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The critical period hypothesis cites a commonly observable phenomenon, the fact that children find language learning much easier than adults, and learn language remarkably quickly, to claim that language learning is more difficult, or impossible after puberty. The concept of "critical periods" was initially introduced in the study of animal behavior, where it was noticed that certain behavioral responses only emerged when stimulus was given within a particular time frame. This concept has been applied to many species, including humans, with regard to the development of specific emotional responses such as stress.  In 1967 Lenneberg proposed that this concept also applied to human linguistic development, seeing language as a response and exposure to language as the stimulus.  There are two versions of this hypothesis: the "strong" version, which claims that no language acquisition is possible after puberty, and the "weak" version, which maintains that language learning will be much more difficult. A distinction is often made between "language acquisition", the way in which children unconsciously learn their native tongue, and "language learning" which implies formal instruction, and Lenneberg maintains this distinction himself.
Age is not the only variable when comparing groups of learners. We cannot simply compare children and adults, but must differentiate between children acquiring their first language, children learning additional language(s) naturalistically, child and adult classroom learners, both in the country where the language is spoken and outside of it, and immigrants immersed in a foreign language and culture, learning language through induction. With these groups, different social and psychological factors, as well as age, affect their language proficiency, and these must be explored alongside the role that age may play.
Some psychologists and psycho-linguists, such as Steven Pinker, claim that language learning stops at puberty because before that an innate language learning mechanism is present in the brain, enabling children to flawlessly acquire any language, given enough input.  Once puberty is reached, this language acquisition mechanism is shut down, and language acquisition becomes impossible. Chomsky claims that another variety of innate device, a "Universal Grammar" capability, allows the child to extract grammatical rules from the input that he receives, and to use them to generate an infinite number of further grammatical sentences.  In claiming that these devices shut down at puberty, these theories make no allowances, for example, for the fact that vocabulary is added to the corpus of language knowledge throughout life, and that adults can be successful in learning foreign languages. If a child acquires substandard forms, this does not mean that as an adult, he or she will not be able to modify these forms. It is also possible to acquire a first language after puberty, as some cases, which will be discussed later, have shown
Both Pinker and Chomsky allude to the ease of language acquisition as proof of an innate device behind it. We do not ascribe innate knowledge to other fields that children excel in, simply because they find them easy. For example, any child who is physically able to can learn to ride a bicycle, yet it is doubtful that a theorist would propose that evolution has provided an innate bicycle-riding apparatus. One element of the skill, balance, is innate, and is controlled by a specific part of the inner ear, the semicircular canals.  Once the child has achieved good enough control over its muscles, it can build on the innate function of balance to learn to ride a bicycle.
To learn language, a child must process the data to which it is exposed, deduce rules from regularities occurring in this data, and apply these. In addition, it must have control over the muscles that govern articulation. There is no need to suppose that a specialized linguistic device to extract these rules exists, however. The entire left hemisphere of the brain is constantly constructing theories regarding the world around it, based on sensory evidence.  Language heard by the child is a part of this sensory data, existing in its aural form and in a visual or aural referent, which must be simultaneously processed. Brain function experiments have shown that the left hemisphere of the brain is indeed more active when grammar is being handled.  Like bicycle riding, language is an ability based entirely on an innate function, but is not entirely innate in itself. Without exposure, language doesn't appear; therefore it is not innate. Rather than supposing that vague, unlocated language acquisition or universal grammar mechanisms exist in the brain, the ability to extract theories from data could be seen as a wider function of the brain itself, rather than being language specific. If these specialized mechanisms cannot be proven to exist, then the decline in language learning ability cannot be accounted for by the "shutting down" of these mechanisms.
Such theories also seem to suggest an erroneous concept of the brain itself. Were the brain "designed" it would doubtless include specific linguistic structures, and some kind of mechanism for language acquisition. But it has evolved to contain not innate knowledge or skills, but the potential for knowledge and skills to develop. The fact that the same areas of the brain handle language in approximately 90% of the population  is no reason to regard these areas as empty "containers" waiting to be "filled" with language, or with their own mechanism to enable this to happen. In any case, the brain is so richly provided with interconnections that it is impossible to describe any boundaries within it. Language is organized in the same way in most humans, but the organization is not rigid. If that part of the brain is injured in youth, language can be relocated to a different place without detrimental effect, suggesting that, although a preference may exist, no sector of the brain is more or less suitable than any other for language processing, and negating the possibility that one particular area of the brain is equipped with a language acquisition device. 
