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The factor of age in second language acquisition is consistently examined and debated within much literature in the field of second language research, the primary focus being to evaluate the extent to which the age of a learner correlates with overall linguistic success in the process of acquisition, and discover whether it influences the fluency of a second language (L2) speaker once the apparent end state of language learning has been reached. This paper will aim to provide a summary of some of the key literature and debated points of discussion within the field, and use this research to consider the aspects of age as a factor in SLA.
In 1956, Chomsky put forth the notion of "Universal Grammar" (henceforth UG) -- an innate blue-print of grammaticality built into the minds of children as a linguistic primer, providing a biological basis for language acquisition. Later, in 1965, Chomsky further hypothesised the existence of a biological tool referred to as the Language Acquisition Device (LAD), a linguistic device that works alongside UG to aid children in deciphering their first language, to acquire its rules and put them into practice in communication as part of a subconscious, innate process. Some theories, such as the nativist approach, support the view that we are able to acquire a second language -- and subsequent languages -- in the same way that we acquire the first, by using the same natural language-acquisition abilities that are hard-wired into the brain from birth. Though many researchers support Chomsky's UG theory, the extent of time to which we are able to access our UG and LAD to acquire languages in such a way is another subject of long-standing debate in the field of linguistics and language acquisition research.
The Critical Period Hypothesis (CPH), first proposed by Penfield and Roberts (1959), argues that the ability to acquire language is biologically linked to age, and that there is a so called "critical period" for language acquisition, after which an L2 learner may no longer have access to their in-built UG. The theory was further explored by Lenneberg (1967), who suggested that after the age of twelve, a learner's brain may retain some flexibility in relation to the functions needed to acquire language, but that "the ability for self-organization and adjustment to the physiological demands of verbal behaviour quickly declines" (pp. 239-240). As the brain reaches the point of lateralization at puberty, the left hemisphere becomes dominant, and therefore it is possible that an adult language learner might become over-analytical in the language acquisition process.
Long (1990) states that the initial rate of acquisition and eventual success depends in part on the age when the learning of a language begins. Humans are introduced to their mother tongue from birth, immersed in some form of communication with parents/caregivers and those that surround them, and thus the process of acquiring the first language naturally occurs in stages from a very young age, with children often speaking their first words around the age of one year, after a babbling period at which they try to articulate sounds close to that of their linguistic environment. As the child grows older and begins acquiring more linguistic knowledge from its first language, they might also begin to acquire other languages that are presented to them alongside their first, provided there is stimulation or a perceived need to do so, e.g. motivation to learn facilitated by, for instance, the need to communicate with a monolingual family member that uses an alternative language.
The CPH suggests that language learning becomes more difficult after puberty, due to changes in the brain. While this theory lacks sound biological evidence to support its arguments, it does seem to be the case that as children grow older, the ease of language acquisition seems to wane. Long (1990) suggests that age related loss in ability is total and affects all areas, not just phonetics, and deterioration can in fact begin as early as the age of six, but is typically complete by the time a child reaches puberty. Additionally, any languages for which rules had been learned by the age of puberty could be considered fully acquired and would continue to develop lexically into adulthood, however, the CPH challenges the idea that adults could ever fully 'acquire' languages, but have to resort instead to the conscious effort of 'earning' them.
Evidence to support the CPH can be found in a study carried out by Newport (2003), in which the sign-language proficiency of deaf individuals was assessed. Some participants within the study were not exposed to sign language until the end cusp of the critical period (thought by Newport to be age 5-6), while others were exposed to it from birth. The study revealed a difference in the language proficiency of the two groups as adults, with those that began their acquisition process earlier, during the critical period, yielding better results. Although this study examines first language acquisition (FLA), we must ask what impact the notion of a critical period may have on the process of second language acquisition, as it is clear that as individuals mature, achieving linguistic competency in an L2 becomes increasingly difficult in much the same way.
According to Krashen (1981), there are two independent systems of second language performance: the 'acquired' system -- in which a child faces a similar process to the one faced in acquiring the first language (L1) i.e. naturally and via "meaningful interaction in the target language ... in which speakers are concentrated not on the form of their utterances, but in the communicative act" (SK.com.br, accessed 25/11/12) -- and the 'learned' system -- in which the second language is a "product of formal instruction" which "comprises a conscious process which results in conscious knowledge 'about' the language"(SK.com.br, accessed 25/11/12). In simplified terms, acquisition is a subconscious development of linguistic knowledge, and learning is achieved through conscious efforts made by the learners of an L2.
