Language Acquisition Theories PDF

5 Theories of language acquisition Introduction...

5 Theories of language acquisition Introduction Environmental theories of language acquisition Nativist theories of language acquisition Social interactionist theories of language acquisition Summary Review exercise Introduction The previous chapter described some studies of language development. The striking feature of these studies is the speed that children acquire Copyright © 2003. Taylor & Francis Group. All rights reserved. both words and grammar. Children largely master the very complex skill of using language by the age of 5 or 6. In just a few years children learn an enormous number of words and the intricacies of how to put these words into meaningful sentences, and they can produce and understand novel sentences. However, as Messer (2000, p.138) points out, ‘there still remains controversy about how children are able to acquire language so quickly, given the immaturity of their other cognitive abilities and the lack of formal tuition’. The theories of how children acquire language fall into three main categories: 1. Environmental theories of language acquisition that stress the role of learning of both words and grammar. 51 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  LANGUAGE AND THOUGHT 2. Nativist theories of language acquisition which suggest that we have an innate ability to recognise and use grammar. 3. Social interactionist theories of language acquisition that stress the social nature of language and its importance in interaction with others. Environmental theories of language acquisition Learning theory One of the earliest psychological theories of how children acquire language was put forward by the behaviourist Skinner (1957). Skinner believed that all behaviour is learned by operant or classical conditioning, described in detail in another book in the series (The Determinants of Animal Behaviour, Cartwright, 2002). In his book, ‘Verbal Behavior’ Skinner argued that language is simply another form of behaviour that can be learned like any other behaviour. He believes the basis of all learning is reinforcement. A reinforcement is something that strengthens a behaviour and makes it more likely to occur again. For example, if a hungry rat gets a food pellet after pushing a lever it is more likely to push the lever again. The food pellet acts as a reinforcement. Many studies have shown that attention and praise are powerful reinforcements for infants and children. Skinner suggested that when infants initially produce babbling sounds they are reinforced by the attention of adults. This reinforcement makes it more likely that the infant repeats the sounds. Skinner also empha- Copyright © 2003. Taylor & Francis Group. All rights reserved. sised the role of selective reinforcement and behaviour-shaping in language learning. Behaviour-shaping is the gradual moulding of a simple behaviour into a complex behaviour. For example, Skinner taught pigeons to hold small bats in their beaks and play ping-pong by shaping a simple pecking behaviour. He suggested that parents use a similar process to gradually shape the sounds the infant makes into words. Thus the sound ‘da’ might be reinforced initially but later the child will only get attention from their father if they say ‘dada’. Skinner calls the verbal labels used to name objects or events tacts. The shaping process continues as the child gets reinforced for putting words together. For example, a young child may be given a drink after saying ‘juice’ but later is more likely to get the desired response by saying ‘more juice’. Later still the child is required to 52 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  THEORIES OF LANGUAGE ACQUISITION produce sentences that become closer approximations of the adult version. Skinner calls verbal behaviours that result in reinforcement in this way mands and they tend to be in the form of requests or commands. Skinner also discusses the role of echoic responses (or imitation) in language learning. Children may learn to name things by imitating adults if the echoic response is reinforced. Thus if an adult points to a duck and says ‘duck’ and the child imitates this they are likely to be reinforced with a positive comment. However, if the child then points to a pigeon and says ‘duck’ the parental response will tend to be negative (‘no, that’s not a duck’). Skinner suggests that echoic responses can also be used in learning how to combine words. For example, a child may imitate the sentence ‘Do you want teddy?’ by saying ‘Want teddy’. If the child is then given the toy it reinforces the sequence and the child is likely to repeat it when requesting the toy in the future. Evaluation of Skinner’s theory Although Skinner’s theory seemed to be a plausible and logical account of language acquisition it soon became apparent that there are alot of problems with it. Firstly, there is little evidence that parents systematically reinforce language (or verbal behaviour) in children (Chomsky, 1959). Chomsky points out that learning language through reinforcement would be a slow process that would require careful shaping. However, children learn language quickly without parents teaching words and sentence structure carefully. Most Copyright © 2003. Taylor & Francis Group. All rights reserved. evidence suggests that parents ignore grammatical errors in young children and respond to the meaning of what they say (e.g. Brown and Hanlon, 1970). Thus if a child had seen some sheep but reports ‘I saw some cows’ they would usually be corrected. However, if they report ‘I saw some sheeps’ they would not be corrected despite the grammatical error. Even if speech is corrected it tends to have little effect. If parents try to correct pronunciation or grammar children often still produce their version (Harley, 2001). This suggests that learning theory cannot explain how children acquire grammar. Chomsky also questioned whether the learning theory