One of the best-known claims from language acquisition research is that the capacity to learn languages is constrained by maturational changes, with particular time windows (aka ‘critical’ or ‘sensitive’ periods) better suited for language learning than others. Evidence for the critical period hypothesis (CPH) comes from a number of sources demonstrating that age is a crucial predictor for language attainment and that the capacity to learn language diminishes with age. To take just one example, a recent study by Hartshorne, Tenenbaum and Pinker (2018) identified a ‘sharply-defined critical period’ for grammar learning, and a steady decline thereafter, based on a very large dataset (of 2/3 million English Speakers) that allowed them to disentangle critical-period effects from non-age factors (e.g., amount of experience) affecting grammatical performance. Other evidence for the CPH comes from research with individuals who were deprived of linguistic input during the critical period (Curtiss, 1977) and were consequently unable to acquire language properly. Moreover, neurobiological research has shown that critical periods affect the neurological substrate for language processing, specifically for grammar (Wartenburger, Heekeren, Abutalebi, Cappa, Villringer & Perani, 2003).

The hypothesis that children surpass adults in long-term second-language proficiency is accepted as evidence for a critical period for language. However, the scope and nature of a critical period for language has been the subject of considerable debate. The controversy centers on whether the age-related decline in ultimate second-language proficiency is evidence for a critical period or something else. Here we argue that age-onset effects for first vs. second language outcome are largely different. We show this by examining psycholinguistic studies of ultimate attainment in L2 vs. L1 learners, longitudinal studies of adolescent L1 acquisition, and neurolinguistic studies of late L2 and L1 learners. This research indicates that L1 acquisition arises from post-natal brain development interacting with environmental linguistic experience. By contrast, L2 learning after early childhood is scaffolded by prior childhood L1 acquisition, both linguistically and neurally, making it a less clear test of the critical period for language.

On the basis of their review of studies, Mayberry and Kluender (2017) propose that the human language learning ability becomes severely compromised if it is not developed in tandem with brain development in early childhood, but that it functions more or less flawlessly, even in adulthood, if language acquisition had at one time proceeded according to the maturational timetable. Mayberry and Kluender therefore suggest that the critical period hypothesis (CPH) for language is unambiguously tied to the timing of L1 acquisition, but that its relevance to L2 acquisition is less clear, the implication being that the well-documented AoA effects in the SLA literature are due to non-maturational (i.e., psychological, experiential, cross-linguistic, etc.) causes.

The idea that there is a critical period (CP) for language acquisition that starts sometime after birth and ends sometime around puberty has been central to language studies since Chomsky's paradigm shift that embedded language acquisition into human biological development (for discussion, see Bialystok & Kroll, in press). From early on, the critical period argument was loosely extended to include second language (L2) acquisition, an idea buttressed largely by anecdote: “children learn foreign languages so much better than adults” in spite of evidence to the contrary (Goldberg, Paradis & Crago, 2008; Snow, 1987). The review by Mayberry and Kluender (2017) describes the existing literature on L2 acquisition and adds recent empirical evidence from investigations of individuals learning American Sign Language (ASL) at different ages, often as a first language (L1) to offer important clarifications and insights to this debate. Their argument rests on a clear distinction between processes used in L1 or L2 acquisition to re-evaluation of some of the more extreme claims of the CP view. In brief, they argue that L2 acquisition is not a good test of the CP argument, a point with which we agree. However, our view is that CP effects on language acquisition are even more limited than Mayberry and Kluender suggest and we offer two further arguments to support their own formidable evidence and conclusion.

This commentary on Mayberry and Kluender (2017) considers the geometry of the function relating Age of Acquisition (AoA) to L2 attainment, the effects of multilingual exposure on one's native language(s) and, briefly, the role of education in language learning.

In a 1997, Noam Chomsky offered this informal observation about language acquisition: “Like other kinds of growth, language acquisition happens easily at a certain age, but not later. There comes a time when the system doesn't work anymore. There are individual differences [. . .] but for most people, after adolescence, it becomes very hard. The system is just not working for some reason, so, you have to teach the language as something strange.” (Chomsky, 1997, p. 128)

For several decades now, research on the acquisition of ASL and other signed languages has contributed to our understanding of language acquisition and of age effects in particular. A strong decline in learning capacity with age has been shown in numerous studies with ASL as L1, and the age range for this critical period phenomenon appears to be very similar to what has been observed in even more studies in L2 (for both spoken and signed languages). Mayberry and Kluender (Mayberry & Kluender) argue that the two phenomena are quite different, however, to such an extent that the concept of a critical period is not applicable to L2. Their two main arguments are that L2 learners are less affected by late acquisition than L1 learners and that some L2 studies have not shown the kind of discontinuity in the age-proficiency function that is predicted by the concept of a critical period. As space is very limited, I will limit my comments to these two issues.

