Chomsky claimed that there was a critical period for language learning which was first proposed by Eric Lenneberg. He claimed, as Cook Newson (1996:301) explain, that there is a critical period during which the human mind is able to learn language; before or after this period language cannot be acquired in a natural fashion. Although the rare cases of feral children who had been deprived of first language in early childhood seems to support the idea of critical period but it is not known for definite if deprivation was the only reason for their language learning difficulties as Sampson (1997:37) points out, “it is not certain if children in cases of extreme deprivation have trouble learning language because they have missed their so-called “critical period” or if it is because of the extreme trauma they have experienced.”Although Chomsky was a very influential and successful nativist, Sampson (1997:159) claims ‘his theories were given a helping hand by external circumstances.’

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Behaviourist Theory and Chomsky’s Innatism
Further, reasoning the success of these new nativist writers Sampson says “When Chomsky originally spelled out an argument, the reader would assess it and might detect its fallacies; but when recent writers refer to something as having been established back in the 1960s-70s, most readers are likely to take this on trust, for lack of time and energy to check the sources.”Finally, on the subject of ‘nature vs. nurture’ debate, which so heavily involves Chomsky, it seems impossible to distinguish whether language is only acquired due to environmental exposure or simply due to innate faculties. From the evidence it seems that humans possess innate capabilities which enable linguistic development, but the correct environment, with exposure to adult language throughout the critical period, also seems to be necessary in order for a child to develop and become a proficient speaker.
Singleton evaluates the scenario in these words, “The one qualification that might be entered with regard to such evidence is that deprivation of language input during the phase in a child’s life when cognitive development is at its most intense may have quite general psychological/cognitive effects, and that it may be these general effects that are reflected in later language development rather that effects relating specifically to a critical period for language” (Singleton 1989, 54).
The behaviourist theory is also contradicted by the fact that young children very often make ‘intelligent mistakes’ as they learn a language. As they progress beyond the two-word stage, they begin to understand and try to express more complex concepts, such as plurality, past tense and negation. Mistakes such as ‘sheeps’, ‘goed’, and ‘I no want to’, are all formed from formulae in the English language. In the first case, the child has grasped the general rule that /s/ at word endings implies more than one, but has not yet distinguished irregularities. The same applies to the second; /ed/ is a regularised past tense ending. In the third instance, the child definitely understands the concept of negation, but cannot yet distinguish between the different forms of expressing it (such as ‘do not, don’t’; ‘will not, won’t’; ‘no’; ‘none’ etc) or completely comprehend the complex syntax involved. (Foster-Cohen, 1999, 89) If children only learned through repetition and imitation, would they be able to produce utterances with grammar applied that they would never have heard their parents use?
Chomsky and the Language Acquisition Device (LAD)
Therefore there must be more to a child’s linguistic development than simply social factors. Theories suggesting that the human brain has a unique capacity for language which is genetic and exists from before birth include Chomsky and the Language Acquisition Device, Piaget and Cognitive theory and Lenneberg and the Critical Period. The Language Acquisition Devise (LAD), according to Chomsky, is an inborn mechanism facilitating the learning of language, consisting of neural wirings and brain structures unique to human beings. A child does not need to be taught language formally, but simply needs to be exposed to adult speech. This effortless discovery of the language system is due to LAD. Chomsky, 1986, 118) The four properties of LAD are:

Ability to distinguish speech sounds from over environmental sounds
Ability to organise linguistic events into various properties
Knowledge that only a certain kind of linguistic system is possible
Ability to engage in constant evaluation of developing linguistic systems, such as generating new sentences and selecting the correct tense.

Chomsky argues that language develops in a uniform way, which is genetically determined, and that learning develops in an individual with the trigger of experience. The human brain is ready for language intake from birth, and exposure to adult speech activates it. An ancient experiment where an emperor ordered for a group of children to grow up in isolation with no exposure to language resulted in none of them developing language for communication. This shows that exposure to adult language is essential for linguistic development, as language will not appear without it. A child’s language develops through hypothesis testing: deciding an utterance and trying it on parents. Through trial and error, their speech will eventually correspond to adult speech. Chomsky’s LAD theory therefore does tie in with the behaviourist theory, but maintains that language acquisition is much more complex then simply imitation because innate faculties are also involved.
If language development is dependent on the environment triggering the LAD, what is it that determined how and when children learn what? Certainly, the same stages of linguistic acquisition occur for all children (i.e. holophrastic, two-word, telegraphic), in the same order and at roughly similar ages.Piaget’s Cognitive Theory identifies language as an element of knowledge acquisition. He believed that “the growth of knowledge is a progressive construction of logically embedded structures replacing one another by a process of inclusion of lower, less powerful logical means into higher and more powerful ones up to adulthood”. This means that children’s logic and thinking methods are originally different to adults.