Krashen believes that the ability to acquire language remains with all learners, regardless of age, depending on the context in which the language is being presented to them, i.e. if fully immersed in the target language, acquisition will occur; if presented with the target language only in a classroom environment, the individual will have limited linguistic opportunities, and be presented with relevant and manageable chunks of language, without the ability to communicate with native speakers using the language naturally, thus they will 'learn'. Krashen (1982, pp. 10) does acknowledge, however, that some advocates of the Critical Period Hypothesis (Asher et al, 1969; Birdsong, 1992, 1999; Flege, 1995; Olson and Samuels, 1973; etc) would posit the contrary, that this distinction made between acquisition and learning reflects on a learners' age i.e. children acquire languages, adults learn them
In line with Krashen's ideas it could be argued, however that a child's ability to acquire language so quickly may not be down to age as a biological factor, so to speak, but instead to the opportunities that children of a young age are gifted with to learn language without pressure. Consider, for instance, the acquisition of the first language; the child is 'immersed' by its target language, is not pressurized to speak and is provided with plenty of natural material and the need and opportunity for meaningful interaction. Adults seeking to acquire an L2 often lack this fruitful environment; they are not immersed in the language - with the possible exception of language lessons for a couple of hours a week - and have formulated structures of what to learn, little time or opportunity for meaningful communication and, often, do not have the need for the language to motivate them in the same way that children do. Furthermore, it could also be argued that during the early stages of acquisition, children have not yet cemented features of their L1 so heavily that mapping an L2 with entirely new features would present much of a challenge. In bilingual children, for example, we often see occurrences of code-switching, in which children communicate using elements of multiple languages in one conversation, sometimes exchanging lexis that they do not yet feel comfortable with in either the L1 or L2, revealing an understanding between the languages and a linguistic maturity, as well as highlighting the parallel development that is taking place at that age.
In sum, it could be argued that the factor of age may not be biological at all, but rather contextual. The ability for adults to acquire language might still be present, but the necessary environment is not. Additionally, Krashen hypothesises that learning rules prior to general target language (TL) exposure might in fact impede language acquisition in adults, as it places the focus on form, and not content. Although Krashen's ideas do not directly criticize the CPH, they do present an alternative outlook on the process that CPH aims to explain with little solid evidence.
Although the Critical Period Hypothesis is not without debate, the vast majority of research carried out to test the CPH has cited that the younger the learner at the time of learning, the more successful the acquisition. Omaya (1967) tested Italian immigrants in the United States by asking them to repeat a sentence muffled by noise and asking them to identify grammatically accurate and inaccurate sentences. The success rate was then correlated with the age of arrival. Omaya concluded that those who arrived at the youngest age did best, with the number of years spent in the US irrelevant, revealing a correlation between the age of arrival and syntactic proficiency, but none between syntactic proficiency and amount of exposure to English. It can therefore be argued that age is a factor in successful SLA, as despite having had a greater amount of exposure to the language, those who were older when they arrived still seemed to encounter greater difficulty.
Similarly, Johnson and Newport (1989) examined the affect of age on second language acquisition through the testing of grammatical rule knowledge in native Chinese and Korean individuals of varying ages, living in the United States and learning to speak English. They found that individuals that arrived before the age of seven scored very highly, performing to the same level as natives, while those who arrived later found the task more challenging. In 1991, Johnson and Newport conducted a further grammatical study in which those who arrived younger once again performed better, further cementing the idea that with maturation, ease of acquisition deteriorates.
Following the emerging pattern that the younger a learner begins to acquire a second language, the better, a study by Yamada et al (1980) tested 30 Japanese children on their ability to learn a selection of English words, also concluding that the mean score of the older participants was less than that of the younger. The evidence for the 'younger is better' theory is vast and cannot be ignored, though it should also be noted that such evidence does not suggest that we ever lose the ability to acquire altogether, but rather that the acquisition process becomes more challenging, and we are less likely to acquire to the same degree of proficiency.
