Bates and MacWhinney (1981, as cited in Saville-Troike, 2006) provide an alternative explanation to the critical period theory for how languages are learnt, called the competition model. This approach suggests that all linguistic performance involves mapping, and competition tends to occur between these mappings. This causes interference between an old language and a new language, making it extremely difficult to learn more than one language.
Another explanation comes from Newport (1990, as cited in Leonard, 2000) who proposed a “less is more” hypothesis which suggests that children have the ability to learn and understand grammatical rules of language in the early stages of language acquisition due to a limited capacity of processing. Therefore the hypothesis would assume that it is more beneficial to be introduced to a new language at a young age, before the critical period ends.
Whether there is a critical period in language acquisition has also been investigated through studies of the development of sign language in deaf individuals. Newport and Meir (1985, as cited in Whitney, 1998) found that when a deaf child is born to deaf parents who use sign language fluently as a means of communication, it is fairly easy for the child to learn sign language. However, more than 90% of deaf children are born to hearing parents who have no knowledge of sign language (Singleton and Ryan, 2004). According to Whitney (1998), many deaf children do not experience any sign language until after the age of four years old, therefore this could make it extremely hard to acquire sign language as the critical period for language acquisition may have been passed.
It has been found that deaf children who are not exposed to sign language develop their own signs and gestures to communicate, which are known as “home-signs” (Singleton and Ryan, 2004). Whitney (1998) says an overlap is caused by the home-sign gestures used by deaf children, and the various signals their parents use to try and communicate back. However, children have a greater ability than their parents to turn simple gestures into meaningful a linguistic system. Whitney (1998) concluded that this evidence provided by deaf children suggest that there is a biological predisposition responsible for language, and not a critical period.
From looking at past literature, Emmorey (2002, as cited in Singleton & Ryan, 2004) has concluded that those who learn sign language in childhood have a significant advantage over those who try to acquire sign language later in life. According to Newport (1984, as cited in Singleton & Ryan, 2004), the critical period for acquisition of sign language appears to be before the age of twelve, as individuals who were exposed to sign language after this age found it very difficult to learn.
Studies on deaf individuals who acquire sign language late in life from hearing parents provide empirical evidence for the existence of a critical period because these individuals were not exposed to sign language early in childhood and find it difficult to learn at an older age. However, these children deprived of sign language are able to develop “home-sign” gestures showing that they do have access to a communication system from a young age (Singleton & Ryan, 2004). There appears to be a great deal of evidence suggesting early learners of sign language have advantages over those who learn it at a later age, but late learners are still able to demonstrate considerable competence over sign language, and it could be that their slow development of language may be due to cognitive problems. Therefore, some evidence on the acquisition of sign language questions the existence of a critical period in language acquisition.
Strong support for a critical period in language acquisition comes from the case studies of isolated children who are deprived of language and social activity. Isabelle was a deaf mute child who had no speech at the age of six and a half, and had spent most of her childhood alone in a small, dark room (Brown, 1958; as cited in Aitchison, 1989). After being found, Isabelle underwent two years of learning that would normally take six years to go through, and by the age of eight, she was not easily distinguishable from ordinary children her age. Isabelle was able to succeed in her language learning because she began acquiring it at age six and a half and had not passed the critical period of language yet (Aitchison, 1989).
Curtiss (1989) reported the case study of Genie, a girl who spent most of her childhood locked in a room, isolated from contact with others and language. She was found the age of thirteen and a half, and it was discovered that she could not stand properly, had not been fed well, and had no social skills. After she was found Genie was given education and assistance and she was able to improve greatly in tasks which did not depends on language, and she reached normal levels on some perceptual tasks (Curtiss, 1989). However, Genie’s language skills failed to improve and reach a normal level. She was only really able to speak in short, ungrammatical sentences and her social skills remained poor. This shows that because Genie started to try and learn a language after the critical period, her progress was slow and difficult causing her to never really acquire a language properly. Genie’s abnormal speech indicates that language acquisition outside of the critical period cause serious constraints and limitation on the development on language (Curtiss, 1977; as cited in Singleton and Ryan, 2004).
To conclude, there is a vast amount of evidence which supports the existence of a critical period in language acquisition. Studies on the acquisition of a second language, sign language, and case studies of children isolated from language suggest that a new language can be highly mastered if it is introduced in childhood, and trying to learn a language after the critical period has proved to be a difficult and slow process. However, some studies on the acquisition of sign language have suggested that difficulties in acquiring a language after early childhood could be due to the specific language experiences individuals have had or the cognitive strategies they have learnt. Therefore, from the evidence presented, it can be assumed that a critical period of language acquisition does exist, but there may be other psychological factors that contribute to the difficulty of learning a language after a certain age.
References
Aitchison, J. (1989). The articulate mammal. New York: Routledge.
Curtiss, S. (1989). The independence and task-specificity of language. In M.H. Bornstein & J.S. Burner (Eds.), Interaction in human development. Hillsdale, NJ: Lawrence Erlbaum Association Inc.
Leonard, L.B. (2000). Children with specific language impairment. Cambridge: MIT Press.
Saville-Troike, M. (2006). Introducing second language acquisition. New York: Cambridge University Press.
Singleton, D.M., Ryan, L. (2004). Language acquisition. Clevedon: Multilingual Matters.
Whitney, P. (1998). The psychology of language. Boston: Houghton Mifflin.