Mayberry and Kluender review evidence that second language (L2) proficiency declines with age of acquisition (regardless of modality), but they also review evidence for variable L2 outcomes for individuals, with factors such as motivation, language aptitude, education, and L2 experience playing a role. They argue that if L2 outcomes were fully under the control of a critical period for language (CPL), these learning variables should not predict L2 outcome, and the outcome of L2 learning would not be consistently observed to be so variable. The questions raised in this commentary are whether there is variation in late L1 proficiency and whether such variation could provide insights into the CPL.

The splendid review provided by Mayberry and Kluender (2017) confirms Lenneberg's 1967 hypothesis that a Critical Period (CP) exists for the establishment of a native language. However, Lenneberg's extension of the CP hypothesis to the later learning of an L2, based on his observation that most individuals who learn an L2 after about the age of 13 speak it with a foreign accent (FA), is doubtful.

Mayberry and Kluender (2017) offer a rich review of empirical research that contributes to the understanding of age-related effects on first and second language acquisition. Their keynote article compiles current, primarily linguistic and neurolinguistic, research on the notion of a critical period for language (CPL). The authors conclude “that the putative CPL applies to L1 learning, and that L2 effects are a consequence of this prior learning” (Mayberry & Kluender, 2017: p. 6). As they propose a clear role for CPL in L1 learning, and because their exact position on its role in L2 learning is, to my mind, not as clearly articulated, I will take the opportunity to argue the following: If a CPL exists at all, it should have identifiable implications for all kinds of language acquisition (cf. Gleitman & Newport, 1995). In the case of L2 acquisition what needs to be identified is how maturational constraints (implicated by a CPL) interact with other conditions that are at hand when the second language comes onto the scene.

The keynote article by Mayberry and Kluender (2017) clearly shows that there are important effects of delayed exposure to a first language (L1), in linguistic comprehension, production, processing, and even in the brain areas that are active for language. This set of findings is of great importance for both theoretical and practical reasons. As Mayberry and Kluender show, one implication of such findings bears on the theory that a critical period for language (CPL) leads to changes in the ways that language develops when accessible exposure is delayed.

Mayberry and Kluender (Mayberry & Kluender) adopt interesting positions on a wide range of issues. Due to space limitations, we will focus on just two where our views differ: (i) the robustness of maturational constraints on second language acquisition (L2A), and (ii) the implications of a role for language aptitudes within the boundaries defined by sensitive periods (SPs).

I am very pleased to comment on this important and thought-provoking keynote article. The evidence reviewed by Mayberry and Kluender (Mayberry & Kluender, 2017) is extremely significant for our understanding of a critical or sensitive period for the acquisition of language, and the hypothesis they suggest regarding a critical period for first but not second language is challenging. The studies of late L1 learning from the Mayberry lab provide very strong evidence for a critical or sensitive period. However, I am less persuaded of their interpretation of L2 learning and the contrast they hypothesize between L1 and L2 learning during adulthood. I will suggest another hypothesis regarding the differences in age effects for first versus second languages, one that is equally compatible with their important data. My focus here is therefore on their interpretation of the L2 literature. I will not comment further on their review of Mayberry's remarkable studies of late L1 acquisition, except to congratulate them on this very important work.

Mayberry and Kluender (2017) present an important and compelling argument that in order to understand critical periods (CPs) in language acquisition, it is essential to disentangle studies of late first language (L1) acquisition from those of second language (L2) acquisition. Their primary thesis is that timely exposure to an L1 is crucial for establishing language circuitry, thus providing a foundation on which an L2 can build. They note that while there is considerable evidence of interference from the L1 on acquisition of the L2 – especially in late L2 learners (as in our work on cascading influences on phonetic category learning and visual language discrimination, e.g., Werker & Hensch, 2015 and Weikum, Vouloumanos, Navarra, Soto-Faraco, Sebastián-Gallés & Werker, 2013, respectively) – there are other examples of ways in which the L1 can scaffold L2 acquisition. Mayberry and Kluender take this evidence of L1 scaffolding L2 as undermining the value of considering CPs as useful in understanding L2 acquisition.