Brain injuries also shed light on another important issue. If a young child suffers a head injury to the region of the brain that controls language, it suffers minor temporary language handicap, followed by normal development. The language centers develop again elsewhere without detriment. An older child will also be able to recover language ability, but other abilities, such as spatial skills, will be compromised. An adult suffering the same injury would be terribly and irrevocably handicapped by aphasia.  As well as suggesting that language can exist normally in other parts of the brain, it suggests that there is a difference between child and adult brains. Also, children become gradually more like adults, reflecting a gradual change in the brain as maturity is approached.
In his original critical period hypothesis, Lenneberg cited this change undergone in the brain as the main physiological basis for the critical period.  Children were constrained in their language ability until the brain was mature enough, but once the brain achieves maturity, as occurs at puberty, language acquisition is again impossible. This is because lateralization is complete, with all brain functions being localized to a particular area, and cerebral plasticity not longer possible. This is supported by the experience of brain injured people. In younger children, brain organization is incomplete, and so if one part of the brain is injured, it is straightforward for the language centre to be re-established. Older children can also re-establish their language ability, but at the expense of other skills, suggesting that language is supplanting these skills in a particular area of the brain. Adults' inability to do this suggests that functions previously flexible have now become immovable, supporting the idea of brain lateralization, and its effect on language.
However, this does not provide proof that language learning is impossible after puberty. Adults learning a second language are not attempting to establish another centre of language function. The same areas of the brain are used to process all languages, even sign language. They are adding to their linguistic knowledge, acquiring new grammatical rules and vocabulary, but the same part of the brain will handle such knowledge. We acquire much of the vocabulary of our native language during our teens and beyond, as well as learning more complicated grammatical structures, so there is no reason to suppose that we cannot do the same with a foreign language.
We can discount these claims, and still be left with the observation that children learn languages much quicker than adults do. If a family immigrates to a new country, the children will pick up the new language quickly, leaving their parent far behind, and probably acting as their interpreters. Children undoubtedly have advantages, yet these are not inextricably linked to their age. When a child is learning language, circumstances are uniquely supportive, and these circumstances are not usually replicated in later life.
Physiologically, children have advantages. Hearing declines with age, and so the child is better able to identify different phonemes, and because children have better control of the articulatory muscles, they are better able to reproduce what they have heard.  Young children also have an amazing ability to learn by rote, and so can retain more data for analysis.
Psychologically, they are unaffected by inhibitions or previous experiences. Young children acquiring a second language may not even have a conception of language itself, since it is many years before children can discuss language reflexively. They are usually free from prejudice against the new language, or any cultural concepts that may accompany it.
In terms of hours spent on language acquisition, it is no small task. Children are attuned to language from birth, and are aware of sound in the womb.  Yet it is many months before they begin to vocalize, and years before they consistently produce grammatical sentences. From a vast amount of input, numbering many hours every single day, the child still takes much time, and much experimentation, before it is consistently accurate in its application of grammatical rules. It would take years of weekly language lessons for an adult to have experienced similar exposure, and undoubtedly the adult would then have acquired a good deal of the language, and would perhaps be approaching native-speaker fluency.
The input is therefore superior on terms of quantity, but also in terms of quality. Caretaker language is a common phenomenon, including foreigner talk as well as Parentheses. This is due to the obvious fact that in communication, we are aiming to be understood, and will naturally produce what we judge that our listener will understand. Parentheses and Foreigner talk share many common features, but Parentheses is superior for language learning for many reasons. Firstly, the parent or other has a much better idea of what the child can understand and can modify their output accordingly. Secondly, Foreigner talk is often ungrammatical, with, for example, infinitives being overused to aid comprehension. Parentheses is mostly grammatical, unlike adult to adult speech,  and therefore provides the child with a large amount of perfect data from which to extract rules, whereas the foreigner is more likely to extract erroneous rules from the false data presented. And of course a parent has a much greater interest in the language skills of their children than a colleague or acquaintance will have in the language learning of their foreign friends.
The child is also favored by the relative unimportance of comprehension. If a baby does not understand what an adult with whom it is playing says, it doesn't matter. If needs be, the adult can physically move the child or otherwise compel it to do something. If the baby is at the top of the stairs, and doesn't heed instructions to move, the adult will simply pick it up. However, an immigrant will have to perform some tasks, such as finding work, shopping, or applying for a driving license, and not understanding warning signs such as "Beware of the Dog" or "Electrified Fence". The need to understand and been understood is much greater, and accompanying stress and frustration may hinder the learner. In addition, the world is favorably disposed towards children, whose mistakes they find endearing, but often hostile towards foreigners with a similar language capability.
