Introduction
The concept of “critical periods” was used in zoology for species’ specific behavior. One of examples, Zoologist Konrad Lorenz found that geese isolated from their mother would develop an attachment to and start following any moving shape that they see first during a certain limited time period since hatching. If that moving shape was Lorenz himself, the geese would recognize him as their ‘mother’ and ignore the actual mother goose. While the nature of the moving object did not seem to matter much, the time of exposure was crucial: geese that failed to attach to the mother during the “window in development” (Pinker 1994:293) seemed unable to do it later.
Interestingly, this critical period also exist in human behavior. For example, learning some musical instrument like the piano or the violin is important to start in early childhood; otherwise, late learners will have difficulty to master the complex or the fast movement of the fingers.
Then how about learning language?
We all know that children are superior language learner. Pinker(1994) noted that although there have been many explanations why children learn language easily, he assumed that a key factor is the solely age. If so, it may be easy to put to a test and prove it. However, the problem with critical period in language learning is not that straightforward as it sounds like. As Bialystok & Hakuta(1994) pointed out that “language is far too complex a system to reveal itself through a single skill, a single experience, or single test. People, too, are complex…” (Bialystok & Hakuta1994;80)
There may be the reasons why the Critical Period Hypothesis for language acquisition has been the hot issue since it was postulated.
In this paper, I will summarize the brief history of the critical period hypothesis and review the available evidence on the idea of critical period hypothesis.
1.The History of the Critical Period Hypothesis for Language acquisition
In 1959, neurologists Wilder Penfield and Lamar Roberts were prompted by their research into the cortical processes responsible for speech to argue that, after the age of nine, the brain becomes increasingly stiff and rigid, which disturbs effective language learning (Singleton2005;271).
In 1967, critical period Hypothesis for language acquisition was popularized by American researcher Eric Lenneberg in his seminal work Biological Foundations of Language. Lenneberg hypothesized that human language is a function of brain maturation and develops “from mere exposure”(Lenneberg1967;176) to a linguistic environment (appropriate stimulation) only during a critical period-from age about 2 years to puberty.(Curtiss 1977;207)
According to Lenneberg, at the beginning, before age 2, language acquisition is not possible because the brain is “lack of maturation” and at the end automatic acquisition from mere exposure to a given language is not possible because of “a loss of adaptability and inability for reorganization in the brain” (Lenneberg1967:179)
Lenneberg (1967) said:
Between the ages of two and three years language emerges by an interaction of maturation and self-programmed learning. Between the ages of three and the early teens the possibility for primary language acquisition constitutes to be good; the individual appears to be most sensitive to stimuli at this time and to preserve some innate flexibility for the organization of brain functions to carry out the complex integration of sub-processes necessary for the smooth elaboration of speech and language. After puberty, the ability for self-organization and adjustment to the physiological demands of verbal behavior quickly declines. The brain behaves as if it had become set in its ways and primary, basic skills not acquired by that time usually remain deficient for life.
(Lenneberg1967; 158)
Since Lenneberg introduced the idea of critical period hypothesis for language acquisition, the idea has led an extensive research in the area of Chomskian linguistics and cognitive psychology.
2. Evidence of the Critical Period Hypothesis
2.1 recovery patterns from aphasia
The evidence Lenneberg suggested for the critical period hypothesis in language acquisition was young children recovering from aphasia. Lenneberg investigated several clinical cases from brain lesions reported by Basser in 1962, and he found that language disturbance as a result of right hemisphere lesions is more likely to occur in children than in adults. Children who lost language under the age 9 can learn language again, even after left hemispherectomy, not adult, children are able to transfer the language function to the right hemisphere. Based on the clinical investigation on brain lesions, Lenneberg concluded that at the beginning of language development, both hemispheres are involved in language learning. However, there is progressive lateralization of the language function to the left hemisphere, after lateralization the brain loses plasticity. Lenneberg believed that process of lateralization is complete by onset of puberty,
Since Lenneberg formulated the critical period hypothesis, however, increasing evidence suggested that the progressive lateralization of the language is complete long before puberty. (Curtiss 1977;213)
First of all, Krashen(1973) reanalyzed the data which formed the basis for Lenneberg’s critical period hypothesis, Krashen found that all the case of fully recovery from right- brain damage were under age of five. Moreover, the number of fully recovery cases at the age of more than five was nearly the same as the number of the case of recovery from right-brain damage in adults. Based on his reanalysis, Krashen concluded that lateralization is accomplished around age 5 (Krashen 1973;65)
Snow(1987; Epstein,Flynn& Martohardjono1996)also noted that
..relying on brain growth as the biological process that times the critical period would predict a critical period ending at 5. This is not an age at which any sharp discontinuities in language acquisition can be observed however (Snow 1987;188; Epstein,Flynn& Martohardjono1996)
What is more, Dennis(1979;Epstein,Flynn& Martohardjono1996) seriously questioned the lenneberg’s observation that the language function can be transferred to the right hemisphere after left hemispherectomy. He claimed that even in children who were believed to have recovered their language after left hemisperectomy, showed permanent language deficits. (Dennis1979;Epstein,Flynn& Martohardjono1996).