The keynote article (Mayberry & Kluender, 2017) makes an important contribution to questions concerning the existence and characteristics of sensitive periods in language acquisition. Specifically, by comparing groups of non-native L1 and L2 signers, the authors have been able to ingeniously disentangle the effects of maturation from those of early language exposure. Based on L1 versus L2 contrasts, the paper convincingly argues that L2 learning is a “less clear test” of sensitive periods. Nevertheless, we believe Mayberry and Kluender underestimate the evidence for maturational factors in L2 learning, especially that coming from recent research.

The issue of critical or sensitive periods affecting the outcome of second language (L2) acquisition has been the subject of intense investigation and debate for many years, with people arguing for or against maturational effects on ultimate attainment. In their influential paper, Johnson and Newport (1989) identify two hypotheses: the exercise hypothesis and the maturational state hypothesis. According to the former, if the capacity for acquiring language is exercised early in life (in first language acquisition), then language learning abilities will remain intact throughout life: in other words, permitting successful L2 acquisition regardless of age. In contrast, according to the maturational state hypothesis, the language learning capacity declines with age, affecting L2 acquisition as well as late L1. Johnson and Newport take their results, which show an age-related decline in performance during childhood and adolescence, to support the maturational state hypothesis. Many L2 researchers have adopted a maturational perspective and have reached similar conclusions as to the presence of critical or sensitive periods (e.g., Abrahamsson, 2012; DeKeyser & Larson-Hall, 2005; Long, 1990; Oyama, 1976; Patkowski, 1980). Some researchers have pointed out that age effects continue into adulthood, contrary to the claim for a critical period (e.g., Birdsong & Mollis, 2001). Others have suggested that what looks like an age-related maturational decline may be accounted for by confounding factors, such as task effects, effects of L1, amount of L2 use, education or input (e.g., Bialystok & Miller, 1999; Flege, Yeni-Komshian & Liu, 1999).

Mayberry and Kluender (2017) make an important contribution to our understanding of the CPL, reporting the striking differences in regions of brain activation in Martin, a deaf man with very late exposure to an L1, compared to other deaf individuals, when processing single signs of ASL. They conclude: “The unique effects of AoA . . . suggest that the hierarchical structure of language and the architecture of the brain language processing system arise from their interaction over the course of early childhood when brain maturation and language acquisition are temporally synchronized.”

We thank the commentators for their thoughtful critiques, which we found both insightful and stimulating to our own thinking. Our first response is that, while debates about the CPL in theoretical contexts are important, the vigor and intensity of these debates should not overshadow the fact that the main goal of our article was to highlight a finding of vital importance: Sufficient language input in early childhood matters deeply because it has long-term consequences (Lillo-Martin, 2018). Woll sums up this point both succinctly and poignantly in her report of a similar case of very late L1 exposure in adulthood who had decades of experience: “For a [deaf] child who, even in the context of early intervention, does not acquire a spoken language, the danger is that they will never have native-like mastery of any L1.” This is what truly matters. Our hope is that our keynote article and the accompanying commentaries might have a positive effect on clinical practice, educational policy, and even parental choice in this regard. In what follows, we discuss the main issues arising from the commentaries. First we note the points of agreement followed by a clarification of what we did not claim in our article. Researchers continue to debate what the shape of the AoA function looks like and its theoretical implications, which we address third. We then address the issues raised as to whether late L1 acquisition and late L2 learning differ in degree or kind, and last we discuss what we mean when we say that language acquisition during post-natal brain growth creates the capacity to learn language.

This study extends research on morphological processing in late bilinguals to a rarely examined language type, Semitic, by reporting results from a masked-priming experiment with 58 non-native, advanced, second-language (L2) speakers of Hebrew in comparison with native (L1) speakers. We took advantage of a case of ‘pure morphology’ in Hebrew, the so-called binyanim, which represent (essentially arbitrary) morphological classes for verbs. Our results revealed a non-native priming pattern for the L2 group, with root-priming effects restricted to non-finite prime words irrespective of binyanim type. We conclude that root extraction in L2 Hebrew word recognition is less sensitive to both morphological and morphosyntactic cues than in the L1, in line with the Shallow-Structure Hypothesis of L2 processing.

Ikizer and Ramirez-Esparza (2017) reported a study suggesting that bilingualism may have a positive impact on people's social skills. They found that a) bilinguals scored higher on a scale that is supposed to reveal social flexibility, and that b) they also report having social interactions more frequently than monolinguals. The authors relate this advantage in social flexibility to the need of exercising language switching in bilingual speakers. In this commentary, we argue that their arguments are not theoretically sound and that their observations are not compelling enough to reach this conclusion.