For children, the vast majority of social interaction is not based on conversation but centered on a particular activity, such as a ball game, or painting. Therefore, a child may gain acceptance into a group of his or her peers without a common language, and through participation is able to learn the language. For adults the reverse is true. Little interaction will take place if adults without a common language meet, reflecting the central role that conversation plays in most adult interactions. Again, this is a sociological factor. On a German exchange, for example, a student would participate in events like family meals, at which they would understand little of the conversation. They would be able to acquire several items of vocabulary, and phrases such as "Guten Appetit". However, it is unlikely that an adult would be able to have the benefit of such an opportunity. In the first place, it is unlikely that he or she would have been able to make such a friendship as to prompt an invitation. Were this possible, conversational conventions would not allow for a silent participant, making the hosts feel as if they were in some way excluding their guest, and the guest as if he was not providing his or her share of the evening's conversation. Used to being able to participate, the guest would no doubt feel frustrated at being unable to express opinions already thought through in the second language. In this way, inhibitions bar the older learner from excellent language learning opportunities.
Our life experience shows us that adults can indeed be successful in learning a new language, whether it is stock phrases from a phrase book, or the entire language. If enough time can be devoted to the language, fluency is achievable at any age. The main area where children are superior to adults is pronunciation. A child can easily sound like a native speaker, yet few adults manage to acquire a perfect accent, however hard they study. This may be because children have superior hearing and better control over their articulatory organs. Even within the native language, accents appear to become fixed after puberty. A Scottish child who moves to England will quickly exchange his accent for the one he hears at school, whereas a Scottish adult may spend the vast majority of his life in England, and yet retain his accent. But adults can alter their accent, through elocution classes, and an actor may possess a vast repertoire of regional accents. In both cases, acquiring a new accent is advantageous; therefore there is a genuine motivation to do it. Research substantiates this: Neufeld developed a successful pronunciation technique that moves gradually from listening to speaking. After eighteen hours of instruction, nine out of twenty students convinced listeners that they were native speakers of Japanese, and 8 out of twenty that they were Chinese.  Where there is no need to alter one's accent it is unlikely that the effort will be made. A native speaker of English, with a regional accent, will have no problem with being understood in any part of the country, and therefore has no motivation to adapt it.
While children are more likely to alter their accent, and have physical advantages enabling them to do so, adults do not lack this ability. Some kind of choice, probably subconscious is made. Children wish to be like their peers, and adults wish to retain their developed sense of personal identity. The importance of accent is perhaps overrated. Fluency is the ability to communicate as well in a second language as in the native tongue, to be able to generate and to understand an infinite number of sentences, not to pass as a native speaker. A foreign accent doesn't often hamper comprehension if the sentence is grammatically correct; therefore accent is not a vital part of language. It is merely a social factor. The fact that adults do not acquire native-like accents is not proof of a "critical period" for language acquisition.
Although children seem to have the upper hand in naturalistically acquiring language, for these various reasons, research shows that classroom learning actually favors the older learner. When language is not taught in a formal context, children excel. Perhaps it seems effortless because the young child is not told that it should be laborious. But a child introduced to language in the classroom, who has experienced school as boring and demanding, and is told that much learning of grammar will be required, and is constantly informed of his mistakes, will see language learning as an effort. Krashen recognizes these factors in his Affective Filter hypothesis,  basically an exploration of the different factors that may influence motivation, obviously an important factor influencing learning of any kind.
Studies concentrating on a variety of first and second languages have shown that when older and younger students learning by the same method are compared, older students make better progress. When immigrants to Holland were compared, adults made faster progress than children did in learning Dutch  ; Swedish pupils were shown to make better progress in their English lessons the older they were  . The Total Physical response method of language teaching is supposedly more suitable for children, yet in a Russian study adults achieved better results.  Even in a supposedly more naturalistic technique, the French immersion system as practiced in Canada, where children acquire French through being taught a range of school subjects only in that language, those who entered the immersion program at a later stage were judged more proficient than those who participated from the start.  The situation is of crucial importance here. Effectiveness in a classroom situation increases with age in all other subjects, and in adulthood, when education is by choice not compulsion, learners are even more motivated. Children's physiological advantages do not produce greater success when they are compared with adults in an identical situation. This suggests that it is the situation in which the language learning is taking place that is of vital importance. Children mostly learn naturalistically, in a supportive environment, whereas adults learning through immersion often find society hostile. The constraints of a classroom mean that language learning is not as effective as naturalistic learning.
