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Steven Pinker in "The Language Instinct" claims that there is strong psychological evidence for the existence of a sharp age cutoff for the ability to acquire a flawless foreign accent (I may dig up the exact reference, if needed). In other words, there is a fairly narrow age threshold (around 20-odd years) below which exposure to a foreign accent is more or less sufficient to acquire it and above which it's physiologically impossible. Has this phenomenon been confirmed or refuted by any systematic study?

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Related: linguistics.stackexchange.com/q/679/325 (When is the end of the critical period?) –  user325 Jan 16 '12 at 16:58
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4 Answers

One of the reasons the feral children data are difficult to assess is that the brains of these children are often underdeveloped or have developed differently because they weren't stimulated with language at a young age. (For a more scientific explanation of the how of this, see Curtiss et al 544-545+.) As I mentioned in my comment to Askalon, while that data supports the idea that there is a critical period for acquiring a faculty for language, it doesn't say much about what factors influence how successful you will be in acquiring native-like competence in a second language. Although age is often speculated to be a factor in acquiring native competence, estimates on the cutoff have ranged from as low as 3-4 to as high as puberty.

Infant Studies of Phoneme Perception
An important question to keep in mind is, why should skill in acquiring a new phonological system decrease because of age? Most cognitive abilities get remarkably better with age, assuming typical development. Of course, there are obvious factors like motivation (generally not as high for L2 learners), phonological interference, or different method of acquisition (generally by analytical study and reflection than immersion for L2 learners), but the biggest theory seems to be currently that it is because, while young infants are like a phonological tabula rasa, after exposure to their native language, they narrow their focus to only pay attention to the contrasts which differentiate meaning -- what are traditionally called phonemes. They lose the ability to learn how to produce and differentiate any sound.

Even very young children show decreased sensitivity to contrasts not present in their native language. For example, Hindi has many contrasts that English doesn't, for example differentiating aspirated and unaspirated stops like [pʰal] and [pal]. Werker and Tees (1984) trained infants to expect a reward (a little puppet show) when they heard aspirated syllables (this is called a conditioned head turn technique). Then, they tested them, playing aspirated and unaspirated stops and measuring how often the infants turned at the appropriate times. If they failed to turn after hearing an aspirated stop, this was counted as a miss, and if they turned after hearing an unaspirated sound, this was counted as a false alarm. At 6-8 months, infants scored well -- they had fewer than 2/10 misses or false alarms -- while those in the 10-12 month group did remarkably poorly -- approximately only 2/10 correct hits. I cannot find the citation, but I have heard vocalic categories are cemented even earlier. And keep in mind that this is before these infants are even babbling!

Immigrant and Foreign Language Learner Studies
There have been studies looking at the acquisition of foreign language by people of varying ages, or by immigrants with varying age-of-arrival, and some (Johnson and Newport 1989) support the idea of a critical period and others (Snow and Hoefnagel-Höhle 1978) do not. It is important to note that these studies often look at both knowledge of grammar and pronunciation and I have unfortunately not found any studies specifically related to accent -- though I will add them if I come across them. These studies generally rely on a battery of tests to assess grammatical competence (e.g. grammaticality judgments) and native speakers to rate the degree of the subjects' accent. In general, it seems that a critical period for phonology is much more supported than a critical period for grammar -- acquisition of grammar is much more linked to cognitive ability in adults, although in young children, cognitive ability has no effect (e.g. Flege et al 1999, DeKeyser 2000).

Conclusion
So yes, there does seem to be evidence for a cutoff in acquiring native-like phonology, from immigrant studies and SLA studies backed up by infant perception studies. What I would take from this, though, is that in addition to lost competence in implicit learning mechanisms, when we already have one language to work with, we will rely more on overt analysis and comparison to learn a foreign language. And while grammars are very good at detailing correct syntax and morphology, they are often abysmally poor and lazy at instructing learners in pronouncing and distinguishing foreign phonemes. (This is because they are often not written by linguists.) Add to that the fact that having an accent is not terribly stigmatized (but poor grammar is) and it's no surprise that L2 learners will often not develop a great accent.