2.2 Feral Children
Probably the most quoted evidence for the critical period hypothesis is the case study of the children who had been deprived of language and tried to acquire the language before and after their critical period.
One of the well-known cases is Genie.(Curtiss1977) Genie was discovered in 1970, a thirteen and half year old girl.(i.e. after “critical period”) She had been completely isolated, deprived, and abused since she was at the age of 20 months. When she was found, Genie had no language. After her discovery, there were great attempts to teach her language. Despite of great attempts, Genie’s language showed many of abnormal features in language development and her language development was abnormally slow. (Lighbrown&Spada1996;12)
Although Genie’s made significant progress in vocabulary learning, she never was able to learn syntactic aspect of language.(Eubank&Gregg 1999;74)
S. Curtiss(1977) who did most of the studies with Genie raised the possibility that Genie seemed to use the right hemisphere of her brain to solve cognitive task and for language development which could account for some of the abnormal features of Genie’s language development.
She said that;
Her language resembles that of other cases of right-hemisphere language as well as the language of those generally acquiring language outside of the critical period. Her case, therefore, supports Lenneberg’s critical period hypothesis and furthermore, suggest specific constraints and limitations on the nature of language acquisition outside of this maturational period. (Curtiss 1977;234)
However, Snow and Hoefnagel-Hohle (1982) empathized a more positive aspect of Genie’s language development;
although her language acquisition is very slow, it is in many respects following the same course as that of young first language learners…and her language skills are continuing to improve several years after puberty (Snow and Hoefnagel-Hohle 1982: 94)
In Genie’s case, many researchers questioned whether her lack of linguistic competence was merely a consequence of the lack of linguistic input during her early childhood. Obviously Genie had many problems. As Pinker pointed out that Genie’s ability to learn language might have been interfered with the sensory deprivation and psychological scars somehow. (Pinker 1994;292)
Lightbrown and Spada (1996) also pointed out that although Genie appear to provide evidence in favors of the critical period hypothesis, it is difficult to argue whether the hypothesis can be supported by the basis of evidence from such unusual cases. (Lightbrown and Spada1996;12)
Another well-known case is that of Chelsea. Chelsea was born deaf but misdiagnosed as retarded or emotionally disturbed without recognizing her deafness. At the age of 31, long after “critical period”, Chelsea was fitted with hearing aids that improved her hearing to near-normal levels, In spite of intensive training, Chelsea’s utterance appeared to have almost no structure at all.(Pinker 1994;293) Her sentences did not even have the Genie’s utterance which seemed to have the structure complexity of a normal 2 year old’s at least.(Birdsong 1999;74).
Pinker addressed that Chelsea case cannot be explained in terms of emotional scars, because Chelsea was brought up by a loving family. Therefore, Chelsea provided genuine evidence that first language acquisition is impossible after a “critical period”.(Pinker1994;292)
However, Sampson(2005) argued that “the quoted documentation on the Chelsea case was based on the reporting an unpublished talk given by a speaker at a local dyslexia society. So it is difficult to judge what weight, if any, to give to this case.” (Sampson2005;There is No Language Instinct)
Furthermore, there is no information about the period which elapsed between Chelsea getting hearing aids, and being observed to produce meaningless utterances. Sampson strongly suggested that the observations may have been made after a period that would not be long enough for a young child to learn to speak. (Sampson2005;There is No Language Instinct)
Anyhow, from such unusual cases, it has to take into account that beside biological maturity other factors might have contributed to their inability to learn language. As Curtiss(1977) herself claimed that
communicative competence may be a separate aspect of linguistic functioning, one not dependent on most other linguistic abilities, but rather dependent on the absence of social and/or psychological disturbance. Genie’s lack of competence in this area, therefore, is probably not a function of her more general linguistic limitation. Her conversational incompetence is instead a manifestation of her social and psychological impoverishment and abnormality. (Curtiss 1977;234)
2.3 Controlled studies
Many of studies strongly support the critical period for language acquisition. Thesestudies(Newport1990,Johnson&Newport1989,Patkowski1980,Pallier,Bosch&Sebastian-Galles1997, among many others) exhibited that there is a close relationship between age of exposure and the ultimate proficiency acquired in target language.
