Psychologists, linguists and teachers have all long debated the manner in which a child acquires language and the existence of a critical period, that is “a period of time during which an organism is optimally ready for the acquisition of specific responses”, here language (Reber & Reber, 2001). Originally, the discussions centred around first language (L1) acquisition, but of course this debate has relevance for ELT and other foreign language teachers!
The hypothesis of a biologically based critical period for language was first overtly suggested by Lenneberg (1967), and asserts that if no language has been learnt by the onset of puberty, it can never be learned in a normal and functional sense. This post will present and evaluate the evidence Lenneberg and later supporters of the critical period hypothesis have cited to verify their ideas, in order to provide an answer to the question as to whether or not a critical period exists for acquisition of language.
As the idea of a critical period was most prominently advanced by Lenneberg, let’s first look at the evidence he cited. His argument can be broken down into two parts. Firstly, that normal language learning occurs primarily (if not exclusively) during childhood. Evidence for this assumption was drawn from studies of brain damage recovery; children who suffer brain damage before puberty typically recover fully and (re-)develop normal language ability, whereas adults rarely recover fully, and often do not regain verbal abilities beyond the point reached five months after impairment (Hurford, 1991, p. 159). Secondly, Lenneberg supports Penfield & Roberts’ (1959) proposal that humans possess a neurological mechanism responsible for this maturational change in language learning abilities: after puberty, the brain loses its ‘plasticity’ and the re-organisational capacity needed for language learning. This coincides with lateralisation of the brain and the specialisation of the left hemisphere for language. In children motor and linguistic skills develop simultaneously, but between the ages of two and thirteen years, the functions of the cerebral hemispheres separate and become fixed, rendering the acquisition of language difficult, if not impossible (Lenneberg, 1967, pp. 389-90).
However, this neurological explanation has,been challenged by later work. Witelson & Pallie (1973), for example, maintained that lateralisation is complete by the age of five years. Krashen (1975) reanalysed the clinical data used as evidence (e.g. from Genesee, 1978), and also concluded that lateralisation was complete much earlier than Lenneberg calculated, therefore the link between lateralisation and the end of a critical period is rendered somewhat unconvincing, considering children can easily learn languages up to the age of about twelve years: if a critical period does exist, it does not coincide with lateralisation.
Despite the concerns with Lenneberg’s original evidence, and the dissociation of lateralisation from the idea of a critical period for language acquisition, the concept of the biologically based critical period remains in itself a viable hypothesis, for which later, more direct and reliable evidence has been cited. This more recent evidence, from studies of feral children, congenitally deaf children, and second language acquisition, will now be discussed.
Feral children are those who were not exposed to human language as a child because of being brought up in the wild (possibly by animals), in isolation and/or in confinement. One early case of this kind is of ‘Victor’ (Itard, 1801). Victor was found aged twelve by a physician who tried for five years to teach him to speak and read. Victor learned to understand many words, but never learned to speak himself. Possibly the most famous example of such case studies is that of ‘Genie’ (Curtiss, 1977). Genie was deprived of social interaction for the first thirteen years of life, because her father judged her as retarded and isolated her from birth. On her discovery she was completely without language, and even after seven years of rehabilitation she still lacked linguistic competence. A similar case study is of ‘Isabelle’, who was incarcerated in a dark room with her deaf-mute mother until the age of six and a half. Although she had learned to communicate with her mother through a system of gestures, when found she had no formal language skills. Unlike Genie, with the specialist attention of psychologists and systematic training, Isabelle quickly acquired normal language ability (Davis, 1949). All of these cases support Lenneberg’s critical period hypothesis: Victor and Genie were both discovered after puberty and did not acquire normal language, whereas Isabelle, who was found before puberty, developed full linguistic competence. This demonstrates that children require exposure to language and normal social interaction before a certain age to acquire language themselves. Critics of the critical period hypothesis, though, highlight that the children in these studies grow up in generally abusive environments, suffering emotional trauma and seclusion, and may have been isolated due to general retardedness; therefore their lack of language acquisition may be the result of these factors, rather than a lack of exposure to language.
Studies of congenitally deaf children have also been applied to support the critical period hypothesis. Newport & Supalla (1987) studied language acquisition in deaf children whose development was otherwise normal, but who were exposed to sign language at different ages: because 90% of children had hearing parents, very few were exposed to sign language from birth, in fact most first learnt it at school. Newport & Supalla’s results showed a “linear decline in performance with increasing age of exposure, on virtually every morpheme tested” (1995, p. 78), that is to say that those exposed to sign language from birth performed best, and ‘late learners’ worst, on all tests of production and comprehension. This study thus provides direct evidence for language learning ability decreasing with age.
