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In general, a critical period is a limited time in which an event can occur, usually to result in some kind of transformation. A "critical period" in developmental psychology and developmental biology is a time in the early stages of an organism's life during which it displays a heightened sensitivity to certain environmental stimuli, and develops in particular ways due to experiences at this time. If the organism does not receive the appropriate stimulus during this "critical period", it may be difficult, or even impossible, to develop some functions later in life.

For example, the critical period for the development of a human child's binocular vision is thought to be between one and three years,^[1] and further critical periods have been identified for the development of hearing and the vestibular system.^[2] There are critical periods in childhood in which imprinting can occur, such as when a greylag goose becomes attached to a parent figure within the first 36 hours after hatching. A young chaffinch must hear an adult singing before it sexually matures, or it will never properly learn the highly intricate song.^[3] These observations have led some to hypothesise a critical period for certain areas of human learning, particularly language acquisition.

Experimental research into critical periods has involved depriving animals of stimuli at different stages of development. Other studies have looked at children deprived of certain experiences due to illness (such as temporary blindness), or social isolation (such as feral children). Many of the studies investigating a critical period for language acquisition have focussed on deaf children of hearing parents.

Contents 1 Linguistics 1.1 First language acquisition 1.2 Second language acquisition 2 See also 3 External links 4 References

Main article: Critical Period Hypothesis

See also: Language acquisition#Critical Period hypothesis

The Critical Period Hypothesis states that the first few years of life is the crucial time in which an individual can acquire a first language if presented with adequate stimuli. If language input doesn't occur until after this time, the individual will never achieve a full command of language — especially grammatical systems.

The evidence for such a period is limited, and support stems largely from theoretical arguments and analogies to other critical periods in biology such as visual development, but nonetheless is widely accepted. One famous case study to support the hypothesis is the case of Genie (feral child).

The nature of this phenomenon, however, has been one of the most fiercely debated issues in psycholinguistics and cognitive science in general for decades.

See also: Second language acquisition#Summary of Critical Period Research to date

The theory has often been extended to a critical period for second language acquisition, although this is much less widely accepted. Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages. David Singleton (1995) states that in learning a second language, "younger = better in the long run," but points out that there are many exceptions, noting that five percent of adult bilinguals master a second language even though they begin learning it when they are well into adulthood — long after any critical period has presumably come to a close.

• Child development
• Universal grammar

• Bibliography of papers on critical periods in second language acquisition

Biology

• ↑  Almli, C. Robert and Stanley Finger. (1987). Neural insult and critical period concepts. In 'Sensitive Periods in Development: Interdisciplinary Perspectives', Marc H. Bornstein (ed.), 123–143. Hillsdale, NJ: Lawrence Erlbaum.
• ↑  Thorpe, W. (1958). The learning of song patterns by birds, with special reference to the song of the Chaffinch, "Fringilla coelebs". Ibis 100:535-570.
• ↑  Moorman SJ; Cordova R; Davies SA. (2002). A critical period for functional vestibular development in zebrafish. Developmental Dynamics 223(2):285-91, Space Life Sciences Publications.

Linguistics

• Bialystok, E. K. Hakuta (1994). In Other Words. BasicBooks, a Division of HarperCollins Publishers, Inc.
• Bruer, John T. (1999). The Myth of the First Three Years. The Free Press, a Division of Simon and Schuster, Inc.
• Johnson, J.S., and E.L. Newport (1989). 'Critical period effects in second language learning: the influence of maturational state on the acquisition of English as a second language.' Cognitive Psychology 21: 60-99.
• Lamendella, J.T. (1977). 'General principles of Neurofunctional organization and their manifestation in primary and non-primary language acquisition.' Language Learning 27: 155-159. [introduces the phrase 'sensitive period'.]
• Lenneberg, E. H. (1967). Biological Foundations of Language. Wiley.
• Marshall, Brad. (2000). 'Is there a 'child advantage' in learning foreign languages?' Education Week 19(22): 39-41.
• Newport, E.L. (1990). 'Maturational constraints on language learning.' Cognitive Science 14: 11-28.
• Oyama, S. (1976). 'A sensitive period for the acquisition of a nonnative phonological system.' Journal of Psycholinguistic Research 5: 261-285.
• Penfield, W. and L. Roberts. (1959). Speech and Brain Mechanisms. Princeton: Princeton University Press.
• Pinker, S. (1994). The Language Instinct. New York: Morrow.
• Robertson, P. (2002). 'The Critical Age Hypothesis.' Asian EFL Journal: http://www.asian-efl-journal.com/marcharticles_pr.php
• Singleton, David, and Lengyel, Zsolt. (1995). The Age Factor in Second Language Acquisition. Clevedon: Multilingual Matters. See also http://www-rcf.usc.edu/~ionin/SLAgroup/Ling527papers/Singleton%20Critical%20Periods%20iral.2005.43.4.269.pdf
• Zhao, A.H. and C. Morgan (2005). "'onsideration of Age in L2 Attainment - Children, Adolescents and Adults.' Asian EFL Journal 6(4): http://www.asian-efl-journal.com/december_04_ahqz_cm.php