**4. Developmental model of phonetic category formation in bilingual children**

Based on the findings of previous work on phonetic category formation, I propose a model called "development model of phonetic category formation" in both simultaneous and sequential bilingual children. In this model, I argue that phonetic category formation continues to evolve during the developmental process rather than emerge all at once in both types of bilingual children. **Figures 1** or **2** shows a schematic representation of phonetic category development in terms of stop and vowel categories for either simultaneous or sequential bilingual children. The direction of arrows shows the language transfer effect. As can be seen in **Figure 1**, in simultaneous bilingual children at 3–4-years of age, phonetic categories for L1 and L2 stops are not distinguished at all regardless of language types. Thus, two circles representing L1 and L2 overlap each other. The size of the circle denotes the development of a stop system in each language. Whether the stop system of each language is fully developed or not depends on the sound system of each language. For example, 3-year-old simultaneous Korean-English bilingual children were able to produce both English and Korean stop phonemes distinctively within a language [38] whereas Spanish-English [25], Japanese-English [27] or Spanish-German [31] bilingual children were not able to produce Spanish or Japanese voiced stops which fall in voicing lead category. This finding was similar to previous research reporting that monolingual children have difficulty to acquire voicing lead stops [47]. The language influence effect also varied depending on languages. While Korean-English bilingual children did not show much interaction effects, bidirectional interaction [31] or unidirectional an influence of L2 on L1 [27]. At 5–6-years of age, phonetic categories for stops across language remains constant. Lee

and Iverson [19] reported that Korean-English bilingual children did not distinguish English and Korean stops across languages; neither Khattab [26] nor Watson [23] reported that bilingual children distinguished stop categories at five years of age. These children still failed to produce voiced stops with lead voicing if any language has voicing lead stops. An L2 influence on L1 still exists at this age [19]. Phonetic category for stops, however, is fully established at age 10 or older in simultaneous bilingual children. It was also noted that interaction effects between L1 and L2 take place at these ages. The interaction direction may be unidirectional in that L2 influences L1 [19, 24, 26] or bidirectional [32]. It is not certain why Whitworth found a bidirec-

**Figure 1.** Developmental model of phonetic category formation in simultaneous bilingual children. *Note*: The size of the circle indicates the phoneme development within each language. Arrow direction denotes the influence of one language

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on the other. Both unidirectional and bidirectional influences are indicated when both claims are reported.

While phonetic category for stops is not fully established until 10 years of age, that of vowels seems developed earlier than stops. At 3–4-years of age, simultaneous bilingual children produced vowels of both languages distinctively with limited interaction effects. The fully separated vowel systems remain the same at 5- and 10-years of age; however, the L2 language started to have an influence on the L1 vowel system at 5-years of age. The developmental model of vowel category formation heavily relies on Korean-English bilingual children [2, 38]. Since limited evidence is available on vowel production produced by simul-

taneous bilingual children, further studies are warranted to verify this observation.

tional influence with these children. Further studies need to verify this aspect.

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Yang and Fox [46] further examined Chinese and English vowels produced by 5–6-year-old Chinese-English bilingual children as a group. Fifteen bilingual children participated; the children were divided into two groups depending on their English language proficiency. The authors found that although no significant difference of vowel formant frequencies among three groups (monolingual English and two bilingual children), bilingual children with low English proficiency showed greater variation and slight positional changes. Furthermore, the bilingual children with high English proficiency showed better separation among the vowel categories, similar to that of the monolingual English-speaking children whereas the bilingual children with low English proficiency showed great overlaps for most vowel pairs than the other groups. In addition, shared vowels of English and Chinese were compared. The authors reported that no significant difference was found for English and Chinese /i/ by monolinguals, but the other shared vowels were fully separated from each other. Two groups of bilingual children showed similar production patterns. The authors concluded that L2 vowel systems in the bilingual children with low English proficiency were strongly influenced by their L1. The bilingual children produced L2 vowels in a near-native manner, but some L2 features were transferred to L1 vowels, suggesting an assimilation process taking place during L1 acquisition. **Table 1** shows a summary of empirical evidence of phonetic