For example, the series of studies was performed on the congenially deaf children. The majority of deaf children, who are not born to deaf parents, often have no access to sign language as they grow up. Therefore, studies of deaf children can provide a unique opportunity to study the language development at different ages.(Pinker1994;37) Newport and Supalla(1987), in their study, they separated subjects into 3 groups in terms of their age exposure to ASL:
(a) native learners (exposed to ASL from birth)
(b) early learners (exposed to ASL between the age of 4-6)
(c) late learners (exposed to ASL at age 12 or later, i.e. after the critical period).
(Newport1990;Lightbrown&Spada1996;13)
Newton and Supalla(1987) found that the late learners performed significantly worse than the early learners. Therefore, the data of experiments directly provided the age effects on language performances. Although the late learners performed equally well on the basic word order tests, the results suggested that increasing the age of language acquisition leads to decreased control over all but the simplest aspects of grammar. (Newport, Bavelier& Neville2001)
Another sign language study in Nicaragua by Judy Kegl, Miriam Hebe Lopez, and Annie Senghas also provided that younger signer’s signing are more fluid and compact, and pack more meaning into their signing.(Pinker 1994;36)
Pinker weighted the result with saying that
because the deaf are virtually the only neurologically normal people who make it to adulthood having acquired a language…. studies of sign language can offer particularly good evidence that successful language acquisition must take place during a critical window of opportunity in childhood. (Pinker 1994;38)
Another study from Johnson and Newport (1989) also showed that the later a language is learned the less its use is proficient. They tested Chinese and Korean immigrants who move to the United States and become exposed to English as a second language. Johnson and Newport found that there are strong effects of subjects’ age of exposure to their language performances to judge its grammatical structure many years later. Interestingly, even the number of years of exposure is matched with their language abilities. (Lightbrown &Spada1996;45)
Johnson and Newport summarized that peak proficiency in the language, both in phonology and syntax, can be found among those who were first exposed to the target language during their infancy or very early childhood. As age of exposure increases, average proficiency in language declines, beginning as early as ages 4 to 6 and continuing until proficiency plateaus for adult learners. (Newport, Bavelier& Neville2001)
However, Newport, Bavelier& Neville(2001) stressed that
age of exposure does not affect all aspects of language learning equally. the acquisition of vocabulary and semantic processing occur relatively normally in late learners, Critical period effects thus appear to focus on the formal properties of language (phonology, morphology, and syntax) and not the processing of meaning. Even within the formal properties of language, though, various aspects of the language may be more and less dependent on age of language exposure. For example, late learners acquire the basic word order of a language relatively well, but more complex aspects of grammar show strong effects of late Language acquisition.(Newport, Bavelier& Neville2001;Perspectives on a critical period for language acquisition)
Newport, Bavelier&Neville also quoted the reporting from Sanders, Yamada &Neville(1999) and noted that although late learners have difficulty to acquire the phonetic information in native-like pronunciation, late leaner may not have problem in acquiring lexical stress.
Those controlled studies strongly suggested that there are great age effects on acquiring language. However, it is important to note that age of exposure does not affect all aspects of language learning equally and individual variation also increases with age (Newport, Bavelier& Neville2001)
The similar case was made for the critical period that applies only to phonoloy and has no impact on the other linguistic skills. Thomas Scovel (1988) asserted that post-pubescent language learners, while still able to improve their accents, “will never learn to pass themselves off as native speakers phonologically” (Scovel 1988:185)
However, Snow and Hoefnagel-Hohle (1978) provided rather different evidence on the critical period hypothesis. Snow and Hoefnagel-Hohle (1978) studied the language ability of two groups of English-speaking subjects:
1) monolingual English speakers who were just starting to learn Dutch (the “Beginner” group: 3-5year olds, 6-7year olds, 8-10year olds, 12-15year olds, and adults)
2) English speakers who had been living in Holland and speaking Dutch for at least 18 months (the “Advanced” group: 6-7year olds, 8-10year olds, 12-15year olds, and adults).