could explain the universality of language development. If learning were respon- sible for language acquisition there would be more variation in language development between and within different cultures. The 53 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  LANGUAGE AND THOUGHT learning theory also appears to have difficulty in accounting for the creativity of language. Children quickly learn to understand and produce sentences they have never heard before. The role of imitation in language learning has also been questioned for a number of reasons (Owens, 2001). Children acquire correct grammar despite being exposed to lots of incomplete sentences and ungrammatical speech. Much of the speech between adults is in the form of fragments of sentences, poor grammar and repetitions yet children learn to use the correct form. The use of overregularisation (see p.44) suggests that children acquire language by applying rules rather than by imitation. If a child uses the word ‘goed’ as the past tense of ‘go’ this indicates that they are applying a rule (‘ed’ indicates past tense) rather than using imitation (adults do not say ‘goed’). Finally, when children do imitate adult speech they tend to convert it to a form similar to their own spontaneous speech. Thus if a young child imitated the adult sentence ‘I am playing with the toys’, s/he would change it to the telegraphic version ‘play toy’. Despite the problems in explaining the development of language Skinner’s theory does seem to be relevant in explaining how children learn the meaning of some individual words. Owens (2001) points out that the processes that Skinner identified (reinforcement and shaping of words) are used in speech therapy programmes for children with language difficulties and are effective in promoting the use of language. It is therefore possible that this process also occurs in normal language learning. Copyright © 2003. Taylor & Francis Group. All rights reserved. Child-directed speech (‘motherese’) Later environmental accounts of language acquisition have suggested that there is a special form of language that parents use to communicate with children and that this facilitates learning. This has been called motherese, but since fathers and other adults also use it the term child-directed speech is often used (Snow, 1994). Child-directed speech is a simplified form of speech in which adults alter both the presentation and content to make the speech accessible to the child. The speech is presented slower and in a higher pitch than adult speech and the phrases are segmented to make them clearer. Sentences are short, with simple forms of grammar and simple, restricted vocabulary. In addition there tends to be a lot of repetition in the speech (e.g. ‘Where 54 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  THEORIES OF LANGUAGE ACQUISITION is teddy? Here is teddy’). The type of child-directed speech changes as the child gets older and both the presentation and content become more complex to reflect the linguistic abilities of the child. Child-directed speech appears in nearly all cultures and it seems that children are drawn to it. For example Werker et al. (1994) found that English and Chinese infants preferred to listen to child-directed speech regardless of whether it was spoken in English or Cantonese. This suggests it has some important function and leads to questions about its role in language acquisition (e.g. Bee, 2000). It is possible that drawing attention to this simple form of language allows the child to learn basic language structure. Once they have mastered the basics the child-directed speech becomes more complex and the child can then learn new, more elaborate structures. Messer (2000) claims there is evidence that it is a particular part of child-directed speech, called adult expansions, which helps in the development of grammar. Adult expansions are used to give feedback about correct grammar when a child uses incorrect grammar. Saxton (1997) has proposed the contrast theory of negative input which suggests that children learn from the juxtaposition of their incorrect form and an adult’s correct form because of the contrast. He uses several examples to illustrate the idea (p.145): a) Child: He SHOOTED the fish. Adult: He SHOT the fish! b) Child: Do you know how Big Foot was BORNED? Adult: No, how was he BORN? Copyright © 2003. Taylor & Francis Group. All rights reserved. Thus there is an immediate contrast between the incorrect and correct version and Saxton believes it is this that helps the child learn. Consequently, each time adults use expansions they are helping to teach grammar to the child. (See Key Research Summaries, Article 2, p.112 for a full discussion of Saxton’s theory). Evaluation of the role of child-directed speech There have been some studies that have found a correlation between children’s language development and the degree of simplification of mother’s speech, but other studies have failed to replicate this (Messer, 1999). Furthermore, even if such a correlation were to exist it does 55 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  LANGUAGE AND THOUGHT not show that child-directed speech caused better development. There are problems with the idea that child-directed speech allows children to acquire language. One is that, although child-directed speech appears in most cultures, it is not universal. For example, Pye (1986) could not detect the use of any child-directed speech in the Mayan culture. There is also evidence that the style of child-directed speech varies greatly across cultures (Lieven, 1994). Since all children acquire language in a similar way regardless of culture this suggests that child-directed speech is not responsible for acquisition. Many researches have concluded that child-directed speech may be useful in developing language but that it is not necessary for language to develop (e.g. Bee, 2000). However, Snow (1995) argues that although some features of child-directed speech are