The question of how much language a child can learn by modelling the patterns she is exposed to in the environment represents an age-old and persistent controversy in language-acquisition research. On a number of occasions researchers felt confident enough to claim that the controversy had been resolved, in favour of their own viewpoint, of course. One such attempt was Skinner's (1957) behaviourist view of language development, which gave a prominent if not exclusive role to input and experience. After Chomsky's (1959) landmark review, however, Skinner's account was left in pieces and was not further pursued by many. A more recent attempt comes from usage-based accounts according to which children directly build linguistic categories and rules from the language they hear around them. Some believe that these accounts have resolved the controversy for good and have ‘overturned’ alternative less experience-driven approaches (e.g., Ibbotson & Tomasello, 2016; Dabrowska, 2015). Yet, casual inspection of recent research articles reveals that this announcement may be somewhat premature; see, for example, Everaert, Huybregts, Chomsky, Berwick and Bolhuis (2015), Boxell's (2016) rebuttal of Dabrowska (2015), and many acquisition studies published in the journal Language Acquisition.

A growing literature on bilingual development explores relationships between language exposure and learning outcomes. Vocabulary size and pace of grammar learning have been claimed to be causally related to amounts or types of exposure to each language. Strong claims are made about the role of exposure on bilingual outcomes. Some researchers posit a unique learning result: a ‘weak language’. In a critical review, I voice reasons for scepticism that quantity or quality of exposure alone will explain findings. Central constructs are not well defined; inappropriate research methods have been used; the right kind of data is not discussed. Crucially, authors prevaricate on the notion of language itself, switching between cognitive and environmental perspectives. Both are needed to interpret bilingual behaviours but play different roles in the construction of learner grammars.

The study of bilingual development has been, and must be, an interdisciplinary endeavor; Carroll (Carroll) presents us with a perspective from within one particular discipline, that of generative linguistics. From this vantage point, she provides us perhaps most importantly with the reminder that language is not a unitary construct, and cautions against extrapolating from findings on the learning of one particular aspect of language, such as vocabulary, to language acquisition more broadly. I wholeheartedly agree (for a similar point, see Paradis & Grüter, 2014). I do not agree, however, with Carroll's implication that such unwarranted extrapolation is characteristic of current research on input and bilingual development. A number of recent studies have looked specifically at the differential relation between input (in a wide sense) and bilingual children's acquisition of different linguistic phenomena. Unsworth (2014), for example, reported different effects of input variation on Dutch–English bilingual children's acquisition of grammatical gender versus indefinite object scrambling.

Before bilingual children can say anything, they must learn to distinguish between the two languages that are spoken to them, and they must learn to make useful perceptual distinctions in each of them in order to understand what is said to them. Conversational interaction and non-verbal communication, such as pointing and gaze, aid children in attending to and processing aspects of their “language input environment” (De Houwer, 2009, 2011), “exposure” in Carroll's terms. As Carroll points out, children must build up their linguistic categories based on their intrinsically category-free “exposure”: they must process speech to acquire language. This reminds me of Wijnen's (2000) notion of language intake, which is the “data base children use to derive hypotheses on the structure of the target grammar” (p. 174), and which constitutes children's selection from what I will continue to call input, pace Carroll.

As pointed out by Carroll (Carroll), our team has investigated the influence of input on the spoken French competence of older Ontario bilinguals. Our research has examined the learning of invariant and variable aspects of French grammar. We focus here on the learning of variation, since it is an under-researched topic not covered by Carroll. Our research examines adolescent speakers of Ontario French from French-medium schools (e.g., Mougeon & Beniak, 1991), same-age immersion students (e.g., Mougeon, Nadasdi & Rehner, 2010) and advanced learners from a bilingual university (e.g., Mougeon & Rehner, 2015). Two key dimensions of input are teacher classroom speech and frequency of use of French in the community for the Franco-Ontarian students and amount of extra-curricular interactions with Francophones for the FSL students. Having collected corpora from these student groups, we compared the output of learners with primarily classroom-based input with that of learners with broader ranging (extra-) curricular input. The availability of teacher in-class recordings for these learner groups has been crucial in identifying additional factors influencing these students’ output.

Carroll (Carroll) purports to contrast one view of language development in bilinguals, based on exposure, with another, based on processing. She claims that some researchers view language acquisition in bilinguals as founded solely on exposure and argues that language acquisition involves more than this, including analogy, associative learning, instance-based learning, rule-learning, and statistical learning.