Interestingly, they found that for all tests except pronunciation and sentence judgment, age groups 12-15 year olds and adult achieved the fastest progress during the first few months of learning Dutch. At the end of the first year, while the age groups of 3-5year olds scored lowest on all the tests employed, the 8-10 and 12-15 year olds showed the best control of Dutch.
(Snow& Hoefnagel-Hohle 1978;Lightbrown&Spada1996;46-48)
Snow and Hoefnagel-Hohle(1978) noted that
The finding that older second language learners are disadvantaged at acquiring the skills associated with giving linguistic judgments, despite their superior linguistic performance, suggests that knowledge of a language comprises several components which are not necessarily related (Snow and Hoefnagel-Hohle1978;School-age second language learners’ access to simplified linguistic input)
From the result, Snow and Hoefnagel-Hohle(1978) conclude that their results does not support the critical period hypothesis.
However, their results were interpreted in rather different way by Lightbrown and Spada.(1996) Lightbrown and Spada argued that some of the tasks provided by Snow and Hoefnagel-Hohle, especially sentence judgement or translation, were too hard for young children. What is more, while adults and adolescents learn at a faster rate, children surpass adults and adolescents in eventual attainment. (Lightbrown &Spada1996;48-49)
3. The Problematic Issue on the Researches of the Critical Period Hypothesis
In research of the critical period hypothesis for language learning, it seems to be difficult to have the reliable hard data available. Moreover, any hard evidence that is available tends to be interpreted in diametrically opposite ways. As we discussed, for example, the results of Snow and Hoefnagel-Hohle(1978) were presented by the authors as strong evidence which provide that there is no critical period for language acquisition. Lightbrown and Spada(1996), however, dismissed this result on methodological grounds
Another problematic issue on the research of the critical period hypothesis is that it is almost impossible to have the distinction whether we should look at the critical period hypothesis in term of first language acquisition or second language acquisition.
Sampson(2005), in his article, empathized the need of distinguishing two cases; acquisition of a first language, and acquisition of a second or subsequent language.
Sampson claimed that as far as second language acquisition is concerned, Lenneberg does not provide the relevant evidence. He quoted the examples of many immigrants acquire the target language rapidly and well.
Sampson noted that
There may be evidence that even successful adult second language learners progress slower than children exposed to a second language in their early years; but it is uncontroversial that learning in general tends to slow with age. Lenneberg gives no evidence that language-learning is “special”. (Sampson2005;There is No Language Instinct)
The similar subject was also questioned by Eubank&Gregg(1999). In their book, it said that “the difficulties encountered attempting to determine whether there are critical period for first language competence are magnified when dealing with adult second language acquisition”(Eubank&Gregg 1999;77)
4.The Critical Period Hypothesis
4.1 The Critical Period Hypothesis within Language theory
Lenneberg’s idea that the acquisition of language is governed by the maturation of the nerve system is one of strong supports of Chomsky’s view that language is essentially innate.(Corballis1991;122)
Therefore, it may be worth looking at how the critical period hypothesis has been applied to the Chomskian’s view.
First of all, Eubank & Gregg(1999) suggested that the idea of critical period hypothesis can be refined within linguistic theory with “the crucial assumption that the innate, domain-specific neurobiological architecture essential for first language acquisition is nothing other than “Universal Grammar”(UG;see,e.g.Chomsky1995;Cook&Newson1996;Eubank&Gregg1999)” Although there is no sufficient division between first language acquisition and second language acquisition, “the debate whether adults second language learner still has access to UG can be recast in critical period terms with little distortion”(Eubank&Gregg 1999;79)
Many of researchers attempted to apply the Chomskian’s frame of reference to the concept of critical period, but the results do not agree on any of single conclusion.