Piaget’s cognitive theory suggests language is one of many cognitive behaviours, and it emerges from cognitive development or learning to think and problem solve. However, this theory is contradicted by the ‘cocktail party chatterer’ children. Observations of these children have suggested that an ability to cope with language structure is largely separate from general intelligence. They talk for the sake of talking and their sentences may not make sense. However, as the behaviourist theory might have suggested, they are not simply repeating phrases they have heard, because they make grammatical mistakes such as ‘three tickets were gave out by a police last year’ which they would not have heard said.
A theory which would explain why these ‘cocktail party chatterers’ could be able to speak fluently even with a very low non-verbal I.Q. is Lenneberg’s Critical Period Theory. He considered language to be an example of maturationally controlled behaviour, like walking and sexual behaviour, which is genetically programmed to emerge at a particular stage in an individual’s life. Learning is required, but cannot be significantly hastened by coaching. Aitchinson Reith tells us ‘no external event or conscious decision causes it, and a regular sequence of milestones can be charted.’ When the brain is in this period, lateralisation occurs, when the two sides of the brain develop specialised functions. Lenneberg recognised that in an adult brain, language is found only in the left hemisphere, whereas in infancy it is more spread out, meaning the infant brain has equipotential.
Feral Children Phenomenon
So-called feral children, those that have had no exposure to language in their ‘critical period’ have helped to credit Lenneberg’s theory. Genie is an example. She was kept in appallingly deprived conditions, with almost no social contact or exposure to language until she was found at age thirteen. Attempts were made to teach her language, but although she progressed to the two-word and three-word stages like most children, her lack of morphology was never remedied. She only grasped simple grammatical concepts. ‘Had’ and ‘gave’ where her only past tenses which were used rarely and her only auxiliary was ‘be’; never ‘have’ or ‘must’. She never used the demonstratives ‘there’ or ‘it’ although there were attempts at the definite article ‘the’. Genie demonstrates that after the critical period, pragmatic skills can still be developed but the structural knowledge of language is lost. She suggests that a child must be exposed to language during the critical period, and that after puberty language acquisition cannot reach its normal end point. (Snow, & Hoefnagel, 1978, 1116) This suggests that universal grammar is no longer operative as an LAD.
About Genie’s case, De Villiers is of the opinion that “It is unnecessary to explain that such circumstances did not leave Genie intact in body and mind. However, although she was malnourished, there was no evidence of physical abnormalities sufficient to account for her behaviour, for she had adequate hearing, vision and eye-hand coordination. She was severely disturbed emotionally, having frequent but silent tantrums, yet there were no other symptoms of childhood autism. The most likely explanation was the chronic social deprivation she had suffered for those twelve years” (De Villiers & De Villiers 1978, p.215)
Critical Period and Language Acquisition
What the critical period theory does fail to emphasise is the way in which children learn language, concentrating more on the scientific explanation for why they are able to. The fact that children definitely need exposure to adult speech in order to learn language has already been discussed. But the reasons that humans, as social animals, need language have not been covered by any of the previously mentioned theories.
Through studying his own son, Halliday identified five main purposes children need language for. These functional frameworks can be distinguished from children’s utterances even before recognisable words can be articulated, i.e. in the child’s ‘proto-language’. Interpersonal functions are when the child is increasing and extending his capacity for engaging linguistically with others, for opening and sustaining dialogue with them. These include Instrumental, to satisfy material needs, for example /na/ when requesting an object. This is the “I want” function of language. The regulatory function is used to control behaviour of others, such as /ee/ meaning “do that again!” This is the “do what I say” function of language.
Ideological functions are the aspects of adult language systems concerned with giving shape to and expressing events and the external world, and also the internal world of consciousness. (Jackson & Stockwell, 1996, 170) The heuristic function is where a child uses language to explore its environment, such as requesting the name of and object. (Harley, 1989, 166). This is the “tell me why” function of language. The personal function enables the child to communicate his identity by expressing pleasure or interest, for example /a/ for “that’s nice”. This is the “here I come” function of language.
Halliday’s language functions propose that both proto-language and later linguistic development can be identified by frameworks. These frameworks are factors which are important for humans to establish relationships with others, satisfy material wishes and to find out about the environment in which they live. All of the theories discussed in this essay have their merits, because the reason humans learn language and the way in which they do so are determined by many factors. Humans do uniquely possess innate faculties which enable linguistic development, but the correct environment, with exposure to adult language throughout the critical period, must be present in order for a child to develop and become a proficient adult speaker.
Lexical Development
Lexical development also continues after puberty, and, according to some suggestions, may continue throughout our lives, as people are continually interested in learning new things (Singleton 1989, p.56). Diller (1971) points out that “twelve year olds have a recognition of about 135,000 words, Harvard freshmen know about 200,000 words, the typical thirty year oold PhD student know about 2,500,000 words. Vocabulary development continues in a natural, almost unnoticed fashion as long as one lives and is interested in new things”.
