1. Preamble

It is undeniable that there exists a relationship between age and success in additional language (L2) learning. However, the precise nature of that relationship has long been a matter of controversy. The main area of contention can be summed up in the following questions:

• • is there a language-related maturationally constrained window of opportunity (or critical period) – ending at some point during or at the end of childhood – at the offset of which language acquisition becomes qualitatively different, more arduous and/or less successful in its outcome?

or

• • do age-related changes in language-acquiring capacity and/or outcome result from the impact of factors other than a specifically language-focused critical period?

In what follows we attempt a critical review of research addressing the age factor in L2 attainment. Obviously, however, an article-length treatment cannot hope to provide anything resembling an exhaustive survey. Instead, we focus on a relatively restricted number of issues that have figured prominently in the recent literature. These are:

1. (i) the extent to which it is appropriate to regard native-like performance as the critical yardstick in assessing L2 attainment;

2. (ii) the extent to which we can speak of a genuinely unitary Critical Period Hypothesis;

3. (iii) the relatively narrow scope of much research in this area, with age of L2 onset typically regarded as the crucial variable and other linguistic and contextual variables often insufficiently taken into account;

4. (iv) the suggestion, which has been the subject of recent research, that late language acquisition may make use of different areas of the brain as compared with early acquisition.

In our discussion we attempt to give due attention to the evidence and arguments deployed in favour of the maturational constraints view, accepting that this view informs many other approaches to this topic. It goes without saying that we fully respect colleagues who espouse such other approaches. Our own review leads us, however, to suggest that the maturational constraints perspective has relied too much on native speaker behaviour as a basis for comparison, ignoring a range of issues calling the reliance on such a comparison into question. Our interpretation of available research is also that divergences among researchers concerning the precise nature of the purported maturational constraints concerned have been in large measure glossed over, and that insufficient attention has been paid to a range of other potentially important factors, such as amount and quality of input, learners' orientations and attitudes, and the specific conditions under which the L2 is encountered. Finally, we conclude that the neurobiological research to which the maturational constraints hypothesis has referred is very far from unequivocal, especially in terms of its recent indications.

2. The native speaker as a point of comparison

The criterion for success in L2 has widely been set as the level of proficiency associated with native speakers, in the sense of persons who have been exposed to the language in question since infancy. In the words of Cenoz & Genesee (Reference Cenoz, Genesee, Cenoz and Genesee1998: 18), ‘bilinguals, in and outside the school, are usually evaluated against “monolingual” competence in their non-native languages’. As pointed out by Andreou & Galantyomos (Reference Andreou and Galantomos2009), the notion of native speaker goes back to the widespread belief that one's particular language, one's lingua nativa, was biologically inherited (cf. Christophersen Reference Christophersen1988). In this section we look more closely at the research and issues relating to use of the native-speaker yardstick in language attainment studies.

It is understandable that native speaker proficiency should have been widely used as a point of comparison. The popular view of success in L2 acquisition is closeness to native-speaker performance, and in relation to the age question the common observation is that, at least in non-instructional settings, early acquirers tend to end up indistinguishable from native speakers whereas later acquirers do not. The question, though, is whether reliance on this particular comparison is really the best way of exploring age effects and maturational issues. The discussion that follows suggests that there are better ways of proceeding – that, while the value of research using this comparison certainly needs to be acknowledged, the question mark attaching to it needs to be borne in mind.

It is interesting to note that in L1 acquisition studies, too, the reference-point for evaluating the attainment of late acquirers has been the language of individuals in contact with the relevant L1 since early infancy. The two kinds of late L1 acquirers that figure in the research literature are, first, children who, until their rescue, were deprived of normal human society and linguistic interaction, and, second, profoundly deaf children who are not given access to language input they can process – i.e. signed input – until a relatively late stage.

The most famous modern case of a language-deprived child is that of Genie, who was rescued from the isolation imposed on her by her parents in twentieth-century California (see, for example, Jones Reference Jones1995). After being brought into care Genie made rapid and substantial progress in her language development, but because her English diverged – in particular in its syntactic anomalies – from that of users of the language who had had normal exposure to English since infancy, some researchers interpreted her case as evidencing ‘constraints and limitations . . . outside of . . . the critical maturational period’ (Curtiss Reference Curtiss1977: 234). This reading has been seen by others as rather over-specific in the light of the recent finding that general neglect during the first five years of life leads to a permanently smaller head circumference, smaller brain size, and impaired ability to learn language and to develop normal social behaviour (e.g. Chugani et al. Reference Chugani, Behen, Muzik, Juhasz, Nagy and Chugani2001; see Uylings Reference Uylings, Gullberg and Indefrey2006).

With regard to profoundly deaf subjects who are deprived of processable language input in their early years and then exposed to sign language at a later stage, when their language is compared to that of early signers, some ‘deficits’ are observable which differentiate them from those in contact with sign language from infancy (see, for example, Mayberry Reference Mayberry1993; Morford & Mayberry Reference Morford, Mayberry, Chamberlain, Morford and Mayberry2000; Emmorey Reference Emmorey2002; Singleton & Newport Reference Singleton and Newport2004). This finding has long been used as an argument in favour of the existence of maturational constraints in language acquisition (e.g. Long Reference Long1990), and at first sight it seems persuasive. On the other hand, late signers do not fail to develop a first (signed) language, but, on the contrary, typically become entirely functional in sign language. Following Chugani et al.'s above-cited (Reference Chugani, Behen, Muzik, Juhasz, Nagy and Chugani2001) finding on the effects of neglect, it can be speculated that deprivation of language input during the phase in a child's life when cognitive development is at its most intense may have serious general cognitive effects, and that it is these general effects that are reflected in later language development. Some research (e.g. Peterson & Siegal Reference Peterson and Siegal1995; Schick & Gale Reference Schick and Gale1997; Woolfe, Want & Siegal Reference Woolfe, Want and Siegal2002) does indicate that deaf children whose access to sign language is delayed have problems in the area of theory of mind, that is, in the understanding that individuals other than themselves have mental states (beliefs, intentions, etc.). Lundy (Reference Lundy1999) has argued that the impact of such problems on language development could be far-reaching.

To return to the L2 domain, the criterion of native-speaker proficiency, widely deployed though it may be, is not unquestioned. Hill (Reference Hill1970: 243f.) suggested that the notion of native speaker is culture-bound, and that, for example, in an area like South India, where many adults speak several languages exhibiting similar phonetic systems, it would not necessarily be easy for native speakers of a given local language to recognize L2 users of their own language who are speakers of other local languages. More recently, Cook (e.g. Reference Cook1999, Reference Cook and Cook2002), Davies (Reference Davies2003, Reference Davies, Davies and Elder2004) and Piller (Reference Piller2002) – among others – have problematized the whole notion of the native speaker model both in L2 research and in the L2 teaching context. Cook consistently argues that the focus should be on L2 users in their own right rather than in comparison with native speakers. He remarks that, while ‘ultimate attainment is a monolingual standard rather than an L2 standard’ (Reference Cook2002: 6), there is no intrinsic reason why the L2 user's attainment should be the same as that of a monolingual native speaker. Davies discusses the difficulty of defining what a native speaker actually is. He expresses the view that ‘the distinction native speaker–non-native speaker . . . is at bottom one of confidence and identity’ (Reference Cook2003: 213). Piller (Reference Piller2002), for her part, reports her subjects' perceptions that ‘passing [for a native speaker] is a temporary performance’, ‘typical of first encounters’, and ‘designed for a particular audience’ (p. 200).

