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【Abstract】： This article is to introduce the Critical Period Hypothesis and the related studies， the analysis of the theory and the opinions from many scholars.
【Key words】： second language acquisition age factor Critical Period Hypothesis
1 The Critical Period Hypothesis
The critical period hypothesis was first introduced by Penfield and Roberts（1959） who argued that language acquisition was most efficient before age nine，when “the human brain becomes stiff and rigid”， and it was later developed by Lenneberg（1967）. He suggested that there was a period when language acquisition took place naturally and effortlessly.
Lenneberg thought that language was the product of the brain. Before puberty， when both hemispheres of the brain participate in language learning， language can be acquired naturally and effortlessly because of the brain plasticity. After puberty， most people’s brain is lateralized and their neurological system loses such plasticity， which makes language acquisition harder and harder.
2 Two Versions of CPH
There are two versions of the CPH. The strong version is that if a language is not learned by puberty the biological endowment which permits successful language acquisition will not be available. Thus the learner will have to use general learning mechanisms which are not designed for language acquisition and thus not as successful. The weak version is that， even though the same learning mechanisms are involved， second language learning will be more difficult and incomplete after puberty because most learners have neither the time nor the motivation to reach the high level of mastery which a child reaches.
3.Studies related to CPH
C. Snow and M. Hoefnagel did profound research about the Critical Period Hypothesis in 1978. According to this hypothesis， first language acquisition must occur before lateralization， and second language acquisition will be relatively fast and successful before puberty. They took a study of longitudinal and naturalistic acquisition of Dutch by three groups of English speakers at different ages--children， adolescents and adults. They found that adults and adolescents outperformed children on tests after three months of residence in the Netherlands， but after ten months， the children had caught up on most measures. These data do not give support to the CPH and leave many unanswered questions.
In general， these results indicate that adults on most tests are able to achieve criterion scores more rapidly than children at the early stages.
Bibliography
[1]Larsen-Freeman， Diane & Michael H. Long. An Introduction to Second Language Acquisition Research. Beijing： Foreign Language Teaching and Research Press， 2000.
[2]Ellis， Rod. Second Language Acquisition. Oxford：Oxford University Press，1997.
[3]Selinker， Larry. Second Language Acquisition—An Introductory Course. London：Lawernce Erlbum Associations， 2001.
【Key words】： second language acquisition age factor Critical Period Hypothesis
1 The Critical Period Hypothesis
The critical period hypothesis was first introduced by Penfield and Roberts（1959） who argued that language acquisition was most efficient before age nine，when “the human brain becomes stiff and rigid”， and it was later developed by Lenneberg（1967）. He suggested that there was a period when language acquisition took place naturally and effortlessly.
Lenneberg thought that language was the product of the brain. Before puberty， when both hemispheres of the brain participate in language learning， language can be acquired naturally and effortlessly because of the brain plasticity. After puberty， most people’s brain is lateralized and their neurological system loses such plasticity， which makes language acquisition harder and harder.
2 Two Versions of CPH
There are two versions of the CPH. The strong version is that if a language is not learned by puberty the biological endowment which permits successful language acquisition will not be available. Thus the learner will have to use general learning mechanisms which are not designed for language acquisition and thus not as successful. The weak version is that， even though the same learning mechanisms are involved， second language learning will be more difficult and incomplete after puberty because most learners have neither the time nor the motivation to reach the high level of mastery which a child reaches.
3.Studies related to CPH
C. Snow and M. Hoefnagel did profound research about the Critical Period Hypothesis in 1978. According to this hypothesis， first language acquisition must occur before lateralization， and second language acquisition will be relatively fast and successful before puberty. They took a study of longitudinal and naturalistic acquisition of Dutch by three groups of English speakers at different ages--children， adolescents and adults. They found that adults and adolescents outperformed children on tests after three months of residence in the Netherlands， but after ten months， the children had caught up on most measures. These data do not give support to the CPH and leave many unanswered questions.
In general， these results indicate that adults on most tests are able to achieve criterion scores more rapidly than children at the early stages.
Bibliography
[1]Larsen-Freeman， Diane & Michael H. Long. An Introduction to Second Language Acquisition Research. Beijing： Foreign Language Teaching and Research Press， 2000.
[2]Ellis， Rod. Second Language Acquisition. Oxford：Oxford University Press，1997.
[3]Selinker， Larry. Second Language Acquisition—An Introductory Course. London：Lawernce Erlbum Associations， 2001.