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.