In specific regard to age-related studies of L2 acquisition, one of the suggestions made by those taking a critical period perspective is that after a certain maturational point the L2 learner is no longer capable of attaining native-like levels of proficiency. For example, Scovel (Reference Scovel1988) makes this claim with respect to phonetics/phonology (cf. Scovel Reference Scovel2000, Reference Scovel, Abello-Contesse, Chacón-Beltrán, López-Jiménez and Torreblanca-López2006), and Long (Reference Long1990, Reference Long2007) makes it more generally. Such claims have been challenged by research focused on later beginners attaining native-like levels of L2 proficiency: see, for example, Birdsong (Reference Birdsong1992); Ioup et al. (Reference Ioup, Boustagui, Tigi and Moselle1994); Bongaerts, Planken & Schils (Reference Bongaerts, Planken, Schils, Singleton and Lengyel1995); Ioup (Reference Ioup, Singleton and Lengyel1995); Bongaerts et al. (Reference Bongaerts, van Summeren, Planken and Schils1997); Palmen, Bongaerts & Schils (Reference Palmen, Bongaerts and Schils1997); Bongaerts (Reference Bongaerts and Birdsong1999); Moyer (Reference Moyer1999); Bongaerts, Mennen & Van der Slik (Reference Bongaerts, Mennen and Van Der Slik2000); Bongaerts (Reference Bongaerts2003); Muñoz & Singleton (Reference Muñoz and Singleton2007); Kinsella (Reference Kinsella2009) (cf. Singleton & Leśniewska Reference Singleton, Leśniewska and Pawlak2009). See Long (Reference Long2005), however, for a critical analysis of the methodological and other problems that some of this research may present.

Hyltenstam & Abrahamsson (Reference Hyltenstam and Abrahamsson2000: 155) assert that no post-pubertal L2 beginner has yet been shown to reach a level of proficiency that in every linguistic detail is identical to that of a native speaker. They also recognize, let it be said, that the more closely we study very early L2 beginners the more we realize that, at the level of subtle detail, they also tend to differ from monoglot native speakers. On this latter point research has shown (see, for example, Hyltenstam & Abrahamsson Reference Hyltenstam and Abrahamsson2000: 161; Hyltenstam & Abrahamsson Reference Hyltenstam, Abrahamsson, Doughty and Long2003a, Reference Hyltenstam and Abrahamssonb) that even very young L2 beginners may diverge from native speakers at the level of lexico-grammatical detail. (cf. also Abrahamsson & Hyltenstam Reference Abrahamsson and Hyltenstam2009, which goes beyond the lexico-grammatical dimension). The same is true at the phonological level: Flege (e.g. Reference Flege and Birdsong1999, Reference Flege, Burmeister, Piske and Rohde2002) cites a number of studies which show that individuals exposed to an L2 in an L2 environment as young children are nevertheless quite likely to end up speaking the L2 with a non-native accent (e.g. Flege, Frieda & Nozawa Reference Flege, Frieda and Nozawa1997; Guion, Flege & Loftin Reference Guion, Flege and Loftin2000; Piske, Mackay & Flege Reference Flege and Liu2001) and to be less good than monoglot native speakers at vowel and consonant perception in their target language (Flege, MacKay & Meador Reference Flege, MacKay and Meador1999; MacKay, Meador & Flege Reference MacKay, Meador and Flege2001).

It could well be, therefore, that the maturational issue is less important in this connection than the very fact of the possession of knowledge of another language (cf., for example, Grosjean Reference Grosjean1982; Cook Reference Cook, Singleton and Lengyel1995). It seems also to be the case that the degree of distance between the L1 and the L2 plays a role (cf. Kellerman Reference Kellerman, Eubank, Selinker and Sharwood Smith1995). McDonald (Reference McDonald2000) certainly found, for example, that learners of English from a Spanish-speaking background who had begun to be exposed to the language before the age of five were able to perform to native levels on an English grammaticality judgement test, whereas Vietnamese speakers with pre-age-five experience of English were not. In any case, what the foregoing would seem to imply from our perspective is that a more appropriate comparison than that between later L2 beginners and native users of the language in question might be one between later L2 beginners and those who begin to acquire an L2 in early childhood (cf. Singleton & Ryan Reference Singleton and Ryan2004:109; Ortega Reference Ortega2010).

Recent work by Abrahamsson & Hyltenstam (Reference Abrahamsson and Hyltenstam2008), however, again uses indistinguishability from native speakers as the criterion for success in late L2 acquisition. They found that high levels of language aptitude were possessed by late L2 learners of Swedish who were judged to be native speakers by native speakers. Their argument is that a high degree of language aptitude is the sine qua non of nativelike attainment in late second language acquisition, and they suggest that the possession of such aptitude by a few individuals ‘does not justify a rejection of the CPH’ (p. 503). They propose (ibid.) a research agenda which would investigate ‘the way in which such learners attain their nativelikeness – for example, through the use of unique psychological processes and an unusual sensitivity to language structure or even through continued access to the innate, implicit language acquisition mechanism that, for some reason, has remained unaffected by maturation’. Finally, they make the prediction that ‘no adult learners should be found who are entirely nativelike in the L2 without having a high level of language aptitude and – we may add – without having worked professionally and successfully with the target language for a significant period of their lives’ (pp. 503f.).

Regarding the detail of the above claim, we should first comment on the idea of the innateness of language. This is an underlying axiom for one aspect of Abrahamsson & Hyltenstam's claim, but in fact, in the sense in which they deploy it is by no means universally accepted (cf. Sampson Reference Sampson2005). On the other hand, the notion that highly successful late L2 learners (indeed all highly successful L2 learners) require considerable amounts of input and experience in the target language is not at all controversial, although it would clearly be unwarranted to suggest that only cases of nativelike late learners without such abundant input and experience could be considered candidates for undermining the Critical Period Hypothesis. In relation to the question of language aptitude, there is certainly evidence that this may play a role in successful L2 acquisition. Bylund (Reference Bylund2009; Bylund, Abrahamsson & Hyltenstam Reference Bylund, Abrahamsson and Hyltenstam2010) have also proposed that high levels of language aptitude may act as a prophylactic against language attrition. Clearly, one may be sympathetic to this proposition without necessarily accepting the authors' claim that there is a critical period, ending at puberty, for avoiding attrition. In order to test detailed predictions in these matters with appropriate rigour, however, one would ideally need a more satisfactory definition of the construct of language aptitude than the fact of doing well on a specific language aptitude test.