Although there seems to be a positive correlation between age and L2 proficiency, there are numerous studies in which adults have performed better at linguistic tasks than children. For instance, Asher and Price (1967) studied 96 individuals divided between four different age groups (one group of undergraduate students, and three groups of children, ages 12-13, 9-10, and 6-7) by exposing the learners to simple Russian commands and testing them. On the contrary to the aforementioned studies, the researchers found that performance correlated with maturity, i.e. the older the participants, the better the result. Ekstrand (1978) also reached this conclusion in a study of 1000 Swedish pupils, aged 8-11, learning English.
This evidence therefore suggests that adults are capable of understanding more complex structures, and are more efficient at learning, however, over time as a child's linguistic knowledge expands, the ability to reach or surpass that same level will become apparent. Piaget's "formal operations" theory supports this in some ways, as it is suggested that adults as formal thinkers will find solutions in terms of rules, and do not need to work through concrete objects - thus learn more effectively than children, as cognitively mature. It is important to note still, however, that this theory refers to the idea of 'learning' rather than subconscious acquisition, but at a young age, children are still learning how to use what they have acquired efficiently.
In an additional study set to test the CPH, Dekeyser (2000) found that adult beginners deemed to be at the same stage of language learning as child beginners displayed higher levels of verbal analytical ability, which he argues plays no role in children's language acquisition. Dekeyser concluded that the age-constraints of acquisition as outlined by the CPH are a valid thing to consider, however, they apply only to implicit language learning mechanisms. In 2003, he summarized this as follows: "somewhere between early childhood and puberty, children gradually lose the ability to learn a language successfully through implicit mechanisms only" as his research found that a child's ability definitively seemed to wane as they grew older, due to cognitive maturation, but there is still a framework in place in the adult mind. Dekeyser therefore does not criticize the CPH, but calls for a reconceptualization of the theory, to consider that some acquisitional abilities remain after the CPH, within what is known as the sensitive period -- the period that precedes the optimal 'critical period' for language acquisition. Furthermore, the author argues that the need for language 'learning' at adult level of L2 acquisition should not be ignored, or considered detrimental; in fact, Dekeyser argues language learning to be necessary for adults, as only adults that achieve the aforementioned high levels of verbal analytical ability will reach near native speaker competency.
McLaughlin (1984) suggests that adults and children alike pass through the same developmental stages in SLA, regardless of age, with the only result of cognitive maturity being a larger vocabulary and more careful application of rules - both, he argues, are expected to make errors between the L1 and the L2 and over-generalize grammatical rules.
There appears to be no definitive answer on the exact affects of age on second language acquisition, but there is an expansive body of research that, regardless of arguments, seems to agree upon the fact that progressive age does have an influence, whether detrimental or otherwise, on second language acquisition processes. It is, however, just one of many factors impacting individuals seeking to acquire an L2, and as a result it is difficult to completely separate the factor of age from other influences, for instance a learner's individual differences, such as intelligence (there is a high correlation between intelligence and language analytical ability), aptitude (a cognitive variable, undoubtedly connected to intelligence and aptitude), and personality (whether the individual is an introvert or an extrovert, afraid or daring enough to take risks in learning an L2).
Additionally, the aforementioned impact of learning experience should not be ignored. Although some of the studies previously mentioned have argued that the length of time an individual is immersed in a linguistic environment does not directly impact the ability of an adult L2 learner in comparison with a child L2 learner, studies have shown that immersion still provides adult L2 learners with more exposure to the target language and more opportunity for practice, which cannot be detrimental but most likely aids learners to improve their language skills, albeit slower than the pace of younger learners. It should also be noted that the formal instruction of a classroom environment, in which adult L2 learners typically begin, the lack of opportunity for natural communication in the target language can also impede progress. Without exposure, without time and without frequent use, linguistic knowledge can diminish. Learners need opportunity for practice, of which children generally have plenty. Adults can often be dismissive of language tasks if they fear rejection or humiliation. Furthermore, if a child is learning an L2 at such an early stage, there is likely to be a purpose and a necessary goal they are intended to reach, such as communicating with a family member, or being raised to speak the L1 at home and the L2 in society/school, or vice versa. Adult learners are more likely to be at a stage in life where motivation is unlikely to amount to the same degree of need.