In the most general sense, the notion of maturational constraints in language acquisition suggests that there is a causal relationship between biologically scheduled changes in the developing brain and language acquisition ability. The concept of a critical period in language acquisition is typically associated with behavioural developments that have sudden onsets and offsets, lead to all-or-nothing results, depend on instinct, are unlearned and irreversible, and for which environmental influences such as motivation do not play any roles (Harley and Wong, 1997; Long, 1990).
A question that has to be addressed how valid such a view is. In this paper, I will attempt to show that while environmental influences may not account for age-related differences in achievements in L2 acquisition, they still play a role in language learning, and can account for the huge variability in achievements by L2 learners after maturation. The declining ability with age to acquire language is attributed to several biological causes.
One such cause is the steady loss of cerebral plasticity, “the ability of neurons to make new connections, and varied connections depending on the stimulus” (Eubank and Gregg, 1999, p. 69). It is postulated that myelination, which involves the wrapping of axons of neuron cells with an insulating nutrient layer (myelin), causes this loss of cerebral plasticity. Myelination allows neutrons to conduct signals more rapidly, but makes connections between neighbouring neurons more difficult.
Since the “trengthening of connections between neurons probably represents the neurobiological basis for learning”, including language acquisition (Pulvermuller and Schumann, 1994, p. 691), and most areas of the brain reach full maturation around puberty (p. 713), a likely result is that the ability to acquire language will decrease as the brain matures, and the cut-off for the critical period will be around puberty.
Pulvermuller and Schumann also argue that maturation (i. e. yelination) has less of an effect on semantics, pragmatics, and vocabulary than grammatical and syntactic aspects, due to the different regions responsible for handling these different aspects of language (p. 713). Increasing lateralisation of language functions to the left hemisphere (as in Long, 1990, p. 278), metabolic differences in pre-puberty and post-puberty brains (references in Pulvermuller and Schumann) and the thickening of the corpus callosum have also been proposed as possible causes for a declining ability to acquire language with maturation.
However, the focus of this paper is not to explore the full biological causes of the critical period hypothesis (if it ever could be done), but to discuss if the notion of a cut-off period for native-like attainment of a language implies that only biological factors are at play. It seems counter-intuitive to some extent that social or psychological factors do not play a role in second language acquisition. For example, an immigrant would be highly motivated to master the language of the host country and blend into the culture of the host country.
Such a driving force, if coupled with advantageous learning environment, should allow the immigrant to pick up the host language. It has also been postulated that children are inherently more motivated than adults to acquire native-like levels of the L2, and that younger learners develop positive attitudes towards the L2, its culture and its speakers more easily than older learners do (e. g. Bialystok and Hakuta, 1999). Besides, there exist adult learners of second language who attain native, or near-native levels of proficiency in the language.
Such examples include Julie, a talented L2 speaker of Egyptian Arabic who had immigrated to Cairo from Britain at age 21, with no formal L2 instruction and Laura, an exceptionally talented speaker of several varieties of Arabic (Ioup et al. , 1994). There are also cases of exceptional adult learners who have either a different psychological set-up in terms of verbal memory and ability to focus on form (Novoa, Fein and Obler, 1988), or a willingness to adopt a new cultural identity (Schneiderman and Desmarais, 1988).
Such cases show that the biologically imposed barrier after the critical period can be overcome with social factors such as a good learning environment (in Laura’s case). The problem with using social or psychological reasons such as those outlined above to account for the differences between children and adults in language acquisition is that there is no evidence to show that these factors are in the first place true. For example, there is no direct evidence that children are inherently more motivated to learn the L2.
Moreover, there are studies to counter the argument that social and psychological factors play a role in accounting for the differences between children and adults (and therefore resulting in differences in levels of proficiency in L2). Several accounts (Johnson and Newport, 1989; Oyama, 1978) in fact show that motivational factors cannot account for the decrease in ultimate attainment with increasing ages of onset (AOs).
As for the cases of competent adult learners of L2, in a study by DeKeyser in 2000, all but one of the late L2 starters who had achieved scores within the range of child starters on a grammaticality judgement test also scored high on a test of verbal analytical ability. This result led DeKeyser to conclude that only adults with such special abilities can reach near-native L2 competence. Ioup et al. also hypothesise that talent for learning languages, such as in Julie’s case, originates from “unusual brain organisation where a greater proportion of the cortex is devoted to language” (Ioup, et al. , 1994, p. 2).
Therefore, it appears that social and psychological factors fail as an alternative to account for a critical period in language acquisition. However, there is no reason to conclude that these factors do not play a role in language acquisition. While maturation would seem to be responsible for age-related decline in learning potential, non-maturation factors can account for why some adult learners of L2 perform better than others. Studies by Moyer (1999) and Johnson and Newport (1989) show that there is huge variation in the level of L2 achievement for learners with AOs after the completion of maturation.
Holding AO constant, factors such as degree of motivation, the amount and type of instruction that post-puberty L2 learners contribute to greater success in L2 acquisition (Moyer, 1999). Similarly, Bongaerts and his colleagues have suggested that intensive training in the perception and production of L2 sounds, high motivation and continued access to ample L2 input may account for the near-native pronunciation of their foreign studies students (Bongaerts, 1999, pp. 154-5). It seems that the available studies still point to biological factors as being solely responsible for the differences between child and adult L2 learners.
For age of onset beyond maturation, native-like proficiency in a foreign language becomes much more difficult. However, innate capability for language learning, high motivation and intense training may still help in overcoming the biological barrier to mastering the L2. Even though the implication that biological scheduling places native-like attainment out of reach sounds pessimistic, the fact that to some extent, social and psychological factors may compensate for this should be encouraging to L2 learners.