The widely shared intuition of language professionals is that language aptitude is likely to contribute to successful L2 attainment at any age. It is noteworthy that Abrahamsson & Hyltemstam themselves find ‘small yet significant aptitude effects in child SLA’ (Reference Abrahamsson and Hyltenstam2008: 481). In the present context, more to the point is the fact that the exclusion as counter-examples to the CPH of cases of high-attaining adult beginners who happen to be possessed of a good measure of language aptitude radically changes the whole critical period concept. In the biological sciences, a critical period is conceived of as species-wide, as transcending individual attributes. In any case, to return to the main theme of the present section, and to emphasize further a line of argument put earlier, given all that we now know about the interaction between languages in the mind (see, for example, Cook Reference Cook1992, Reference Cook2003, Reference Cook2006), using native speaker behaviour as the yardstick of L2 attainment for L2 acquirers beginning at any age may require some re-evaluation, especially in relation to adult beginners, whose L1 is long established and deeply entrenched.

Let us return to the above-mentioned issue of the innateness of language. The Chomskyan version of the innateness hypothesis throws an interesting sidelight on the use of a native speaker yardstick in the discussion of age effects in L2. This relates to the question of whether the criterion to be adopted in assessing L2 attainment of late acquirers should be the degree of native-like performance or behaviour in the L2 or whether we should simply be looking at the extent to which late L2 acquirers, in common with native speakers and early acquirers, are able to derive particular kinds of mental representations from purportedly bio-endowed mechanisms.

The claim made by some Chomskyans is that there is a fundamental qualitative difference between child L2 acquirers and post-pubertal L2 acquirers in terms of access to the innate bio-endowment – ‘Universal Grammar’ (UG) – which, according to Chomsky, informs and guides language acquisition (see, for example, Cook & Newson, Reference Cook and Newson2007: 237f.). Researchers such as Bley-Vroman (Reference Bley-Vroman, Gass and Schachter1989) have suggested that the child L2 acquirer is on a par with the child L1 acquirer in benefiting from access to innate mechanisms, but that the post-pubertal L2 learner has no access to UG. The empirical basis for this perspective was never solid (cf. Martohardjono & Flynn Reference Martohardjono, Flynn, Singleton and Lengyel1995; Hawkins Reference Hawkins2001: 353–359), which is doubtless why other views on this issue abound within the Chomskyan paradigm – the notion that full access to UG remains throughout life, as well as various claims regarding the continuation of partial access beyond childhood (see, for example, Mitchell & Myles Reference Mitchell and Myles2004: 78f.).

Some UG researchers, adopting a partial access standpoint, have attempted to isolate precisely which linguistic modules, submodules, features, or interface areas are deleteriously affected by maturation. One oft-cited example of such an approach is Hawkins & Chan's (Reference Hawkins and Chan1997) Failed Formal Features Hypothesis (FFFH), which posits a critical period for the selection of parameterized formal features, while suggesting that the principles of UG remain available beyond this critical period. The hypothesis suggests that formal features not selected during the course of L1 acquisition become inaccessible in respect of L2 acquisition in adulthood. Hawkins & Chan claim to find evidence within their own study that speakers of Chinese (a language without wh-operator movement in overt syntax) learning L2 English (a language with wh-operator movement in overt syntax) beyond the childhood years establish L2 mental representations which involve pronominal binding rather than operator movement. Montrul & Slabakova (Reference Montrul and Slabakova2003), on the other hand, focusing on the post-pubertal acquisition of the Spanish aspectual system by English speakers, found that purportedly universal features of functional categories in this domain that were not selected in the L1 in early childhood remained accessible beyond childhood.

As is evident from the foregoing, the UG perspective on the critical period is less concerned with performance – the overt use of the language – than with underlying competence, in the Chomskyan sense, and the associated mental representations. As Rothman puts it:

The nature of such mental representations is the nub of the question for UG-informed research – not the capacity or otherwise of L2 acquirers to perform like native speakers. In his own work, Rothman reports that the late L2 acquirers he investigates, while certainly not native-like in terms of the detail of their behaviour in the L2, remain capable of overcoming poverty-of-the-stimulus conditions and of arriving at particular UG-derived mental representations at the syntax–semantics interface. Given the different concerns of UG-based research into maturational constraints, the issue of whether or not to use native speaker performance as a yardstick arises to a much lesser degree than in more ‘applied’ areas of the CPH debate. The mental representations under scrutiny are, of course, identified on the basis of conclusions reached concerning the nature of native-speaker competence, but the question here is not about late L2 acquirers’ capacity to pass for native speakers, but rather about whether or not these underlying mental representations can continue to be induced from L2 data with advancing maturation.

A final dimension to the question of the use of the native speaker as a reference point for L2 acquirers is given by cases where in functional terms the L2 becomes the L1 (cf. Bialystok Reference Bialystok1997). Where, for example, a child migrating to a particular community has an L1 which is different from that of the majority of members of the host community, where his/her home language receives little or no support from the community in question and where his/her parents make little or no effort to support it either, the language in question may significantly attrite. One such case, reported by Kouritzin (Reference Kouritzin1999: 75–96), is that of Lara, who had migrated with her family from Finland to Canada at age two, and subsequently lived for four years in a small town within a tight-knit Finnish community, continuing to develop in Finnish. At age six, however, her parents decided that the time had come to integrate with English-speaking Canada, and the entire family moved to a large city. According to Lara's account, her development in Finnish came to a halt at this point and English progressively took over. She reported that the last time she tried (and failed) to converse in Finnish was when she was eighteen years old. Her perception was that she had lost her original L1 and that her language was now English.

Researchers have been aware of this kind of phenomenon for a long time. Lambert, writing about the negative image of bilingualism in the first half or so of the twentieth century pointed out in 1975 that relevant negative research findings emanating from early studies mostly focused on sequential bilinguality in immigrant or minority children whose L1 was effectively being replaced by the language of the majority population. This point remains valid for more recent studies showing negative effects of bilinguality (Hamers & Blanc Reference Hamers and Blanc2000: 93). It has been observed (notably by Jia & Aaronson Reference Jia and Aaronson1999, Reference Jia and Aaronson2003) that whereas older arrivals in an L2 environment often make choices which bring them into frequent contact with fellow native speakers of their L1, which accordingly restrict their contact with the L2, fewer choices are available to younger arrivals, because of compulsory schooling. In any case, their linguistico-cultural identity is not as fully formed as that of their elders, so their motivation to maintain it is likely to be weaker. According to Jia & Aaronson the consequence of these contrasting circumstances is that, whereas immigrants arriving after age ten on the whole maintain their L1, immigrants arriving before age ten tend to switch their dominant language from the home language to the language of the host country.

Likewise, Flege writes of a frequent trade-off in migrant children between L2 and L1 proficiency specifically in the phonetic–phonological domain, and a switch of dominance from L1 to the L2, his line being that in bilingual children ‘the phonic elements of the L1 subsystem necessarily influence phonic elements in the L2 system, and vice versa’ (Reference Flege and Birdsong1999: 106). Thus, depending on their experience and input, young children may acquire a good L2 accent at the expense of their L1 accent. Alternatively, they may develop an authentic accent in their L1 at the cost of a non-native accent in their L2. Flege further contends that in migrants arriving as adults in the L2 environment, L1 phonology has continued to be refined, untrammeled by other language influences, and that its influence on L2 phonology acquisition is accordingly much stronger than in the case of child migrants.