Child language researchers have often assumed that progress in first language learning depends heavily on language exposure. For example, Hart and Risley (1995) compared children in middle class families with children in lower class families. Based on recordings made across several years in the home, they estimated that by the time the children from lower SES families entered first grade they had heard 30 million fewer words than the middle class children. Researchers and educators have argued that this ‘30 million word gap’ is a primary cause for academic failure of lower SES children in the primary grades in the United States. Researchers in second language acquisition (SLA) research have often postulated a similar linkage between exposure and attainment, both for early and simultaneous bilingual children and later second language learning. Carroll (Carroll) expresses justifiable skepticism regarding such claims regarding the effect of amount of exposure on language attainment. Despite some important differences in conceptualization of the nature of the input, I find her overall analysis compelling and important.

Carroll (Carroll) takes issue with the use of parent report to obtain quantity of language exposure measures in research on bilingual development. When discussing parent questionnaires, Carroll writes “Temporal units are crude measures of exposure and they tell us nothing about input”. While I agree that temporal units do not tell us much about the fine-grained details of the input within the temporal units, importantly, parent-report-based measures of input quantity have predicted variation in bilingual development of phonology, vocabulary and morphosyntax. These are robust and reliable findings across numerous studies, and yet, Carroll skates over them as if they did not matter, or dismisses them as trivial. Furthermore, Carroll seems to lead readers to believe that only coarse-grained, language-use temporal units have been obtained through this method; on the contrary, researchers have obtained fine-grained input quality details via parent report that also predict bilingual children's development. Finally, in some circumstances, parent report data is the only feasible method for obtaining language exposure information.

Carroll's paper (Carroll) is most welcomed and shows that the weaker language is still to be explored from various points of views. Over twenty years after the publication of Schlyter's paper, “The weaker language in bilingual Swedish-French children”, not only is the topic of the weaker language of simultaneous bilinguals still a clearly under-researched area, but it is also, as Carroll shows, a crucial domain of research in the discussion of properties and constraints of the human Language Making Capacity (LMC).

Carroll's critique (Carroll) of the field of bilingualism yields strong directives. Let us not study specific groups of bilinguals, and make big claims about bilingual learning. Let us not study one domain, say vocabulary, and generalize to bilinguals’ language. These are all valid points. She also voices strong skepticism about how current literature deals with language experience:

Exposure and input are often used interchangeably in the study of child bilingualism to describe the learner's interaction with her environment. Carroll (Carroll), in her thought-provoking keynote article, challenges the equation between amount of exposure and amount of input necessary for acquiring a language, and in particular a second language. Carroll, whose diverse research borrows from both generative and psychological models of learning, brings these two perspectives into her exploration of the role of exposure and input in language acquisition, calling for a clearer definition of each. Carroll argues that the input to the language acquisition mechanism is the abstract construct necessary in order to solve the learning problem, and it is not a function of the amount of exposure: very little input is necessary in order to solve a learning problem, e.g., identifying word boundaries, or acquiring a syntactic structure. While scrutinizing the equation often drawn between exposure and input, Carroll further argues that the amount of exposure further misses the quality of this exposure as well as the societal context in which this exposure occurs.

At its best, the field of bilingualism is a dynamic discipline informed by diverse perspectives. Carroll's critical review – situated within a linguistics framework – contends that current research on bilingual exposure and language development suffers from a lack of theoretical clarity, and makes little contribution to our understanding of bilingual acquisition. Carroll's push (Carroll) towards greater precision in our thinking about the relation between input and outcomes is an important and welcome challenge. However, it is also critical to keep in mind the social context that motivates much of the current research on bilingual development, and to leave room for studies whose main goal is to provide answers to societally important questions about bilingual children's health and development.

I had several goals in writing my keynote “Exposure and input in bilingual development”. The first was to emphasize that there are two components to the study of environmental effects on language learning. The first is the stuff ‘out there’ (exposure) that we want to observe and count and whose effects we want to assess; the second is the internal, mentally represented stuff (my input) that is logically related to a particular learning problem. Both exposure and input are indissociable from assumptions about what language acquisition mechanisms do and the nature of linguistic cognition. Accordingly, for example, a decision to count ‘words’ in child-directed speech (CDS) or via a parental questionnaire is not an innocent one. Not only can one find radically different views on what a ‘word’ is (Krause, Bosch & Clahsen, 2015), one can find work that questions the need to postulate such a unit at all (see discussion in MacWhinney, 2000). It follows that adopting a clear position, about which abstract mentally represented elements are crucial cues to learning some phenomenon, is an essential step in deciding what to count in CDS.

Many bimodal bilinguals are immersed in a spoken language-dominant environment from an early age and, unlike unimodal bilinguals, do not necessarily divide their language use between languages. Nonetheless, early ASL–English bilinguals retrieved fewer words in a letter fluency task in their dominant language compared to monolingual English speakers with equal vocabulary level. This finding demonstrates that reduced vocabulary size and/or frequency of use cannot completely account for bilingual disadvantages in verbal fluency. Instead, retrieval difficulties likely reflect between-language interference. Furthermore, it suggests that the two languages of bilinguals compete for selection even when they are expressed with distinct articulators.