**4. Developmental model of phonetic category formation in bilingual** 

Based on the findings of previous work on phonetic category formation, I propose a model called "development model of phonetic category formation" in both simultaneous and sequential bilingual children. In this model, I argue that phonetic category formation continues to evolve during the developmental process rather than emerge all at once in both types of bilingual children. **Figures 1** or **2** shows a schematic representation of phonetic category development in terms of stop and vowel categories for either simultaneous or sequential bilingual children. The direction of arrows shows the language transfer effect. As can be seen in **Figure 1**, in simultaneous bilingual children at 3–4-years of age, phonetic categories for L1 and L2 stops are not distinguished at all regardless of language types. Thus, two circles representing L1 and L2 overlap each other. The size of the circle denotes the development of a stop system in each language. Whether the stop system of each language is fully developed or not depends on the sound system of each language. For example, 3-year-old simultaneous Korean-English bilingual children were able to produce both English and Korean stop phonemes distinctively within a language [38] whereas Spanish-English [25], Japanese-English [27] or Spanish-German [31] bilingual children were not able to produce Spanish or Japanese voiced stops which fall in voicing lead category. This finding was similar to previous research reporting that monolingual children have difficulty to acquire voicing lead stops [47]. The language influence effect also varied depending on languages. While Korean-English bilingual children did not show much interaction effects, bidirectional interaction [31] or unidirectional an influence of L2 on L1 [27]. At 5–6-years of age, phonetic categories for stops across language remains constant. Lee

category formation.

92 Multilingualism and Bilingualism

**children**

**Figure 1.** Developmental model of phonetic category formation in simultaneous bilingual children. *Note*: The size of the circle indicates the phoneme development within each language. Arrow direction denotes the influence of one language on the other. Both unidirectional and bidirectional influences are indicated when both claims are reported.

and Iverson [19] reported that Korean-English bilingual children did not distinguish English and Korean stops across languages; neither Khattab [26] nor Watson [23] reported that bilingual children distinguished stop categories at five years of age. These children still failed to produce voiced stops with lead voicing if any language has voicing lead stops. An L2 influence on L1 still exists at this age [19]. Phonetic category for stops, however, is fully established at age 10 or older in simultaneous bilingual children. It was also noted that interaction effects between L1 and L2 take place at these ages. The interaction direction may be unidirectional in that L2 influences L1 [19, 24, 26] or bidirectional [32]. It is not certain why Whitworth found a bidirectional influence with these children. Further studies need to verify this aspect.

While phonetic category for stops is not fully established until 10 years of age, that of vowels seems developed earlier than stops. At 3–4-years of age, simultaneous bilingual children produced vowels of both languages distinctively with limited interaction effects. The fully separated vowel systems remain the same at 5- and 10-years of age; however, the L2 language started to have an influence on the L1 vowel system at 5-years of age. The developmental model of vowel category formation heavily relies on Korean-English bilingual children [2, 38]. Since limited evidence is available on vowel production produced by simultaneous bilingual children, further studies are warranted to verify this observation.

Vowel category formation in sequential bilingual children also showed a similar pattern as compared to simultaneous bilingual children. At 3–4-years of age, a sequential bilingual child showed separation of two vowel systems after short exposure duration to L2, suggesting that this child tends to distinguish two systems although there was an influence of L2 on L1 [45]. However, this finding was based on a single bilingual child without direct comparisons between the two languages. Further studies are warranted to confirm their findings. At 5–6 years of age, sequential bilingual children continue to manifest two systems. L1 production is also influenced by L2 at this age [46]. The distinctive vowel categories remain in separation at 10 years of age. Unlike younger aged sequential bilingual children, 10-year-old sequential bilingual children showed either bidirectional influence for children with longer exposure duration or L1 influence on L2 for shorter exposure duration [35]. L1 influence on L2' vowels were not observed in research with bilingual children, but the effect is commonly found in adult L2 learners. These differences may suggest that phonetic category formation and the effect of interaction between L1 and L2 may be different between child and adult bilingual speakers. In short, phonetic category formation in bilingual children is established progressively using multi-dimensional representations for each sound category, and continues to evolve in the developmental process. Interaction between L1 and L2 varied depending on