If Jia & Aaronson and Flege are right, the favourable comparison made between the L2 proficiency of early L2 acquirers and that of native speakers of the language in question is explicable in many cases by the fact that these acquirers’ erstwhile L2 has in fact switched its status to that of L1 – understood not in the classic sense of the language of infancy, obviously, but in terms of dominance, functionality and users’ perceptions. There is a sense, therefore, in which making a comparison between the proficiency of such language users in their dominant language and that of native speakers is not a matter of comparing L2 attainment with an L1 baseline, because for such language users their dominant language for all practical purposes is itself their L1. As for late acquirers, the continuing dominance (and influence) of their language of infancy, if viewed in the terms proposed by Jia & Aaronson and/or by Flege, emerges in most cases as an age-related phenomenon, but not strictly a maturationally determined phenomenon.

3. Variability in proposed offset points for the critical period

So far we have been talking about early and late, younger and older beginners, as if the underlying basis for such polarities were clear and agreed upon. This is far from the case, however. From the outset of the discussion of age-related factors in language acquisition there have been disagreements about when the optimum period for acquisition ends. Thus, the pre-cursor of critical period thinking, Penfield, suggested that ‘for the purposes of learning languages, the human brain becomes progressively stiff and rigid after the age of nine’ (Penfield & Roberts Reference Penfield and Roberts1959: 236) and that ‘when languages are taken up for the first time in the second decade of life, it is difficult . . . to achieve a good result’ (Penfield & Roberts Reference Penfield and Roberts1959: 255). Lenneberg (Reference Lenneberg1967), on the other hand, proposed puberty as the offset point for the critical period. With respect to L2 acquisition, he asserted (p. 176) that after puberty ‘the incidence of “language-learning-blocks” rapidly increases’, ‘foreign languages have to be . . . learned through a conscious and labored effort’, and ‘[f]oreign accents cannot be overcome easily’.

Other researchers have suggested that different areas of language (phonology, syntax, lexicon, etc.) have different critical ages associated with them (for discussion see, for example, Singleton & Ryan Reference Singleton and Ryan2004: 84–94; Singleton Reference Singleton2005). Such multiplicity implies a multiplicity of mechanisms, which is entirely plausible, but which clearly runs counter to the notion of a unitary critical period. A further group of L2 researchers assert that the critical period ends progressively over a number of years, a process that begins around the age of six or seven (e.g. Johnson & Newport Reference Johnson and Newport1989; Long Reference Long1990). More radically, Hyltenstam & Abrahamsson (Reference Hyltenstam, Abrahamsson, Doughty and Long2003a: 543–544) cite, in this case in respect of L1 evidence, Ruben's (Reference Ruben1997) review of studies of children who had experienced temporary hearing impairment in their first year of life and who subsequently showed deficits in verbal memory and phonetic perception in L1. Ruben concludes that the critical period for phonetics/phonology ends around the twelfth month of infancy. He further interprets the relevant research as indicating that the critical period for syntax ends in the fourth year of life, and for semantics in the fifteenth or sixteenth year. To return to L2 evidence, Hyltenstam & Abrahamsson's own review of the evidence leads them to have doubt about the critical period itself, and to state in one publication that it may be une chimère (Reference Hyltenstam and Abrahamsson2003b: 122). Elsewhere they speculate that the language learning mechanism may be ‘designed in such a way that it . . . inevitably and quickly deteriorates from birth’ (Reference Hyltenstam, Abrahamsson, Doughty and Long2003a: 575) and, for this reason, ‘nativelike proficiency in a second language is unattainable’ (p. 578) (cf. also Abrahamsson & Hyltenstam Reference Abrahamsson and Hyltenstam2009).

Referring to L1 evidence, Aram et al. note (Reference Aram, Bates, Eisele, Fenson, Nass, Thal and Trauner1997: 85) that ‘the end of the critical period for language in humans has proven . . . difficult to find, with estimates ranging from one year of age to adolescence’. The impact of such uncertainty regarding the critical age at which, purportedly, the language acquisition capacity abruptly deteriorates is twofold. First, it is taken by some to undermine the plausibility of the entire notion of a critical period for language acquisition, whether L1 or L2. Second, it deprives the concepts of ‘early’ and ‘late’ L2 learning of any kind of stable reference point and, therefore, of any real meaning.

With regard to the former observation, if there were clear evidence for the closing of a window of opportunity for language aquisition, surely it ought to be possible for researchers to agree where it is situated. The fact that there is so much disagreement about this matter casts severe doubt on the whole idea of a critical period for language. After all, zoologists seem to have no hesitation in determining the critical period for the development of binocularity in various species – including humans (Almli & Finger Reference Almli, Finger and Bornstein1987: 126). In fact, scepticism regarding critical periods affecting higher-order functioning in humans is not confined to the language domain. Thompson (Reference Thompson, Bailey, Bruer, Symons and Lichtman2001: 87), for example, talking about child psychology in a general way, comes to the conclusion that ‘the complexity of the behavioral systems to which these concepts are applied in young children makes it difficult, if not impossible, to identify the parameters of sensitive periods with appropriate specificity’. Bailey (Reference Bailey2002) argues similarly that proof of the existence of developmental critical periods in human beings has been elusive, and that their invocation by child educators may in fact be a distraction from the real issues.

Concerning the point made in the previous paragraph about the stability of reference points, if twelve years is taken to be the critical age, L2 learning at age four should presumably be regarded as ‘early’ learning, but if twelve months is taken to be the critical age, L2 learning at age four would already be ‘late’ learning. At the very least, this implies that any comparison of early and late L2 beginners must be discussed relative to a particular timing of the critical age, and that any such comparison risks falling foul of other views on the offset of the critical period.

Two further points need to be made in the context of the above discussion. The first is that variability in the views on the critical period concern not only the timing of its offset but also the scope of its effects. Whereas, for example, Lenneberg (Reference Lenneberg1967) saw such constraints as affecting language in general, for Scovel (Reference Scovel1988, Reference Scovel2000, Reference Scovel, Abello-Contesse, Chacón-Beltrán, López-Jiménez and Torreblanca-López2006), as we have seen, they are relevant only to the phonetic/phonological sphere. Other researchers limit the purported domain of the age factor quite differently. Cummins, for example, explains certain findings from critical period-oriented research in terms of age-related effects affecting basic interpersonal communicative skills (BICS) rather than cognitive academic language proficiency (CALP) (Cummins Reference Cummins1979: 199ff.). Other researchers have drawn a distinction between implicit and explicit learning. DeKeyser (Reference DeKeyser2000, Reference DeKeyser, Doughty and Long2003a, Reference DeKeyser2003b, Reference DeKeyser, Abello-Contesse, Chacón-Beltrán, López-Jiménez and Torreblanca-López2006) suggests that maturational constraints apply only to implicit language learning mechanisms. Clearly, the divergences evident in the approaches outlined above further fragment the maturational constraints hypothesis.