not found in all cultures it has yet to be established whether any culture fails to modify adult speech at all to communicate to children. The idea that children learn rules of language because of immediate feedback is supported by research into language using connectionist networks. Connectionist networks (or neural networks) are computer simulations that are designed to mimic the way information is processed in the brain. These programs respond to feedback and in effect learn from the feedback. Although they are very simple it has been shown that connectionist networks are capable of learning grammatical rules such as past tenses of verbs (Shanks, 1993). The networks not only learned the past tenses of regular verbs (e.g. add ‘ed’ to the verb) but irregular forms as well (e.g. ‘went’ not ‘goed’). It has been suggested that the network learned irregular forms quickly because they tend to Copyright © 2003. Taylor & Francis Group. All rights reserved. be verbs that are often used (Messer, 2000). Since these relatively simple networks are capable of learning grammatical rules through feedback it is possible that children (who are much more complex than networks) can also learn language from feedback. 56 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  THEORIES OF LANGUAGE ACQUISITION Progress exercise Try to arrange a time with family or friends when you can unobtrusively observe parent/s talking to a young child. Note whether there is any evidence of: 1. Systematic reinforcement of the child’s language. 2. The adult/s using child-directed speech. 3. The adult/s providing feedback using adult expansions. Do your observations support either of the environmental theories? Nativist theories of language acquisition Nativist theories argue that the ability to acquire language is innate and that children are programmed to learn language. The most prominent supporter of this view is Chomsky (1957, 1959, 1965), who argues that only some form of pre-programming can explain the speed with which children learn the complex skill of using language and the similarity of language acquisition across cultures (as described in the previous chapter). Chomsky noted that all human languages share certain features (i.e. they all have nouns and verbs), which are called ‘linguistic universals’. He suggested we have an innate mechanism called the language acquisition device (LAD) that allows children to identify the grammar of the language to which they are exposed by picking out the linguistic universals. He believed that Copyright © 2003. Taylor & Francis Group. All rights reserved. only something like the LAD could explain how children are able to learn correct grammar from the fragments of sentences and incorrect grammar they hear. Chomsky believes that the LAD is unique to humans and that no other animal uses language or can be trained to use language. Chomsky also proposed that there are two levels to every sen- tence. One level is simply the words we speak or hear that make the sentence: the surface structure. The other level is the meaning of the sentence: the deep structure. When we speak we have to transform an idea or thought into a sentence (e.g. from deep to surface), but when we listen we have to transform the sentence into a meaningful idea (e.g. from surface to deep). Sentences are changed from one structure to another by using transformational grammar. Sentences can have 57 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  LANGUAGE AND THOUGHT very different surface structures yet the same deep structure. For example, The dog chased the man The man was chased by the dog are different in surface structure but conjure up the same image or deep structure. On the other hand, one surface structure can have a variety of deep structures (e.g. hearing someone say ‘I can see a man eating fish’ has very different connotations, depending on whether you are walking past a restaurant or swimming in the ocean!). Chomsky believes that ability to transform sentences from deep to surface or vice versa (transformational grammar) is a major function of the LAD and is innate. Over the past twenty years or so Chomsky has expanded on his original ideas to produce a theory that recognises the complexity of language (e.g. 1986, 1995). In the government and binding theory he suggests that, although there are linguistic universals, languages vary in a number of parameters (hence the alternative name ‘principles and parameter theory’, or PPT). Language acquisition consists of learning the correct version of any parameter from hearing the adult speech. Messer (2000) illustrates this by comparing word order in English and Japanese. Sentences in English are usually in subject–verb–object order but in Japanese they are subject–object–verb order. Thus any child exposed to English speech would have the word-order parameter set for subject–verb–object but a child exposed to Japanese would have the same parameter set for subject–object–verb. Copyright © 2003. Taylor & Francis Group. All rights reserved. Evaluation of Chomsky’s theory Chomsky’s theory has been the most influential theory of language acquisition and has dominated psycholinguistics for the past forty years. There is evidence from a variety of sources that supports the theory. For example, the process of language acquisition discussed in Chapter 4 is very similar in all cultures. When a process is similar in every culture regardless of environment this suggests it is maturational (i.e. biologically based). Furthermore, all languages share features (the linguistic universals) and this suggests that there may be a common underlying feature such as the LAD. Lenneberg (1967) is a strong proponent of the idea that language 58 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  THEORIES OF LANGUAGE ACQUISITION has some form of biological basis. He has suggested the critical period hypothesis, which consists of two strands. One idea is that certain events must happen within a critical time