This Special Issue of Applied Psycholinguistics includes the third Keynote Article in our Journal. This format has been very successful, and thus far it has generated fruitful discussions in two areas of research: the relationship between nonword repetition and word learning (Gathercole, 2006) and grammatical processing in language learners (Clahsen & Felser, 2006).

Research at the interface of bilingual development and child language disorders has increased greatly in the past decade. The purpose of this article is to highlight the theoretical and clinical implications of this research. Studies examining the similarities in linguistic characteristics between typically developing sequential bilingual children and monolingual children with specific language impairment (SLI) the same age are reviewed in light of predictions from a maturational model of SLI. Studies examining the linguistic characteristics of bilingual children with SLI compared to monolinguals with SLI and their bilingual peers with typical development are reviewed in light of predictions of limited processing capacity theories of SLI. It is shown that data from bilingual children pose interesting challenges to both theoretical perspectives, although in different ways. Finally, the findings from this research are discussed in terms of their relevance for assessment of SLI in bilingual children.

Only a decade ago, a very few researchers considered the study of language disorders in bilingual population worth pursuing. It was mostly argued that there were enough challenges in studying bilingualism, and even more challenges in the study of specific language impairment (SLI). So why complicate things and combine the two domains?

The cross-overs between monolingual–bilingual and typically atypically developing children are a goldmine for research on language development. The four permutations of language exposure and language abilities create “natural experimental conditions” for investigating the nature of the language capacity and how this is shaped by input in typical development, and importantly, in specific language impairment (SLI). Paradis has played a prominent part in identifying and exploring this rich source of evidence. Her Keynote Article provides a valuable review of findings from her own and others' research, a carefully considered evaluation of their compatibility with competing theories of language development, and an excellent catalyst for debate.

The Keynote Article is a very welcomed, timely, and incisive review article that examines theoretical and clinical implications of research on bilingual development and specific language impairment (SLI). Paradis provides a considered examination of the available evidence and takes into account a number of potential influencing factors. In particular, I want to commend her discussion of potential compensatory mechanisms that may play a role in bilingual development in SLI. This is often a neglected area in discussions concerning SLI. I also want to highlight that I agree wholeheartedly with her conclusion that the evidence to date “leans toward a positive attitude toward dual language learning for children with SLI who are in a supportive context for bilingualism.” This message is by no means universally accepted, nor is it put into action in therapeutic contexts across the United Kingdom and Europe. There is still much debate as to what type of advice should be given to parents and what the target language or languages of therapy should be (Harulow, 2008). This review article has therefore the potential not only to make a significant contribution to theory but also to practice.

Johanne Paradis' Keynote Article can be read as a concise critical review of the research that focuses on the sometimes strained relationship between bilingualism and specific language impairment (SLI). In my comments I will add some thoughts based on our own research on the learning of Dutch as a second language (L2) by children with SLI.

In her Keynote Article, Paradis reviews evidence from bilingual language development to assess the claims of two opposing theoretical views of language disorders. Specifically, she examines the evidence for similarities in language profiles of typically developing (TD) sequential bilingual (second language [L2]) children and monolingual children with specific language impairment (SLI) with respect to Rice's extended optional infinitive (EOI) account. A limited processing capacity (LPC) account of SLI, Leonard's surface hypothesis, is evaluated within the context of comparisons among bilingual children with SLI, monolingual children with SLI, and TD bilingual children. Paradis concludes that the evidence from bilingual children poses challenges for both accounts of SLI.

What makes a child's language development trajectory have the patterns that it has, and what causes differences across children in those patterns? These fundamental questions have for over half a century been at the heart of research on language development in monolingual children, on the cross-linguistic development of language in children from distinct language communities, on bilingual language development, and on development in cases of language disorders in children. Paradis has taken the important step of carefully comparing the trajectories of two populations of children—children with specific language impairment (SLI) and children growing up bilingually—who both differ in interesting ways from normally developing monolingual children. Her Keynote Article highlights striking similarities and potential differences between these two groups. This Commentary focuses on three issues: what influences developmental profiles in language development, bilinguals' development of vocabulary and syntax, and assessment issues related to treating monolinguals as the “standard.”

