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2. Literature Review
In 1960s, Lenneberg propositioned the critical period hypothesis. He believes from two to adolescence (10 ~ L2), the human brain has plasticity, they learn language easily. Lenneberg attributed differences in learning between children and adults to the physiological phenomena. He thought that after 12 years old, children’s accent has been finalized and is difficult to change; they must spend more effort and may not learn as well as their mother languages. However, Krashen believes that older children acquire quicker than younger children in early period of learning vocabulary and syntax, when time and language contact are the same. He also pointed out that adults can learn foreign languages better, and if the time of learning and using foreign language is same, adults can make progress more than children and older children make more than younger children. Stern (1985) had made summary on age in foreign language learning long ago. He believed that the critical period did not exist, and learners of all ages have their own merits and drawbacks. Younger children can easily respond to language acquisition in communication context, while the older children and adults are more likely to use cognitive and class learning approaches. That is to say people of any age can learn a second language. Ellis (1985) believed that adults initially has advantages in speed of acquisition, especially in the light of syntax; in the formal language learning environment, only children can acquire native language; whether or not be able to reach proficiency of native language, children can reach higher level in pronunciation and ultimate level.源[自*751^`论\文"网·www.751com.cn/
2.1 The Concept of Critical Period Hypothesis
People noticed that childhood is the best age to learn the language long ago. American neurophysiologist Penfield proposed in 1959 that 4-8 years old is the best age for foreign language learning in the life of the people. By 1967 the psycholinguists Lenneberg developed the idea. He put forward the famous critical period hypothesis in language learning: natural language acquisition can only occur in the critical period, which is from two years old to the puberty. The critical period hypothesis led to various research and debate in the field of second language acquisition.