besides general CEFR-proficiencies, the case studies concern determiners, subject–verb agreement, tense and aspect, word order, and lexical cross-linguistic influence.

Monolingualism, bilingualism, and multilingualism represent concepts of individual upbringing and social organization of extreme impact and scope. All in all, the book attempted to guide the reader from a multilingualism-as-problem to a multilingualism-as-resource perspective. However, it also argued that multilingualism cannot work wonders and should not be considered a goal in itself. Running a multilingual society can produce many beneficial effects, but maintaining several languages at the same time also incurs costs that a society must be prepared to burden and share. It is crucial to know which boundary conditions tip the balance from burden to benefit, or vice versa. The book further argues for a continuum from monolingualism to multilingualism based on the dimensions of homogeneity and heterogeneity. It further introduces a novel typology of English in multilingual contexts, distinguishing between English in heritage contexts, English in bilingual heritage contexts, English in contexts of balanced bilingualism, English in indigenous multilingual contexts, English in postcolonial multilingual contexts, and English as a lingua franca in modern multilingual immigrant contexts.

The question of putative multilingual advantages is still being hotly debated. Following some initial euphoria, the field is currently characterized by a wave of sobering criticism, as key findings fail to be replicated. Here we offer a summary of the current controversy, followed by a discussion of new evidence drawn from a questionnaire study of 1,454 students and 341 instructors at the University of Hamburg. This study measures self-assessed proficiencies in English among subjects who regularly use English as a lingua franca in the context of tertiary education. We compare self-assessed English proficiencies between monolingually and multilingually raised ELF users in five CEFR domains. The results attest slightly higher scores for multilingually raised ELF users, of statistical significance in some domains, which we interpret in terms of a multilingual advantage. However, we wish to be cautious about generalizing these findings, as they need to be substantiated by tests that objectively measure proficiencies.