The second point has to do with the nature of the offset. In order to qualify as a critical period as this is understood in the biological sciences, the purported optimum period for language acquisition would have to end in a relatively abrupt manner. That is to say, the observed decline in capacity would have to be qualitatively different from the overall, gradual, linear decline in physical and cognitive capacities that is associated with the ageing of any organism. So we must ask: is this the case? Classic critical period-friendly studies such as that of Johnson & Newport (Reference Johnson and Newport1989) have claimed to find a sharp discontinuity between the performance of L2 acquirers who began to be exposed to the L2 before or after the supposed offset of a critical period. The recent investigation by DeKeyser, Alfi-Shabtay & Ravid (Reference DeKeyser, Alfi-Shabtay and Ravid2010), focussing on morphosyntax, also suggests a steep decline until the age of 18 followed by a levelling-off. Other researchers, however, have placed a question-mark over the notion that any age-related decline in language-acquiring capacity has an ‘elbow-shaped’ slope of the kind that one would expect if it were determined by a critical period (Bialystok & Hakuta Reference Bialystok, Hakuta and Birdsong1999; Flege Reference Flege and Birdsong1999; Birdsong Reference Birdsong, Davies and Elder2004, Reference Birdsong2006). Bialystok & Hakuta's re-analysis of Johnson & Newport's data (Bialystok & Hakuta Reference Bialystok and Hakuta1994; Bialystok Reference Bialystok1997) suggests ‘that the tendency for proficiency to decline with age projects well into adulthood and does not mark some defined change in learning potential at around puberty’ (Bialystok Reference Bialystok1997: 122). Bialystok and her colleagues also analysed census data on reported age of arrival in an L2 English-speaking environment and reported proficiency in English; they detected a steady linear decline of reported proficiency as age of arrival increases, but no indication of a dramatically sharper rate of decline at any given point (Bialystok & Hakuta Reference Bialystok, Hakuta and Birdsong1999; Hakuta, Bialystok & Wiley Reference Hakuta, Bialystok and Wiley2003; Wiley, Bialystok & Hakuta Reference Wiley, Bialystok and Hakuta2005). We should note that Bialystok et al.'s interpretation of such census-derived data has been heavily criticized on methodological grounds (DeKeyser & Larson-Hall Reference DeKeyser, Larson-Hall, Kroll and DeGroot2005), but, on the other hand, data on the relationship between L2 accent and age of arrival show a similarly continuous decline (cf. Flege Reference Flege and Birdsong1999). Birdsong (Reference Birdsong2006: 12) comments that ‘a recurrent finding is that a linear function captures the relationship between AoA [age of acquisition] and outcome over the span of AoA’.

Reichle's publications (Reference Reichle, Arabski and Wojtaszek2010a, Reference Reichle2010b) provide further evidence of the continuity of a linear decline in L2 learners’ capacities well into adulthood. This research focuses on information structure (IS) – the ways in which a speaker deploys syntactic cues to signal the important elements in a sentence, with particular reference to the French clefting construction involving c'est – for example (Reference Reichle, Arabski and Wojtaszek2010a: 24), C'est un marteau qui se trouve sur la table: ‘a hammer is on the table’. Reichle administered an acceptability judgment interpretation of the c'est construction to L2 users of French all of whom were at least 18 years old, and who had resided in France for periods ranging from four to 32 years. In his results he found evidence for a decline in the accuracy of acceptability judgments as age of arrival increased, but he also found that ‘[t]he relationship between judgment task proficiency and age of arrival continues to decline past the years of physical and cognitive maturation and well into adulthood’ (Reichle Reference Reichle, Arabski and Wojtaszek2010a: 28). He interprets these findings as running counter to a strong version of the Critical Period Hypothesis. In a second study, Reichle again looked at acceptability judgments relating to the use of c'est. His experimental groups on this occasion consisted of low-proficiency and high-proficiency L2 users of French, all of whom were at least 18 years of age and whose average age of exposure was the mid-teens. The results of this experiment were more difficult to interpret, suggesting that under controlled conditions truth value takes precedence over focus structure in acceptability judgments. Reichle concedes that in the light of such differences, further studies are needed, involving greater numbers of participants. Nevertheless, he affirms (Reference Reichle2010b: 82) that the high level of native-like behaviour among late acquirers in both studies ‘is contrary to one of the criteria for the critical period’.

A final comment on the shape of the decline of age effects: whereas an elbow shape would certainly strongly argue for the end of a privileged period for L2 acquisition, the smooth linear decline that is observed in a number of studies suggests an age-related decline more in keeping with what we know about the contour of general cognitive deterioration (see Birdsong Reference Birdsong2006). Moreover, given the high degree of interrelation of the factors that influence L2 acquisition, it seems reasonable to hypothesize that the effects observed may result from the interplay of a range of variables (including age of acquisition) rather than the decline of any specific faculty. Of course, in order to verify such a hypothesis a great deal more research is required.

4. Contextual and individual factors

Comparisons of the effects of age of onset or age of acquisition (AoA) and of length of residence (LoR) in the target language community have frequently shown that the correlation between age of initial learning and proficiency is higher than that between length of residence and proficiency. This has been used in support of the proposition that age effects are the most important, or even the only, determinant factor of variable proficiency in SLA. Recently, however, a number of researchers have highlighted the role of contextual and individual factors that may mediate or interrelate with age effects in SLA. In this section we begin by reviewing recent research that has focused on the impact of L2 input and contextual factors. We then review studies that have looked at socio-psychological and cognitive factors in relation to age.

5. Neurolinguistic dimensions

Recently, the use of sophisticated neuroimaging techniques in age-related SLA research has raised high expectations regarding their potential as a source of evidence relative to the CPH. However, the interpretation of neuroimaging-based research evidence is difficult at this stage, because we still know very little about, for example, the exact functions of brain areas involved in language production and perception, or about such issues as the relationship between the localization of neural substrates and language learning outcomes (cf. Fabbro Reference Fabbro1999, Reference Fabbro and Cook2002; De Bot Reference De Bot2000, Reference De Bot2004). In what follows, a brief account of neurolinguistic work is presented, identifying the foci of concern for researchers working in this area of inquiry.

A critical or a sensitive period for language acquisition accords well with the idea that a late-acquired L2 is represented in the brain differently from the L1. Specifically, under the strong version of the hypothesis, the classical language areas are not available for the learning of an L2 after the critical period. In fact, since the early days of the anatomo-clinical approach the claim of differential localization for L2 has dominated neurolinguistics, following the discoveries of Broca (Reference Broca1861). For example, as noted by Abutalebi (Reference Abutalebi2008), Scoresby-Jackson (Reference Scoresby-Jackson1867) based his account of the selective loss of an L2 in an aphasic patient after brain damage on the idea that Broca's area is the language organ only for native languages, whereas the remaining part of the left inferior frontal gyrus might be responsible for L2s. In more recent times, a clear distinction has been made between the cognitive system mediating the meaning of words on the one hand, and the grammar of an L2 on the other. While the former tends to be seen as common across languages (Kroll & Stewart Reference Kroll and Stewart1994; Francis Reference Francis1999), the representation of L2 syntax has become a controversial issue. The line that argues for the existence of a critical period and a consequent different representation (i.e. at a cognitive level) of a language acquired after puberty is well represented by Paradis (Reference Paradis and Ellis1994, Reference Paradis2004).