period for normal development. Specifically children must be exposed to a language before puberty (10–12) or they will not be able to learn to use language. The other is that certain features of language (and other aspects of development) can only happen when the critical period is reached. There is evidence for both these ideas. The first idea is difficult to test since nearly every child is exposed to language well before puberty. However, there are a few cases of children who, because of severe deprivation, have not been exposed to much language until they were older. Perhaps the most famous case is that of Genie who from the age of 20 months until she was nearly 14 heard very little language (Curtiss, 1977). Genie’s father disliked noise and there was very little speech in the house and no radio or TV. Genie was kept isolated in a small room and virtually her only human contact was when her mother fed her each day. Unsurprisingly, when she was taken into care she had very few linguistic skills although she could understand a few words. Despite the intensive training she has been given Genie never learned to speak fluently and could not master many of the grammatical skills that most children pick up so easily. She did learn to say words and could produce sentences but it was as if she was speaking a second language poorly. However, the fact that she learned to use language at all suggests that there is not a fixed critical period for language learning; rather there is a sensitive period (Harley, 2001). Lenneberg’s second idea was that language development is a Copyright © 2003. Taylor & Francis Group. All rights reserved. maturational process akin to motor development and that certain behaviours can only be shown when critical stages have been reached. He observed that certain motor developments seem to correspond to language developments (e.g. crawling–babbling, standing–first words, etc.). In children where development was delayed the delay affected both motor and language skills. This again suggests a maturational not environmental process. This is further supported by studies of language acquisition in deaf children. Deaf children seem to learn sign language in similar stages as children learning spoken language. Goldin-Meadow and Feldman (1977) studied four deaf children who had not been taught sign language but who nevertheless communicated initially by using one sign then a combination of two signs etc. This suggests some need or drive to use language. 59 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  LANGUAGE AND THOUGHT It is also argued that only something like the LAD can explain how children can learn a language from the poverty of what they hear. Despite hearing fragments of speech and ungrammatical speech children still manage to pick out the correct use of grammar. Chomsky argues that this is because of the LAD. Finally, phenomena such as overregularisation suggests that rules are at the heart of language acquisition, and it is these rules which are the centre of Chomsky’s theory. However, the emphasis on the rules of grammar (or syntax) can be regarded as one of the problems of Chomsky’s theory since it ignores the use of language to communicate (or pragmatics). As we will see in the next section some researchers feel that it is the need to communicate that is the driving force in language acquisition. There is evidence that children do not learn language merely by exposure but by interacting with people using language (see social interactionist theories p.61). Owens (2001) points out that sentences can be syntactically correct but meaningless or nonsensical. However, children do not use grammatically correct meaningless sentences but sentences that communicate meaning. The nativist account of language acquisition also largely ignores the importance of envi- ronment. However, the environment clearly is important in the learning of word meaning, accent and the grammar of the language. There is growing evidence that children do not learn ‘rules’ of grammar but, as environmental theories suggest, gradually learn to use different verbs, adjectives, etc. (e.g. Tomasello, 1992; Pine and Lieven, 1997). Tomasello (1992) has suggested that children learn ‘verb islands’ which Copyright © 2003. Taylor & Francis Group. All rights reserved. represent the most commonly used verbs but that the rules of these verbs are not applied to others. One of the problems of the government and binding theory is explaining bilingualism or second-language learning. If language acquisition consists of setting the parameters of language how do children manage to learn two languages with different parameters? The studies of language learning using connectionist networks (see p.56) has also challenged Chomsky’s claim that language learning is so complex that there must be a unique innate mechanism to enable children to do so. Connectionist networks are designed to mimic the way humans learn and are able to learn seemingly complex ‘rules’ of grammar. The networks are trained by giving feedback about whether its responses are correct or incorrect. Messer (2000) 60 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  THEORIES OF LANGUAGE ACQUISITION suggests the connectionist network research into language challenges Chomsky’s theory in three ways. Firstly, the way the networks learn is not specialised for language but can be applied to non-linguistic learning. Secondly, the networks do not require any specific ‘innate’ knowledge to learn rules of grammar. Finally, although the network behaves as if it is using rules no linguistic rules were pre-programmed. Chomsky’s claim that language is unique to humans because only humans inherit a LAD is subject to intense debate that centres on definitions of ‘language’ (for a discussion of this debate see the chapter on animal language in another book in this series: Animal Cognition, Lund, 2002). However, a number of studies suggest that some animals such as chimpanzees and bonobos are