As it is often difficult to directly test the critical period in native language acquisition, mainly due to ethical objections, many researchers have turned to the area of second language acquisition to clarify and refine the critical period hypothesis. Indeed, this is the research that is probably of most interest to language teachers. If, as an extreme interpretation of the critical period suggests, “the biological fact of adulthood is enough to establish an insurmountable obstacle” to comprehensive language acquisition (Seliger, 1978, P. 11), then adults should find it almost impossible to learn a second language. Asher & Garcia (1969) substantiate this belief with their study of Cuban immigrants, which showed that children who had arrived in the U.S. aged between one and six did much better on a test of reading English than those were aged seven to nineteen years on arrival.
Nonetheless, studies of foreign language learning and the application of their data to first language acquisition has been questioned by some researchers. The extent to which first and second language acquisition are similar forms the basis for this counter-argument, but it also highlights the different circumstances under which younger and older language students learn languages, including the amount of exposure and nature of instruction, if any.
Aside from the biological view of a critical period, Chomsky (1965)’s theory of language acquisition based on a ‘transformational grammar’ has been used to explain why children acquire language easily. Initially, the ‘learner’ possesses innate principles und rules which build a ‘black box’ in the brain. These principles denote restricted possibilities for variation within the target language, and enable the learner to construct a grammar from the ‘raw input’ collected from the environment. Although he did not name a specific period for optimal language learning, it has been drawn from Chomsky’s ideas that younger children can learn languages more easily than older children or adults. The theory maintains that children can learn several languages simultaneously, as long as their ‘black box’ principles are still active. Adults would have to use the same principles developed at a young age for language acquisition to re-learn language (e.g. after brain damage) or to learn a foreign language, and will therefore have more difficulty in doing so.
Yet another view, the behaviourist approach, argues that language is learned in the same way as other behaviours; through learning reactions to environmental stimuli by conditioning or imitation. The theorists who first applied this idea to language acquisition are Skinner (1957) (concerned with classical conditioning) and Mowrer (1960) (concerned with imitation and operant conditioning). New connections between behaviour and the environment are formed and reformed throughout life and it follows that it is possible to gain new skills, including language(s), at any age. Although Skinner and Mowrer do not discuss the question of a critical period for language directly, some theorists (e.g. Felix, 1985) have drawn from their ideas that children, whose brains create countless new connections every day, will simply cope better with (re-)learning a language than adults. This assumption is however untested and can not be taken as an explanation of the ease with which children acquire language in comparison to adults. The biggest criticism this theory faces, is that it assumes that all learning, whether verbal (languages) or non-verbal (general learning), takes place through the same habit forming processes (Chomsky, 1969).
In summary then, the hypothesis of a critical period for language acquisition is not a new idea. Since it was hinted at by Penfield & Roberts and vigorously advocated by Lenneberg, a biological base of the idea has been thoroughly researched and, to a certain extent, validated by the results of studies, such as those by Newport & Supalla, Curtiss and Asher & Garcia. Although the evidence comes from a variety of areas, namely feral children, deaf children and second language acquisition, no one study can provide critic-free conclusive evidence. The feral children’s lack of language may be due to abusive environments; deaf children do not show total incapability for language learning once they reach puberty; and second language learning studies rely on a questionable generalisation of their data to native language acquisition. Despite this, all the studies discussed here do demonstrate differences between younger and older learners of a language, and the majority of theorists agree that children have an advantage over adults in language acquisition.
Biological and neurological approaches are not the only tools to explain this age different in language learning abilities. Chomsky’s language acquisition theory of transformational grammar has also been applied to this purpose, as have Skinner’s and Mower’s behaviourist ideas. Both approaches have some solid supporting evidence, but also weaknesses and disadvantages. The problem faced by all theories and studies is that they seem to focus on just one aspect of the relationship between language acquisition and age; in order to completely prove the existence of a critical period for language acquisition a theory would need to be able to explain the maturational changes as well as the general language acquisition process.
It seems then, that evidence generally supports the notion of a period prescribed by age where language is more easily learned, but with some considerable modifications to Lenneberg’s initial idea, and with various explanations. One modification that has often been discussed is the ‘weakening’ of the concept to a sensitive period. This is a period where “an organism is sensitive to particular forms of stimulus inputs and physiologically and psychologically ready for the acquisition of a particular response”, here language (Reber & Reber, 2001). This would mean learning a language is easier as a child, but does not imply that later learning is out of the question, rather that it is more difficult. Indeed, the various studies and approaches discussed here point to the likelihood of a period with relatively steep, but not absolutely abrupt boundaries. As Hurford explains, “the evidence available seems to indicate a curve [in language learning ability] which rises early, maintains a high level before puberty, and the falls, with the steepest downward slope coinciding roughly with puberty.” (p. 160). This idea is clearly evidenced by Newport & Supalla’s results, and by the fact that students of foreign languages do not experience complete failure after puberty, and makes many theories and approaches to explaining language acquisition more feasible.
In answer to the specific question of whether there is a critical period, then, it seems sensible to conclude that, as Lenneberg and others suggest, there is an optimal window of opportunity for language acquisition, but this is more correctly labelled as a sensitive period, rather than a critical one.
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