According to Paradis, there is a fundamental distinction between L1 and L2 learning in that the L1 grammar is acquired implicitly whereas an L2 grammar, if learned after the end of the critical period, is acquired explicitly. This implies that individuals may have declarative knowledge of the L2 grammar, even if the processes involved in production and comprehension may remain inaccessible to conscious awareness. A rationale for this position is provided by Ullman's Declarative/Procedural model (Reference Ullman2001). This model proposes that, in monolinguals, words are represented in a declarative memory system, whereas grammatical rules are represented in a cognitive system that mediates the use of procedures. Following Ullman (Reference Ullman2001, Reference Ullman2004), procedural and declarative knowledge are mediated by distinct neural systems involving a fronto-striatal network (i.e. Broca's area and the basal ganglia) for the first type and left temporal areas for the second. An L1 is acquired implicitly and mediated by an innate language learning mechanism only triggered during a critical period, whereas an L2 is generally acquired explicitly via formal instruction and represented declaratively in a left temporal area along with L1 and L2 vocabulary. In consequence, grammatical knowledge for an L2 learned after the critical period may not be processed through the neural structures related to implicit processing such as Broca's area and the basal ganglia, as is the case for L1 grammar, but, rather, be subserved by the temporal memory system. (Paradis Reference Paradis and Ellis1994, Reference Paradis2004; Ullman Reference Ullman2001, Reference Ullman2004). This is the line taken by Clahsen and his colleagues (cf. Clahsen & Felser Reference Clahsen and Felser2006; Felser & Clahsen Reference Felser and Clahsen2009) in their investigation of grammatical processing by adult language learners. In their studies, adult L2 learners are observed to under-use syntactic information during the processing of morphologically complex words and sentences and to rely more on lexical-semantic cues to interpretation. This is taken as support for ‘the shallow structure hypothesis for L2 processing according to which mature language learners interpret structurally complex sentences by assigning . . . representations to the input that lack grammatical detail . . .’. As underlying reasons for this, these researchers suggest ‘maturational changes in the brain during childhood and adolescence that make it harder for late language learners to acquire procedural processing routines, as suggested by Ullman (Reference Ullman and Sanz2005)’ (Felser & Clashen Reference Felser and Clahsen2009: 315).

An alternative account that challenges the claim of differential localization for the L2 is Green's convergence hypothesis (Reference Green, van Hout, Hulk, Kuiken and Towell2003). According to this hypothesis, ‘the acquisition of the L2 arises in the context of an already specified, or partially specified, language system and L2 will receive convergent neural representation within the representations of the language learned as the L1’ (Abutalebi Reference Abutalebi2008: 468). Following Abutalebi & Green (Reference Abutalebi and Green2007: 247) ‘the neural representation of L2 (i.e. the brain structures implicated in its representation) converges with the neural representation of that language in native speakers – in other words, there is common neural network subserving L1 and L2’. Further, according to their dynamic view of bilingual speech production, the single network mediating the representation of a person's L1 and L2 is modulated by a control structure (integrated by separable neural systems including the anterior cingulated cortex, the basal ganglia, the inferior parietal lobule and, most prominently, the prefrontal cortex), and the manner in which this network operates depends on a person's proficiency in L2. More specifically, an increase in proficiency is accompanied by a shift from controlled to automatic processing and this will be accompanied by a reduction in prefrontal activity. In the end, neural differences between native and L2 speakers may disappear as proficiency increases. The convergence hypothesis has succeeded in bringing L2 learners’ level of proficiency to the foreground so that both the current major strands of neurolinguistic investigation (those concerned with the where question and those concerned with the when question) have tended to focus on both AoA and level of proficiency as the potential predictors of degree of L1-like processing of the L2 (see Birdson & Paik Reference Birdsong, Paik, Spolsky and Hult2008: 427).

Research concerned with the where question using functional neuroimaging techniques has found ample support for the notion that the same structures underlie the acquisition of L1 and L2. In his overview of neuroimaging studies focusing on grammatical and lexico-semantic processing in bilinguals, Abutalebi (Reference Abutalebi2008) notes three types of studies: those that use an artificial grammar, those concerned with L1 processing (in particular with syntactic encoding for the characterization of the neural structures mediating grammar), and those focusing on grammatical processing in L1 and L2 in bilinguals. As an example, the activation exerted by different natural languages in bilinguals has been investigated by a large number of functional neuroimaging studies, and Abutalebi (Reference Abutalebi2008: 469) lists 12 functional neuroimaging studies that seem to have contradicted the predictions of Ullman's Declarative/Procedural model, showing that overall both low and high proficiency bilinguals engage for L2 the same neural structures responsible for grammatical processing in L1.

However, when the L2 system is weak, neural differences between L1 and L2 may exist, it seems, for both grammatical processing and lexico-semantic processing. The lexico-semantic domain has been well studied by means of neuroimaging techniques, and the detailed findings can be found in comprehensive recent overviews (Indefrey Reference Indefrey, Gullberg and Indefrey2006; Abutalebi & Green Reference Abutalebi and Green2007). Abutalebi (Reference Abutalebi2008) summarizes the main findings. In contrast to the results from studies that failed to take account of L2 proficiency level (e.g. Kim et al. Reference Kim, Relkin, Lee and Hirsch1997), it transpires that when L2 proficiency is comparable to L1, neuroimaging studies have reported common activations in similar left frontal and temporo-parietal brain areas which are engaged when monolinguals perform the same tasks, both in single word production tasks (e.g. Hernández et al. Reference Hernández, Dapretto, Mazziotta and Bookheimer2001; Ding et al. Reference Ding, Perry, Peng, Ma, Li and Xu2003) and in retrieval tasks (Chee, Tan & Thiel Reference Chee, Tan and Thiel1999). On the other hand, bilinguals with low proficiency in L2 engaged additional brain activity in the latter type of tasks, mostly in prefrontal areas (e.g. Briellmann et al. Reference Briellmann, Saling, Connell, Waites, Abbott and Jackson2004), as well as in lexical decision tasks (Illes et al. Reference Illes, Francis, Desmond, Gabrieli, Glover, Poldrack, Lee and Wagner1999; Pillai et al. Reference Pillai, Allison, Sethuraman, Loring, Thiruvaiyaru, Ison, Balan and Lavin2003) and semantic judgment tasks (Chee et al. Reference Chee, Hon, Ling Lee and Soon2001; Wartenburger et al. Reference Wartenburger, Heekeren, Abutalebi, Cappa, Villringer and Perani2003; Rueschemeyer, Zysset & Friederici Reference Rueschemeyer, Zysset and Friederici2006).