There have been several studies that strongly support Lenneberg’s hypothesis. Among the most prominent are feral children. Feral children are persons that have been linguistically isolated. When they were found, most were unable to pick up on language abilities. These cases propose that there may be a ‘critical age’ in which any child who has somehow missed out on learning a language will never fully master one. (Macwhinney, 2004, 910) Genie never learned proper grammar or sentence structure. Other cases of feral children include: Victor, the wild boy of Aveyron (who was found at age 11) and Kamala of Midnapore (who was found at age 8), both never learned language correctly either. Therefore, although Lenneberg’s hypothesis is not proven, feral children forcefully support it.
Critical Period Hypothesis and Empirical Evaluation
The Critical Period Hypothesis is further supported by experiments about second language acquisition. Lenneberg believed that “the language acquisition device, like other biological functions, works successfully only when it is stimulated at the right time” (p. 19).
Aitchison (1998, p.88) goes for a general outcome by saying that young children who start signing early because of their deaf parents end up more proficient that those who have hearing parents. The Ildefonso case is probably used against the hypothesis of upper limit critical period because he has shown nearly full competence in sign language despite his late start which was far beyond puberty.
Psycholinguistic, a branch of linguistics theory covers the cognitive process that discusses the process as how to generate a grammatical and meaningful sentence out of vocabulary and grammatical structure, as well as the process to make it possible to understand utterances, words, texts, etc. It concentrates in the internal process of acquiring language and their effect on language learning. Whereas, adults can perform and develop many aspects of new ways of language and vocabulary (Singleton:1989,p.55) One internal factor is the development of Interlanguage, which is developed by learners of a second language who have not become fully proficient, but are only approximating the target language. Learners use some features of their first language during speaking and writing and they also create their own innovations; it is a system based upon the best attempt for learners to provide order and structure to the linguistic stimuli surrounding them. Interlanguage is particularly based on the learners’ experience on the target language.
The learner creates an interlanguage using different learning strategies such as language transfer, overgeneralization and simplification. Language learning acquisition is a gradual process where errors will be unavoidable, but learners will slowly and tediously succeed in establishing closer and closer approximations to the system used by native speakers. Educators can give appropriate feedback after checking learners’ interlanguage, and assure learners that making mistakes is a procedure of development from the mother tongue to the second language.
Universal Grammar Orientation
Universal Grammar is another theory that explains failure of second language competence. It is said that Universal Grammar is a natural unconscious ability present at birth, that is a knowledge of grammar. This does not imply that a person does not make mistakes, but that they do make mistakes but irregular types of errors, and somehow a person has the ability to accept these errors and re-apply them. The set of grammatical rules are learned through conditioning, meaning that if a person when learning their mother language never heard anyone make mistakes, then he or she learns it that way. With Universal Grammar set in place at birth, the person is able to take on whichever language he or she is exposed to, as all languages have common elements and are inter-adaptable. This theory does not state that all human languages have the same grammar, or that all humans are encoded with a structure that underlies all surface grammatical expressions of each and every specific human language. But it implies that Universal Grammar suggests a set of rules that would explain how a person acquires their language or how they construct valid sentences of their language. (Chomsky, 1972, 37) This theory explains that grammar is the system of principles, conditions, and rules that are properties of all human language.
Second Language Learning And Critical Period
Learners begin by transferring the sounds and meaning, words order into the learning of the second language. By doing this an obstruction of the second language acquisition starts to raise, the learner starts to confuse the grammatical rules of the first language with the second language creating an incorrect language learning process. When learning a second language, a learner uses this language transfer concept as a strategy to have an uncomplicated language learning process. The learner uses parts of the analysis of one language in order to manage with the unclear grammatical rules of the target language. Learners when presenting or expressing their ideas, opinions and statements in the target language, tend to consider and apply the same grammatical structures of their first language; this transferring interferes with the proper application of the grammatical structure of the target language. The Monitor Model is another likely explanation for this second language incompetence. This theory is characterized of five hypotheses.
The Acquisition-Learning Hypothesis involves that there are two independent systems of second language performance; acquisition is the result of a subconscious process similar to children’s mother language acquisition and requires life-like second language interaction, which concentrates on communication rather than accuracy. On the other hand, learning occurs through formal instruction and includes a conscious process, which results in knowledge of the target language. Learned knowledge permits learners to read and listen more so that the acquisition is effectual. The Natural Order Hypothesis; suggests that just as first language learners, second language learners seem to acquire the characteristics of the target language in an obvious order. Contrary to intuition, the rules which are easier to state and to learn are not necessarily the first to acquire. It is observed that the natural order is independent of the order in which rules have been learned in class.