Joanne Paradis' Keynote Article on bilingualism and specific language impairment (SLI) is an impressive overview of research in language acquisition and language impairment. Studying different populations is crucial both for theorizing about language acquisition mechanisms, and for practical purposes of diagnosing and supporting children with language problems. If the child is a second language (L2) learner it is assumed that the support should differ from the kind of language therapy given to children with language disorders. The article deals with the interface of bilingual development and child language disorders in two dimensions: the discussion concerns the disentangling of bilingual and L2 development from children with SLI, as well as monolingual and bilingual children with diagnosis of SLI. Traditionally, discussions of language impairments have only dealt with children in monolingual environments, but during the last decades an increasing number of studies have extended the analyses to bilingual populations. Contrary to expectations, grammatical development of monolingual children with SLI has been found to be more similar to L2 children than to monolingual first language (L1) children (e.g., Crago & Paradis, 2003; Grüter, 2005; Håkansson & Nettelbladt, 1993, 1996; Paradis & Crago, 2000). These findings are intriguing, and they have inspired new research on both bilingualism and language impairment.

In her very interesting Keynote Article, Johanne Paradis gives a clear overview of recent research at the interface of bilingual development and child language disorders, and highlights its theoretical and clinical implications. She raises the challenging question of “whether bilingualism can be viewed as a kind of ‘therapy’ for SLI.” At first sight, this is perhaps a surprising question, because one of the predominant views in the literature is that bilingual children with specific language impairment (SLI) will exhibit difficulties and perhaps a “double delay.” It is this challenging question that we consider in more detail here.

In the Keynote Article, “The Interface Between Bilingual Development and Specific Language Impairment,” Johanne Paradis considers issues and evidence at the intersection of children learning two languages and primary or specific language impairment (SLI). The review focuses on morphosyntactic evidence and the fit of this evidence with maturational (domain-specific) and limited processing capacity (LPC; domain-general) theories of language impairment. We agree with Paradis that studies that systematically and simultaneously investigate the behavioral profile of dual-language learners and children with language impairment are of significant theoretical and practical value. In our commentary we aim to broaden the behavioral profile to be considered in these populations, beyond the level of morphosyntax. In line with this aim we use the term primary language impairment (PLI) for the same population referred to as SLI by Paradis.

I commend Johanne Paradis not only for her interesting Keynote Article but also for the careful research that she has conducted along with her collaborators in the area of bilingual language development and disorders. Her contributions have been significant and are sure to shape our theoretical as well as clinical understanding of specific language impairment (SLI). In this Commentary, I focus on three issues. The first stems quite directly from ideas raised in the Keynote Article; the second and third deal with factors that we need to consider when conducting research involving comparison groups of bilingual and monolingual children with SLI.

The debate over the characterization of specific language impairment (SLI) is fundamental to theoretical linguistics and, more broadly, to the whole of cognitive science. It is built directly out of the pervasive question regarding the extent to which language ability is best considered as a domain-specific set of skills or as the outcome of various domain-general processes. Therefore, an examination of this issue in conjunction with bilingual language acquisition, a situation that naturally entangles both linguistic and cognitive systems, is a powerful forum for exploring these basic theoretical questions. Paradis' Keynote Article is a substantial contribution to this enterprise: it provides a thorough review of the literature on bilingualism and SLI, and in so doing, evaluates the evidence in terms of its consistency with the maturational model that follows from the tradition of domain-specific language acquisition and the limited processing capacity (LPC) theory, a more domain-general approach. Her extensive review of the literature shows that both second language (L2) learning and bilingualism produce language proficiency profiles that are not identical to those found in SLI, and therefore support neither approach. In our view, the problem is in the attempt to dichotomize language ability as being controlled by either domain-specific or domain-general factors. A more inclusive approach to language ability, especially regarding bilingualism and L2 learning, would set out different criteria for evaluating language development other than the strictly linguistic features used in Paradis' analyses. Such an analysis may lead to a clearer identification of how these experiences uniquely affect language outcomes.