Exposure to the L2 may also have a strong influence on language inter-dependency in the bilingual lexico-semantic system. Perani et al. (Reference Perani, Abutalebi, Paulesu, Brambati, Scifo and Cappa2003) investigated the effect of ‘differential exposure’ to L2 in a functional Magnetic Resonance Imaging (fMRI) study of two groups of early highly proficient bilinguals (Spanish-born or Catalan-born). Speakers in the group that had Spanish as L1 and lived immersed in Catalan, as assessed by an extensive questionnaire, activated less of the left prefrontal cortex for word generation in L2 than speakers in the group that had Catalan as L1 and were less exposed to Spanish (their L2). Following Abutalebi (Reference Abutalebi2008), these exposure-related differences, observed in the left dorsolateral frontal cortex, are in line with evidence from previous studies in monolinguals, reporting that experience and practice on language task performance might result in decreased neural activity within the left prefrontal cortex (Thompson-Schill, D'Esposito & Kan Reference Thompson-Schill, D'Esposito and Kan1999). More evidence for the role of exposure is provided by the study by Pallier et al. (Reference Pallier, Dehaene, Poline, LeBihan, Argenti, Dupoux and Mehler2003), which indicates that when L1 input is no longer available, the L2 seems to be able to take over the role of the L1. In this study Pallier et al. tested eight Korean participants who were adopted by French-speaking families between the ages of three and eight. Behavioural and fMRI data revealed that the Korean participants did not differ significantly from the French controls. Pallier et al. (Reference Pallier, Dehaene, Poline, LeBihan, Argenti, Dupoux and Mehler2003: 158) interpret their results as evidence ‘in favour of the reversibility of plastic changes associated with language acquisition in the first few years of life’. However, in work also comparing Korean adoptees with highly proficient Swedish learners of Korean, Hyltenstam et al. (Reference Hyltenstam, Bylund, Abrahamsson and Park2009) challenge Pallier's results on methodological grounds. Their principal argument is that the attrited language can be reactivated through intense relearning, which explains the advantage shown by some of the adoptees in their own work. In contrast, they say, the lack of a relearning programme in Pallier el al.'s study had not allowed L1 remnants to re-emerge.

It is suggested that L2 proficiency and exposure may be the main determinants for lexico-semantic processing, whereas the age of L2 acquisition seems to have no major role in this domain (Indefrey Reference Indefrey, Gullberg and Indefrey2006; see also Perani & Abutalebi Reference Perani and Abutalebi2005). In contrast, in the grammatical domain, the neural substrate is argued to be more dependent on age of acquisition effects than proficiency effects. Following Abutalebi (Reference Abutalebi2008), the available evidence points to the role of age of acquisition, since late L2 learners recruit more neural resources around the areas mediating L1 syntax (e.g. Wartenburger et al. Reference Wartenburger, Heekeren, Abutalebi, Cappa, Villringer and Perani2003; Jeong et al. Reference Jeong, Sugiura, Sassa, Haji, Usui, Taira, Horie, Sato and Kawashima2007). However, Abutalebi also notes that many of the studies showing that late learners engage the prefrontal cortex more extensively were carried out with relatively low L2 proficiency subjects. The more extensive activity along the left prefrontal cortex could then be due to the degree of L2 proficiency rather than to age of acquisition. An exception is the study by Wartenburger et al. (Reference Wartenburger, Heekeren, Abutalebi, Cappa, Villringer and Perani2003) that investigates late bilinguals with a nativelike proficiency. The study reports that these highly proficient bilinguals activate the prefrontal cortex more extensively, suggesting that grammatical processing may be neurologically wired in. Abutalebi (Reference Abutalebi2008) concludes that more studies are needed that investigate very high proficiency bilinguals, in order to see if the neural basis for grammatical processing depends on age of acquisition. Another problem with studies such as that of Wartenburger et al. (Reference Wartenburger, Heekeren, Abutalebi, Cappa, Villringer and Perani2003) is that the analysis does not allow us to see if the age effect has an elbow shape – showing abrupt discontinuity after a certain age – or if, on the contrary, there is a continuous decline with age. In this study, late learners have an AoA greater than six and a mean AoA of 19 (with a standard deviation of 6.6); that is to say, there is a wide variation in age. In fact, the authors themselves conclude that the exact time-frame of any critical period remains unspecified.

In this context, Snow (Reference Snow, Galaburda and Kosslyn2002) argues that the real question about age differences in brain localization is whether it implies anything about behaviour or about critical periods. Besides, even if differences clearer and greater than those shown in research so far were found, they could still be irrelevant to the issue of the critical period for language acquisition. As Marinova-Todd, Marshall & Snow (Reference Marinova-Todd, Marshall and Snow2000: 17) put it, ‘it is entirely possible that adult and child learners localize their learning differently without showing different levels of learning or alternatively show similar localization but different learning outcomes’. One may also note that neuroimaging studies concerned with location have been criticized from the point of view of theory construction. Beretta (Reference Beretta, Long and Doughty2009: 70) suggests that findings generated by PET (Positron Emission Tomography) or fMRI ‘may be merely epiphenomenal, just as linearity is epiphenomenal in linguistic theory – every sentence has to be heard or produced in a linear string, but that observation has no theoretical status’.

Fewer studies using neuroimaging methods have been carried out for the when question. Existing findings point out that, in general, the same neural resources are used in processing L2 and L1, even when the L2 is learned relatively late. However, Stowe & Sabourin (Reference Stowe and Sabourin2005) conclude their review of studies that have examined ERP (event-related potential) responses pointing out that L2 processing may show more subtle differences, particularly in the extent to which the ERP waveform is modulated by the amount of early input or input over the lifetime, by proficiency, and by differences in the linguistic systems of the L1 and the L2. It is also claimed that for less proficient learners, there appears to be evidence of an overuse of some parts of the system used by the native speakers (Sabourin Reference Sabourin2003).

Not many studies have both recorded ERPs and contrasted different stages of L2 exposure or proficiency. One was the study by Weber-Fox & Neville (Reference Weber-Fox and Neville1996) in which ERP responses to linguistic anomalies among Chinese–English bilinguals were examined. The participants had been exposed to English at various ages, ranging from one to sixteen years, and differences in terms of delays of response, amplitude and scalp distribution were reported. It was not clear, however, that the groups had been matched in proficiency, and the younger group showed a more nativelike profile but also seemed to be the most proficient group. Uncontrolled subject variability has been recognized as a serious problem with cross-sectional research in this paradigm. The more recent work by Osterhout and colleagues (Reference Osterhout, McLaughlin, Pitkänen, Frenck-Mestre, Molinaro, Gullberg and Indefrey2006) attempts to overcome this problem by using a longitudinal design. Specifically, these researchers used ERPs to assess changes with increasing L2 exposure or proficiency in online comprehension of an L2 in novice student learners. By means of longitudinal studies, the researchers found that remarkably little L2 instruction was needed before learners incorporated L2 knowledge into their online comprehension processes. The apparent contradiction between this fast rate and the conventional belief, based on production data, that adult L2 syntactic learning is slow is resolved by separating comprehension from production: as in L1 learning, a learner's ability to understand the language develops in advance of his or her ability to produce it. Importantly, the researchers argue in favour of their choice of longitudinal studies because it minimizes between-subject variability, suggesting that the lack of consistency across experiments reflects, in part, uncontrolled subject variability. For example, if the variability across subjects is large enough, the researchers argue, the grand average might reflect only the accidental overlap of effects that were present in many learners in the sample. In this case, comparisons among ‘L2 learners who were exposed to the L2 at different ages might produce reliable differences among the groups but might not represent true effects of age on acquisition’ (Osterhout et al. Reference Osterhout, McLaughlin, Pitkänen, Frenck-Mestre, Molinaro, Gullberg and Indefrey2006: 207).

