Lenneberg hypothesized that cerebral dominance for language did not characterise the initial state of children, but developed gradually from about age two, complete only at puberty. This hypothesis accorded with suggestions that there was a ‘critical period’ for language acquisition, which terminated about the time of puberty.
Critical Period Hypothesis
The original formulation of the CPH came from Lenneberg (1967) who noted that “automatic acquisition from mere exposure to a given language seems to disappear after puberty, and foreign languages have to be taught and learned through a conscious and labored effort. Foreign accents cannot be overcome easily after puberty” (p. 176).
Lenneberg (1967) saw a correlation between the process of lateralization, which is not completed until puberty, and a critical period for language learning. MacWhinney (2006) reviewed the evidence for lateralization as an explanation for the critical period and concluded that there was little to show that lateralization can account for age-related effects in language learning after the first three years of life.
It is supposed that after a given age, it becomes impossible to acquire full native-speaker competence in an L2. The review of the literature suggested no agreement on this issue.
While there is substantial evidence to show that many L2 learners who begin learning an L2 at puberty or beyond fail to acquire native-speaker competence, there is also evidence to show that at least some learners appear to be able to do so and also that the ability to acquire an L2 declines gradually with age rather than abruptly when a specific age is reached.
Birdsong (1999) admitted that after many years of supporting the Critical Period Hypothesis, he had become a doubter.
A somewhat different argument is advanced by Paradis (2004). He argued that the critical period needs to be understood as relating only to procedural memory for language. He proposed that the acquisition of implicit competence declines with the gradual loss of plasticity that begins at 5 years of age and with the increased reliance on declarative memory which begins at 7 years. Thus, the critical period applied only to those aspects of language that depends on procedural memory (i.e. prosody, phonology, morphology, and syntax). As there is no maturational effect evident in declarative memory, there is no critical period for vocabulary. According to this view, then, learners switch from a reliance on the neural mechanisms that feed incidental, implicit learning to those that govern intentional, explicit learning once the critical period is ended. What is clear is that there is a general decline in the neural capacity for language learning with age (Ellis, 2008).
Bialystok (1997) argued against maturational factors as a determining factor in the success or nonsuccess of second language learning. She found that age of onset of learning does not have significant effects and that there is some support for the importance of length of study (or length of stay in the target culture). Patkowski (1980, p. 462) discusses the ‘Conrad phenomenon’, named after Joseph Conrad, the native Pole who learnt English at the age of 18 and became one of the greatest English novelists.
Bilingualism and Critical Period Hypothesis
Bilinguals, like monolinguals, demonstrate left hemisphere dominance for language knowledge.
The early work on hemispheric differentiation in bilinguals (Albert & Obler, 1978) suggested that L1 and L2 learning drew on different mechanisms. Studies of aphasia indicated that bilinguals were less hemisphere dominant than monolinguals. Albert and Obler claimed that language is organized in the brain of the bilingual in a manner different from that which might have been predicted by studies of cerebral organisation for language monolinguals.
Studies of monolinguals have indicated that the left hemisphere is dominant for language in most individuals. Studies on bilinguals demonstrate not only the left hemispheric role in language but also a major right hemispheric contribution. Galloway (1980) also noted differences in the aphasias of monolinguals and bilinguals. She found that 98 percent of aphasias in monolinguals were due to left-hemisphere lesions and only 2 percent to right-hemisphere lesions, while for bilinguals the ratio was 85 percent and 15 percent.