For examples, researcher like Hawkins(2001) suggested that only particular subparts of Universal Grammar, such as mapping of abstract syntactic features, are impacted by age-related degradation. Hawkins(2001), referred to Lardiere’s suggestion (1998a, 1998b), argued that late second language learning have problems “mapping the abstract feature specifications of lexical items onto their morphological realizations” (Hawkins 2001;358) He concluded that late second language learner may not have certain features of Universal Grammar functional categories.(Singleton2005;274)
Other view is that of Johnson & Newport (1989), who argued that there has no access to Universal Grammar in post-pubertal second language learning and such post-pubertal learning has to take place via general problem-solving abilities, which are not constrained by maturation.( Eubank&Gregg 1999;80)
Still Martohardjono and Flynn (1995), among others, maintained that maturational constrains does not affect the innate language device. In terms of the proficiency of second language, Martohardjono and Flynn (1995) distinguish between the innate language device and the non-innate components of our language ability. They argued that the latter maybe susceptible to age related degradation. (Singleton 2005;274))
However, Enbank&Gregg(1999) noted that there is a fundamental difference between post-critical period first language acquisition and post-critical period second language acquisition. “This different along suggest that an effect as plainly gross as no access or full access is not particularly plausible in the latter case”(Enbank&Gregg1999;80)
4.2 The Critical Period Hypothesis within Cognitive Factors
A number of explanations for the critical period Hypothesis have drawn on Jean Piaget’s cognitive approach to language acquisition. In his 1975 paper, Krashen(1975) related the critical period hypothesis to the Piaget’s “formal operations” stage and suggested that after puberty, acquisition of second language learning rely on theories rather than ad hoc solution. (Krashen1975;Singleton2005;276)
In 1981, Felix also referred to the Piaget’s “formal operations” stage along with the chomsky’s idea of an innate language faculty. Felix(1981) claimed that “problem solving cognitive structures” emerging at the formal operations stage, which was referred by Piaget, compete with the language acquiring structures of the language faculty. (Felix1981;Singleton 2005:276)
Another explanation in terms of the role of general cognitive factors relating to the age question was provided by Robert Dekeyser(2000, 2003a). Based on his evidence(2000) that adults beginners used their verbal analytical ability, which did not apply to the performance of child beginners. DeKeyser noted that, children are able to acquire a complex abstract system such as language implicitly whereas adults can no longer do that, which results in more effort-consuming and less effective language learning. Dekeyser concluded that maturational constraints applies only to implicit language learning mechanism-that “somewhere between early childhood and puberty children gradually lose the ability to learn a language successfully through implicit mechanism only” ( Dekeyser2000,2003a;Singleton2005;274)
4.3 The Critical Period Hypothesis within Social and Psychological Factors
Lastly, many of researchers attempted to understand the critical period hypothesis in terms of social and psychological factors.
Researchers like Bialystok&Hakuta (1994) suggested that such constraints on second language acquisition as adults’ reluctance to take on a new language identity, social distance, and attitudes towards the target language.
Sagerdahl(2005) closely linked first language acquisition to “enculturation”: “Language is so thoroughly intertwined with how we function spontaneously together that it cannot be learned through planned and explicit instruction.” (Sagerdahl 2005;9) He also claimed that language can be learned “by living together” rather than in a planned manner; this is why adults and children learn languages in fundamentally different ways. (Sagerdahl 2005;10)
Krashen(1985) also claimed that the “affective filter” is strengthened at puberty due to the onset of formal operations:
While the filter may exist for the child second language acquirer, it is rarely, in natural informal language acquisition situations, high enough to prevent native-like levels of attainment. For the adult it rarely goes low enough to allow native-like attainment. (Krashen1985:13; Singleton2005;277)
Conclusion
It is said that the ancient Egyptian king Psammetichus placed two infants in an isolated cabin and allowed nobody to speak to them. Two years later, the children spoke Phyrigian. Like this tale, if we can possibly experiment such language deprivation, that may give us the answer for the critical period for first language acquisition. However, when it comes to second language acquisition, the sheer numbers of variables are applied.
Age-related factors can impact on language learning in several of ways; and in some cases we cannot even be sure that these factors are age-related in the first place. Neither is it quite clear what it means to be a better language learner (Gass&Selinker 2001: 335): Should we take into account initial and medium-term progress, or is it only ultimate attainment that matters? In the former case, many of studies showed that adult beginners outperform child beginners. In the latter, how exactly do we decide at which point attainment becomes ultimate?
It is obvious that the notion of critical periods as it is understood in zoology cannot be applied to human language. It is also obvious that particularly high standards of methodological precision are needed to study any interaction between language and its neurological and social environment. If these requirements are met, however, critical period-related research can help us make important progress towards the ultimate goal of linguistics – complete understanding of how language works.
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