In sum, it may be concluded that findings from a number of neurolinguistic studies conducted in the 1990s (e.g. Weber-Fox & Neville Reference Weber-Fox and Neville1996; Kim et al. Reference Kim, Relkin, Lee and Hirsch1997) cannot provide decisive evidence concerning the existence of a critical period, because they fail to relate differences in brain activation patterns to differences in target language proficiency. More to the point are the findings from some recent studies reviewed above that, in considering apparent age of onset effects, take due account of target language proficiency and use. However, studies which meet this criterion are very few in number, and the problem remains of how to interpret the findings and draw relevant inferences.

6. Concluding summary

In this review we have critically explored the widely-held view that ultimate attainment in an additional language is predictable overwhelmingly or even solely on the basis of the chronological age at which exposure (whether significant or other) commences. We have subjected to close scrutiny the notion that the maturational constraints of a critical period for language acquisition are the crucial operative factor.

We began by raising questions about how age-related L2 research has deployed the criterion of the proficiency exhibited by monolingual native speakers as the yardstick for ultimate L2 attainment. We have gone on to detail the vast amount of variability associated with critical period advocacy. We have then subjected to close scrutiny the notion of age of onset, and have highlighted the widely neglected importance of quality and amount of input and learners’ attitudes and orientations. Finally, we have addressed the claim that maturational constraints manifest themselves in terms of changes in the organization of neural substrates for additional languages, countering this claim with evidence that many of these differences seem to be washed out with increasing L2 proficiency, although more research is needed to understand differences in grammatical processing in particular.

We have taken the line throughout that the evidence to be found in a number of recent L2 attainment studies of factors other than maturational should be brought more to the fore and treated more seriously. This is not to deny the reality of age-related effects in L2 acquisition, but rather to suggest that some of those effects relate to variables other than maturation as such, and that to focus exclusively on maturation impoverishes and distorts discussion of the relevant issues.

We have referred in our treatment of the topics specified above to the lack of confirmation in the literature of the existence of an abrupt maturational cut-off point in L2 learning capacity of the kind that would normally be associated with the ending of a critical period as usually understood. This question will no doubt continue to be debated, but our view is that until and unless an ‘elbow’ can be seen as clearly associated with the purported offset of any postulated maturational window of opportunity for language acquisition, age-related factors in L2 acquisition will need to be interpreted in the same light as age-related factors in every other domain of learning.

Overall, our suggestion is that a loosening of the association between ultimate L2 attainment research and Critical Period Hypothesis issues would open the way to a richer perspective on L2 attainment and a fuller harvest of empirical findings and theoretical insights. This would – in our judgment – remain the case even if at some future date some version of the Critical Period Hypothesis were to be put in place with a less variable definitional profile and less questionable empirical foundations than the version(s) we currently have before us.

7. Further research

It is customary in proposing future lines of research in the L2 acquisition domain to call for more longitudinal research. We shall not diverge from this pattern, since such research is needed, especially in the probing of the role of age in respect of ultimate attainment. If discussion in this area is ever to get beyond the stage of disputes about the interpretation of statistics, we require research which examines in real time and in a thorough and detailed manner the relationships among (i) onset variables, (ii) the full array of L2 learners’ experience, contexts, attitudes and orientations and (iii) L2 development across the life-span.

It would also be valuable if further studies were to replicate the classic critical-period research design, but comparing the attainment of later L2 beginners against that of early L2 beginners rather than native speakers of the language in question. It is clear from our earlier treatment of the matter, and for the reasons given, that we are highly doubtful about the value of using monolingual native-speaker proficiency as the yardstick for evaluating the attainment of early and late L2 beginners. A more useful comparison would be simply between the ultimate L2 attainment of high-proficiency late and early beginners, although any such comparison would have to ensure that the context and conditions of exposure, as well as attitudes and orientations, were similar in all cases. Another possibility would be to compare very high-achieving late beginners with high-proficiency, simultaneous, balanced bilinguals with competence in both the L1 and the L2 of the late beginners. Presumably, such bilinguals’ language processing would be subject to similar kinds of interaction between the two languages in question as that of the late beginners.

With respect to input-limited instructed language acquisition, it would be interesting for future research comparing older and younger L2 starters to replicate the typical comparative design but with the important modification of treating the teacher's input (rather than some general conception of native speaker norms) as the model against which to measure learners’ achievement. The area of pronunciation would be especially interesting in age-related studies, since it would allow us to elucidate in a precise manner whether children's purported advantage in the phonetic/phonological domain is confirmed, by means of a better imitation of the accentual particularities of the specific model presented by the teacher's speech.

A research area in need of further elucidation is whether a high level of language learning aptitude is a prerequisite for high levels of proficiency in late learners only. Such elucidation will require serious theoretical work on the construct of language aptitude and its components, as well as the development of a broader range of valid and reliable testing instruments. Work examining the potential predictive role of psycholinguistic factors such as working memory could also make a valuable contribution in this domain. Furthermore, a longitudinal design would be of particular interest here for tracing the predictive power of individual aptitude factors.

We have repeatedly emphasized the need in ultimate attainment comparisons to attend very closely to the amount of input received by the L2 learners under scrutiny and to the manner in which it has been delivered, as well as to the quality of the target language experience. Studies which have delivered on this are relatively thin on the ground. We strongly suggest that such attention should become the norm in future research in this area, and in any replications of existing studies. Although very close scrutiny to input would certainly necessitate keeping numbers of participants low, this would be compensated for by a clearer understanding of the factors that impinge on individuals’ language learning processes and outcomes.

Finally, with respect to instructed learners, it seems particularly timely at the moment to examine the age-related benefits that a stay abroad may bring them. Whereas studies in this area to date have usually examined a number of factors, such as learners’ orientations or learners’ proficiency level, and have typically focused on young adults, more and more adolescent and also child school learners are spending some time in the target language country. There is a need to investigate the role played by age in the outcomes of stay-abroad experiences, especially on the types and amount of L2 contact that learners at different ages have while in the new setting. In addition to stays abroad, formal learners now have sources of unlimited target language input at their disposal from media and internet use. The degree and manner in which this L2 exposure influences learners’ proficiency over time may also give new insights into the mediating role of input and learning context in the effects of age on SLA.