The cognate facilitation effect (i.e., a processing advantage for cognates compared to non-cognates) is an evidence of language non-selectivity in bilingual lexical access. Several studies using behavioral or electrophysiological measures have demonstrated that this effect is modulated by the degree of formal overlap between translations. However, it has never been tested with a psychophysiological measure such as pupillometry. In the present study we replicate the cognate facilitation effect by examining reaction times and pupil responses. Our results endorse pupillometry as a promising tool for bilingual research, and confirm the modulation of the cognate effect by the degree of formal similarity.

The Flanker and Number Stroop tasks, and the Wisconsin Card Sorting Test (WCST) were adopted to examine how bilingualism and public speaking training would contribute to cognitive control differences among young adults. Four groups of participants (of similar cultural and language backgrounds) were tested: monolinguals, general bilinguals, L1 public speaking bilinguals, and L2 public speaking bilinguals. Both ANOVA and multiple regression analyses showed that public speaking experience (esp. in L2) significantly contributed to conflict monitoring as tested in the global reaction times in the Flanker and Number Stroop tasks, whereas bilingualism (L2 verbal fluency, to be more specific) significantly contributed to mental set shifting as tested in the WCST. These results suggest that specific aspects of language experience, either in L1 or in L2, may incur enhancement in specific aspects of cognitive control, which has implications for bilingual advantage research.

This study investigated whether lifelong bilingualism can be associated with enhanced executive control, particularly mental flexibility, and with a modulation of an age-related decline in these functions. We compared performance of middle-aged and elderly speakers of German and bilingual speakers of Dutch and Frisian in a cued task-switching paradigm. All bilinguals were fluent in the same, closely-related language pairs. Bilinguals incurred significantly lower switching costs than monolinguals, and elderly bilinguals were less affected by an age-related increase in switching costs than monolinguals. Bilinguals did not differ from monolinguals in the size of the mixing costs. Our findings suggest that lifelong bilingualism correlates with enhanced ability to shift between mental sets, as well as increased resistance to proactive interference. The fact that we found significant group differences – while some previous studies did not – may be attributable to the choice of our task and to the cognateness of the languages involved.

Whether bilingualism affects executive functions is a topic of intense debate. While some studies have provided evidence of enhanced executive functions in bilinguals compared to monolinguals, other studies have failed to find advantages. In the present study, we investigated whether high opportunity of language switching could contribute to bilingual advantage. Advantages have been consistently found with Catalan–Spanish bilinguals who experience frequent opportunities of language switching. Fewer opportunities are experienced by speakers of Italian and one of the Italian dialects, the participants of our study. We anticipated reduced or no advantages with these participants. In Experiment 1, subjective estimates of familiarity with dialect failed to show a relationship with performances in different tasks involving executive control. In Experiment 2, we compared Italian–Venetian dialect bilinguals to Italian monolinguals in the flanker task, and no advantages were found either. Contrasting with results from Catalan–Spanish bilinguals, our results suggest that language switching plays a role in bilingual advantages.

Differences between monolinguals and bilinguals are often attributed to crosslinguistic influence. This paper compares production of discourse connectives by Dutch–Russian bilinguals (Dutch-dominant), typically-developing Dutch/Russian monolinguals and Russian-speaking children with SLI. If non-target-like production in bilinguals is due to crosslinguistic influence, bilinguals should perform differently from both impaired and unimpaired monolinguals. However, if differences between bilinguals and monolinguals are due to other factors (e.g., input quantity, processing capacities), bilinguals’ language production might be similar to that of children with SLI. The results demonstrate that language dominance determines the direction of crosslinguistic influence. In terms of frequency distributions of Russian connectives across pragmatic contexts, the bilingual group performed differently from both monolingual groups and the differences were compatible with the structural properties of Dutch. However, based on error rates and types bilinguals could not be distinguished from the SLI group, suggesting that factors other than crosslinguistic influence may also be at play.

Previous studies have shown effects of bilingualism on inhibitory control in preschool children. However, these effects only held for ‘conflict tasks’, and not delay of gratification tasks, and other domains of executive functioning were not investigated. For older children, previous studies have found relationships between bilinguals’ advantages and home language environment. This study investigates effects of bilingualism and bilingual home language environment on executive functioning in three-year-old children. 200 bilingual and 829 monolingual three-year-olds performed tasks of inhibitory control, working memory, and selective attention. Home language environment characteristics were assessed through a parental questionnaire. The bilinguals outperformed the monolinguals on a conflict task only, and this effect was very small. Further analyses showed broader effects on inhibitory control that were related to home language environment: Bilinguals whose parents spoke different languages outperformed bilinguals whose parents spoke the same language on both the conflict task and a delay of gratification task.