capable of learning some aspects of language (e.g. Savage-Rumbaugh and Brakke, 1996). Social interactionist theories of language acquisition Language is not an abstract skill but, during childhood, develops into our primary means of communication and social interaction. The social interactionist theories of language believe that it is the need to communicate and interact with others that is the driving force behind language acquisition. In contrast to the nativists the social interaction theorists do not believe that mere exposure to language is sufficient to acquire it but that it is a skill learned by interacting with others. They concentrate on the pragmatics of language not the syntax. Bruner (1983) describes a language acquisition socialization system (LASS) which, in contrast to the LAD, focuses on the social nature of Copyright © 2003. Taylor & Francis Group. All rights reserved. language. There is evidence that the skills of interacting with others precede both the comprehension and production of words or language. For example, one vital aspect of using language to communicate is turn- taking. We cannot hold a conversation if everyone talks at the same time. A lot of research suggests that infants learn to use ‘turn-taking’ in their interactions with adults long before they start using speech (Harley, 2001). Evaluation of social interaction theory Harley (2001) believes that few people would disagree with the central idea of the social interactionists; that is, that effective language acquisition must take place in a social setting. There is also evidence 61 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  LANGUAGE AND THOUGHT that this is the case. For example, Messer (2000, p.140) notes that when the main source about language is television ‘children make little or no progress with the language they hear’. This is illustrated by the case of ‘Jim’ who, until the age of 3, only heard language on television because his parents were deaf (Sachs et al., 1981). Jim had learned to use words but both his articulation and grammar were very poor. One of the first rules that most children learn is the use of an ‘s’ to make a plural, but Jim still could not do this at the age of 3. Television exposes children to language but children do not learn well from it because it does not interact with them. Another source of evidence for the social interactionist theory is the language development of twins. The language acquisition of twins is often slower than average until about school age when they catch up. One reason for this could be that, although they hear the same amount of language as any other child, less of it is used in interacting with each individual twin (Messer, 2000). There are a number of problems with the social interactionist theory. Firstly, the style of interaction between infants and adults varies across cultures and if language acquisition depends on this interaction there should be differences between cultures. However, as the nativists note, many aspects of language acquisition seem to be universal. Secondly, this approach has been criticised because there is a tendency to be vague about exactly how social interactions influence language acquisition (e.g. Harley, 2001). Copyright © 2003. Taylor & Francis Group. All rights reserved. Summary There are a number of different theories of how children acquire language, but they fall into three main types: environmental, nativist and social interactionist theories. The environmental theories concen- trate on the role of learning in language acquisition. Skinner proposed that reinforcement and behaviour shaping of the child’s vocalisations could explain acquisition of language (or verbal behaviour). However, parents do not seem to reinforce children’s speech selectively and this theory does not seem to explain the universality and creativity of language. The simplified version of speech adults use to children (or child-directed speech) may help in the acquisition of language but since it does not appear in all cultures it is unlikely to be responsible for language development. Nativist theories, such as Chomsky’s, 62 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  THEORIES OF LANGUAGE ACQUISITION emphasise the role of innate mechanisms in language acquisition. Chomsky suggests that humans have a language acquisition device (LAD) that enables children to learn any language simply by being exposed to it. This is supported by the existence of linguistic universals and the universal nature of language acquisition. However, the theory concentrates on the syntax of language and ignores its social and communicative aspects. The social interaction theory stresses the importance of language in communication and suggests that language is acquired through using language whilst interacting with others. However, this account of language acquisition has problems in explaining why children from all cultures learn language in a similar way. Review the three approaches discussed in this chapter and complete the Review exercise following table: Central concept Main evidence Main limitation Environmental theory (Skinner) Nativist theory (Chomsky) Interactionist Copyright © 2003. Taylor & Francis Group. All rights reserved. theory (Bruner) Further reading Owens, R.E. (2001) Language Development: an introduction (5th edn). Needham Heights, Mass: Allyn and Bacon. This book covers the theories of language development clearly and thoroughly. Harley, T.A. (2001) The Psychology of Language: from data to theory (2nd edn). Hove: Psychology Press. A comprehensive book on all aspects of language, including theories of development. 63 Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.  This page intentionally left blank Copyright © 2003. Taylor & Francis Group. All rights reserved. Lund, Nick. Language and Thought, Taylor & Francis Group, 2003. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/cityuhk/detail.action?docID=199320. Created from cityuhk on 2024-10-29 07:14:29.