An important part of the debate over the "critical period" has focussed on the "Wild Children" - the few children who have been raised entirely without language and have been later discovered. These children cannot provide proof of anything in scientific terms. They are unfortunate human beings rather than a scientific experiment, with their lives before discovery shrouded in mystery. We could not use the fact that these children do not always acquire language to prove that no child would be able to under the same circumstances, since we cannot discount factors such as mental retardation, or the effects of their often brutal earlier lives. However, their achievements do challenge some theories advanced.
History has provided us with two known examples of children who did not begin to learn their native language until past puberty. Two cases, Caspar Hauser and Victor, the wild boy of Aveyron, are distant in time and complicated by claims of hoaxing in the first instance and mental retardation in the second, which obviously cannot be proved or disproved. For what it is worth, both cases acquired language, Caspar (aged 16) perfectly, and Victor (aged around 12) imperfectly, but he is said to have attained a useful communicative ability. 
More recently, other cases of children being raised without language have occurred, and have been subject to more stringent scientific exploration, and can therefore be more useful in a discussion of the "critical period". Genie is perhaps the most notable example, being the eldest at the time of discovery, at thirteen years old, and subject to complete isolation before that time.  The case of Isabelle, aged six, is also notable for her perfect acquisition of language within two years, to the normal child's five.
Genie was discovered in November 1970, having been isolated in the same room between the age of twenty months and 13 and a half years. Her only human contact was with her father and brother, who used only growling noises and violence to communicate with her. Genie was handicapped not only in her language development, but also in all forms of socialization. She had no reaction to temperature, no concept of ownership or of personal space, and could not even chew her food. At first, she was disinclined to vocalize at all, having been beaten by her father for making any noise whatsoever, and even in her tantrums she tended to use items of furniture to make sound, remaining eerily silent herself. Children begin to use words to describe a world they have already become familiar with, but Genie was faced with the task of acquiring words to describe an environment she could barely even understand. Under these circumstances it is not remarkable that she did not develop complete language competency over the first five years of her rehabilitation. However, her achievement in these years was not inconsiderable, as one researcher, Susan Curtiss points out:
Genie's language is far from normal. More important, however, over and above the specific similarities and differences that exist between Genie's language and the language of normal children, we must keep in mind that Genie's speech is rule-governed behavior, and that from a finite set of arbitrary linguistic elements she can and does create novel utterances that theoretically know no upper bound. These are aspects of human language that set it apart form all other animal communication systems. Therefore, abnormalities notwithstanding, in the most fundamental and critical respects, Genie has language. 
Here we have an example of an individual past puberty who has made considerable progress in mastering her first language, has succeeded "in the most fundamental and critical respects" even if not completely. At the very least, Genie shows that the term "critical period" is misleading, since language exposure before puberty is not critical, and no fixed amount of time or developmental stage can be cited. 
Isabelle, like Genie, was completely isolated from the speaking world, being imprisoned with her deaf-mute mother. Unlike Genie, she was not treated cruelly, and interacted with her mother using gestures of their own devising. On being discovered aged six, in 1938, she was thought to be uneducable, but within a week had begun to use words. She was noted to pass through the normal developmental stages of language acquisition, but at a vastly accelerated rate, catching up with her age group two years later.  The fact that Genie was more than twice Isabelle's age is not necessarily the decisive factor here. Isabelle was not traumatized by her upbringing, and was not an unsocialised creature, like Genie. She understood the concept of communication, and had developed a language of a sort, the gestures she used with her mother. Her task was not so great as Genie's, and so her greater achievement is proof only that a six-year-old can acquire language, and not that a thirteen-year-old cannot.
The Critical Period hypothesis suggests that age is the primary reason for children's apparent superior language learning ability. This may be a factor, but is far from being the only reason. Language is more complex than a simple response to a simple stimulus, as Lenneberg's hypothesis may suggest. Even if it were proved that children's ability proceeds entirely from their more suitable circumstances, the impossibility of exactly replicating these circumstances for an adult would mean that the discovery would not be of much functional use. As an adult language student it would be most useful to note that it may be my inhibitions rather that my inabilities that hinder me most in my studies, and that regarding grammar as laborious may indeed make it so!