Monitor Hypothesis
The Monitor Hypothesis; states that a learner’s learned system acts as a monitor to what they are producing. In other words, while only the acquired system is able to produce spontaneous speech, the learned system is used to check what is being spoken, this occurs before or after the interaction is made, but not at the same time. The interlocutor therefore monitors their spontaneous speech using what they have learned. The Monitor Model then predicts faster initial progress by adults than children, as adults use their monitor when producing statements in the target language before having acquired the ability for natural performance, and adults will input more into conversations earlier than children.
Input Hypothesis
The Input Hypothesis states that only understandable input will produce acquisition of the target language. It is implied that learners must be exposed to input that is higher than their actual level of knowledge of the target language in order to produce a result in the acquisition of the target language. Because some instances learners do not posses that higher level of input, the development and improvement of the target language will not occur. The Affective Filter Hypothesis affirms that the learner’s emotional state acts as flexible filters that freely permit or obstruct input necessary to acquisition. It is said that adolescence and puberty are not effective periods for second language acquisition due to the hormonal changes that occur during that development period.
The Sociolinguistic theory is concerned with the effect of all aspects of society in the learning process of the target language such as cultural norms, expectations and context on the way language is applied. It also concentrates on the different types of language variation, ethnicity, religion, status, gender, level of language knowledge and how all these factors have an impact on second language learning process. The Acculturation Model which is a likely reason for the failure of second language competence refers to the process of adjustment to a new culture, on how new language learners acclimatize not only to the target language but also to the culture of that language. This model refers to immigrants that are in second language surroundings, but it could also apply to new language learners and the way it affects them to also acquire a new culture. (Sagae, MacWhinney, 2004, 119). For some learners the idea of modifying and adding another culture in their life could cause difficulties in their language learning process.
One important distinguishing factor in the acquisition of second language is fossilization which refers to the permanent cessation of progress towards the target language, despite all attempts and exposure to the target language. (McLaughlin, 1985, 209). An aspect that has a relevance to fossilization is motivation; there exists lack of empathy with the target language, the culture and the native speakers.
Accommodation Theory
The Accommodation theory implies the way learners may adapt their speech in multilingual settings. This theory suggests the way learners adjust their way of interacting with people of different culture with different languages in order to facilitate communication. It also states that: is the way the learner’s social group (ingroup) identifies itself in the target language community (outgroup). If these two groups do not have a positive effect on one another, or they do not connect and identify themselves with each other, there will be a consequence in the acquisition of the second language. When learning a new language, learners need to feel that they belong in this outgroup, they need to feel that they are part of this new culture and that they feel a cordial welcoming from this outgroup; but if none of these intentions are met then learners will not have an effective and productive learning acquisition process.
Social identity is another sociolinguistic issue that has a great impact in second language competence. Social identity is the way in which learners understand themselves in relation to others, and how they view their past and their future. Structural inequalities can limit leaner’s exposure to the target language as well as their opportunities to practice it. (Asher & Garcia, 1969, 338). The input that learners acquire could be received from different sources such as; the teacher, textbooks, individuals, radio, television, newspapers, magazines, etc. And if these resources are not accurate in their language transmission then the input might not be logical to the learners and the learners’ intake will be erroneous which will result in the failure of language competence. (Collier, 1989, 523).
Conclusion
In conclusion, numerous linguists believe that there is a ‘Critical Period’ in which humans can acquire language. The theory proposed by Lenneberg about a limited time in which language can be acquired is supported by large amounts of research. Although many challenge this hypothesis with the theory of ‘Universal Grammar’, both are unproven.
Studies have determined that invalid conversational interaction affects the acquisition of second language, which in result could decrease the pace of language acquisition. Invalid interaction is the product of inaccurate input, which could cause a decline in the learners’ participation and motivation to produce language properly. The role of instruction has been examined as the role of focus on the grammatical forms of language in instruction.
Many factors such as; internal or external factors can modify the way learners acquire and learn the target language; the above stated factors can obstruct second language acquisition competence. However, learners can manipulate and be capable of attaining the knowledge of the second language if their perspective of the target language is adjusted. Learners with low motivation and low language confidence will have high filters which will obstruct the achievement of second language acquisition. There are other variables in second language acquisition process. Aptitude refers to the ability and capacity that a learner has towards the language. The attitude the learner posses towards the target language, how the learner feels, the posture they have towards the target language.
Age has also an influence in second language acquisition, as it is stated above adolescents might have an increase difficulty level when it comes to acquisition of language due to the hormonal changes, but at the same time they perceive language faster than adults due to their neurological development. As for adults, they become more appropriate for the study of a second language because of their maturity, but at the same time, their learning process struggles due to their developmental age.