In her Keynote Article in this issue, Paradis explores the nature of bilingual language acquisition by examining the question of possible similarities between children learning a second language (L2) and children with specific language impairment (SLI) who are monolingual or bilingual. She evaluates the maturation model of Rice (2004), the extended optional infinitive (EOI) model, that focuses on children's acquisition of finiteness marking during the early childhood period. Paradis alludes to the issue of how to deal with the nonparallels between chronological age and acquisition in the comparison of L2 and SLI language acquisition within maturational models. I explore that issue further in this Commentary, using the available growth templates drawn from the work on English-speaking typically developing children and children with SLI for projected possible growth trajectories for bilingual and L2 children, with and without SLI.

In her Keynote Article, Paradis discusses the role of the interface between bilingual development and specific language impairment (SLI) on two different levels. On the level of theoretical explanations of SLI, Paradis asks how domain general versus domain-specific perspectives on SLI can account for bilingual SLI, as well as what bilingual SLI may contribute to the discussion of these theories. Paradis argues in favor of domain-specific deficits (in addition to well-documented processing deficits in SLI), and especially for the maturational model (Rice, 2004). She argues against a mere processing deficit of input information and against deficits in working memory and processing speed as sole sources of SLI. On the practical level, Paradis focuses on the question of whether and how language tests that have been standardized for monolingual children are valid for the assessment of SLI in bilingual children. Both discussions, on theory and on practice, are based on empirical data from Canadian studies on bilingual children with and without SLI carried out by Johanne Paradis, Martha Crago, and Fred Genesee (e.g., Crago & Paradis, 2003; Paradis, 2007; Paradis, Crago, & Genesee, & Rice, 2003).

One of the main conclusions that we (Vainikka & Young-Scholten, 1994) make in regard to the second language (L2) German development of uninstructed Korean and Turkish adults was the resemblance of their morphosyntactic development to that of the German children under study at the time by Harald Clahsen and colleagues (see, e.g., Clahsen, Eisenbeiss, & Vainikka, 1994; Clahsen & Penke, 1992). Data from these L2 learners also indicated initial transfer of the headedness of their native language verb phrases (VPs), a claim then strengthened by research on L2 learners whose first language (L1) headedness differed from German, namely, Italian and Spanish (Vainikka & Young-Scholten, 1996). L2 learners' initial grammars were argued to consist of just a “bare” VP, based on comprehensive lack of inflectional morphology and complex syntax, and similar to children acquiring L1 German, these L2 learners' nonfinite verb forms were typically in final position, either early on (for head-final Korean and Turkish speakers) or a bit later, once headedness shifted to the German value from head-initial (for Italian and Spanish speakers). Similar to child L1 learners, the L2 morphosyntactic data pointed to subsequent projection of a head-initial underspecified functional projection and, with sufficient input, projection of higher functional projections. Apart from some details, the claim was that for children and adults learning German, acquisition is defined by the emergence of syntactic projections and the morphology associated with them.

In her Keynote Article, Johanne Paradis does a service for the research and the clinical communities with this comprehensive review of the literature encompassing bilingualism and specific language impairment (SLI). Her work and the work of her colleagues for more than a decade have been persistent in bringing these two threads together: using bilingual (BL) data to speak to theoretical issues and using research findings to inform clinical practice. I am not alone in my appreciation (and admiration) of her many contributions, which are nicely pulled together here and placed in better perspective than in the several articles that have presented much of the work in isolation.

The Keynote Article examined research situated at the interface of bilingualism and specific language impairment (SLI) primarily to evaluate theories of SLI and secondarily for clinical considerations. The purpose was not to argue for one theoretical perspective over another, but instead to demonstrate how they both need some refinements and extensions to account for data from bilingual children. The breadth and depth of the Commentaries on this Keynote Article were impressive, and common themes emerged across many of them. This response article is organized into sections according to these themes. I hope that the review and synthesis in each section reveals how this collection of contributions has moved us forward in our understanding of the interface between bilingual development and SLI. The final section of this article includes more longitudinal data from English second language (L2) children to explore ideas arising from the discussion of cross-linguistic transfer, cognitive processes, and age effects.