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Well, I am no expert, but Indian call centres contain tons of guys who speak in a perfectly British/American accent. It' true that most Indians learn English before the end of critical period, but the accent can be learnt considerably later also. I have first hand evidence of this as I have seen many such persons. –  Nikhil Bellarykar Jan 17 '12 at 12:31
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@NikhilBellarykar It's true some people develop great accents -- I met an Estonian who I never would've guessed wasn't a native speaker of English -- though they might not be 100% native (I recall a study of white rappers which found that all mimicked AAVE but only one -- form whom it was his native idiolect -- had percentages of stop deletion equivalent to that of AAVE speakers.) As I allude to in the answer, I don't think it's necessarily a cutoff so much as the difficulty of learning phonology through nonimplicit mechanisms coupled with the loss of said mechanisms and decreased motivation. –  user325 Jan 17 '12 at 15:10
    
That I agree with indeed. –  Nikhil Bellarykar Jan 17 '12 at 15:58
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Native "accent acquisition" means successful acquisition of the language's phonology (i.e. the system of sounds in a language). The cutoff you're referring to is the critical period hypothesis, which claims that there's a critical period (from birth up to around 7-ish or puberty, give or take) during which a person is capable of acquiring a language and achieving native-like competence, but after this critical period it becomes very difficult or impossible to achieve fully native-like competence. It's thought that there isn't one single cutoff point though, but rather that it varies depending on what part of language you're talking about: phonology is thought to have the earliest cutoff age, while syntax is thought to have the latest (it's much more common to see non-native speakers with excellent syntax but errors in their phonology than vice versa). The phonology is therefore most often the place where unsuccessful native-like acquisition most often manifests itself, in the form of a non-native accent.

So to get to your question about evidence for this. There is some evidence, but it's limited, and some of the proposed evidence has been criticized as not valid. There are some cases of feral or deaf children/adults who grew up without a language (surpassing at least part of the critical period before they were discovered). These people are often used as evidence that the difficulty with successfully acquiring a language (including phonology) is biologically linked to age. It's rather clear that normal adults have trouble acquiring a new language and gaining native-like competence as well, but one could claim that that's perhaps because of interference caused by the language they already know. With feral or non-lingual deaf people though, they don't know any language (so there's clearly no interference from a native language), so some linguists claim that their lack of or limited success in acquiring a language must be linked to their age.

A frequently mentioned case is that of Genie, a girl that was brought up in isolation by abusive parents and never learned a language as a child. She was found at age 13, after which a linguist worked with her and she started to learn English. She was never able to learn English beyond a rudimentary level. Studies with deaf children who started learning a sign language later in their childhood (and having never learned the spoken language of their parents) have also shown a correlation with age and the level of their success in their acquisition of the language.

Some of this evidence has been criticized. For example, feral children such as Genie have also obviously suffered traumatic experiences that have affected them psychologically, which could be a factor in their unsuccessful language acquisition. Cases of late learners of sign language provide better evidence because they were generally well cared for, despite lacking a language.

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I +1'd this, though it's worth pointing out that feral children provide evidence for a cutoff point for the acquisition of the faculty of language -- which turns out to be use it or lose it -- but it does not point to what factors, if any, control acquisition of a second language, and there is much more debate on a critical period for second language acquisition than there is for first language acquisition. –  user325 Jan 14 '12 at 21:41
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Yes, this is a quality answer but doesn't discuss L2 accent acquisition, which is what the question enquires about. –  Floating Tone Jan 15 '12 at 2:55
    
@FloatingTone I'm afraid I might have misunderstood the question, I haven't read the section of the Language Instinct that was referenced. I assumed "ability to acquire a flawless foreign accent" meant ability to acquire native-like phonology in their L2--which would require the speaker to begin acquiring the language before the end of the critical period (basically it would need to be a child L2er). That's why I gave an explanation for the CPH and supporting evidence. L2A can still take place prior to the end of the critical period (going on the definition of L2A starting at 3;0+). –  Askalon Jan 15 '12 at 5:45
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I believe that the existance of a cut off period is imposible to measure due to the large number of factors affecting language accuisition. As an English language teacher and a person who has very near native competence in a foreign language, I believe that older people are less likely to achieve native competence in pronunciation because to mimick a native accent perfectly, in the majority of cases, would necessitate presenting oneself as having a different identity to that you have been raised with.

Children are less concerned about changing their identity than adults are and will more willingly mimick a native accent.

That is why they aquire native pronunciation much more efficiently - and the same is true for adults who are not shy or ashamed to assume a different identity - they also master foreign accents.

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Purely anecdotally: We live in Taiwan and my son learned Chinese from age 4. Today, he is often told that if he speaks over the phone, it cannot be determined that he is not Chinese. Even the very best of adult-learner speakers of Chinese I know, even when 100% fluent, cannot shake the accent/inflection that marks them as "non-native".

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