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The developmental model proposed in this chapter is based on current empirical evidence. Some research studies are a single case study without employing rigorous statistical analysis. Thus, this model should continue to develop based on more empirical findings in the future. Future studies should consider following aspects when phonetic category formation is examined in bilingual children. First, more group studies are expected in the future. Among 20 studies examining phonetic category formation in bilingual children, only half of the studies employed group comparisons. In order to lead to a more solid theoretical model of phonetic category formation, findings should be based on group studies. Second, when studies examine phonetic category formation in bilingual children, it is necessary to employ monolingual control groups of each language. Without understanding the phonetic development of monolingual children, it is not certain whether such a pattern shown in bilingual children is a natural developmental consequence or a bilingual effect. For example, several studies reported that bilingual children whose stops are produced with voicing lead often produced voiced stops with short lag VOT instead of voicing lead. It is not certain whether such production is attributed to the fact that these children acquire two languages or one language influences the other. Third, compared to studies examining stops, vowel studies are relatively limited. Only vowels produced by Korean-English and Chinese-English are currently available. In addition, no fricative or other consonantal study has been conducted. Thus, future studies are warranted to examine vowels and other consonants in simultaneous or sequential bilingual children. Fourth, although recent studies examined more diverse

types of bilingualism.

**5. Limitations and directions for future research**

**Figure 2.** Developmental model of phonetic category formation in sequential bilingual children. *Note*: The size of the circle indicates the phoneme development within each language. Arrow direction denotes the influence of one language on the other. Both unidirectional and bidirectional influences are indicated when both claims are reported.

It seems that phonetic category formation in sequential bilingual children develops similarly with simultaneous bilingual children; but some differences are also observed. At 3–4-years of age, sequential bilingual children did not manifest fully distinctive phonetic category for stops, similar to simultaneous bilingual children. While no transfer effect was observed in simultaneous bilingual children, a language transfer effect appears in that there was a strong effect of L1 on L2 language in sequential bilingual children. Similar to simultaneous bilingual children, sequential bilingual children did not manifest distinctive phonetic categories for stops at 5–6-years of age. Although voiceless stops were distinguished from each other, voiced stops across languages remains undistinguished by this age. Similar to 3–4-years of age, a unidirectional L1 influence on L2 exists during this age [40, 44]. Phonetic category formation; however, is fully acquired at 10 years of age or older in sequential bilingual children. There was also L1 influence on L2 during this age [37]. It is interesting to observe that L1 influence on L2 on stops in sequential bilingual children because L2 typically influences L1 in simultaneous bilingual children. It is not certain why this happens. It may be due to the fact that sequential bilingual children fully develop a stop system of their L1; thus, it may affect stops of L2, which is not fully developed yet.

Vowel category formation in sequential bilingual children also showed a similar pattern as compared to simultaneous bilingual children. At 3–4-years of age, a sequential bilingual child showed separation of two vowel systems after short exposure duration to L2, suggesting that this child tends to distinguish two systems although there was an influence of L2 on L1 [45]. However, this finding was based on a single bilingual child without direct comparisons between the two languages. Further studies are warranted to confirm their findings. At 5–6 years of age, sequential bilingual children continue to manifest two systems. L1 production is also influenced by L2 at this age [46]. The distinctive vowel categories remain in separation at 10 years of age. Unlike younger aged sequential bilingual children, 10-year-old sequential bilingual children showed either bidirectional influence for children with longer exposure duration or L1 influence on L2 for shorter exposure duration [35]. L1 influence on L2' vowels were not observed in research with bilingual children, but the effect is commonly found in adult L2 learners. These differences may suggest that phonetic category formation and the effect of interaction between L1 and L2 may be different between child and adult bilingual speakers. In short, phonetic category formation in bilingual children is established progressively using multi-dimensional representations for each sound category, and continues to evolve in the developmental process. Interaction between L1 and L2 varied depending on types of bilingualism.

### **5. Limitations and directions for future research**

It seems that phonetic category formation in sequential bilingual children develops similarly with simultaneous bilingual children; but some differences are also observed. At 3–4-years of age, sequential bilingual children did not manifest fully distinctive phonetic category for stops, similar to simultaneous bilingual children. While no transfer effect was observed in simultaneous bilingual children, a language transfer effect appears in that there was a strong effect of L1 on L2 language in sequential bilingual children. Similar to simultaneous bilingual children, sequential bilingual children did not manifest distinctive phonetic categories for stops at 5–6-years of age. Although voiceless stops were distinguished from each other, voiced stops across languages remains undistinguished by this age. Similar to 3–4-years of age, a unidirectional L1 influence on L2 exists during this age [40, 44]. Phonetic category formation; however, is fully acquired at 10 years of age or older in sequential bilingual children. There was also L1 influence on L2 during this age [37]. It is interesting to observe that L1 influence on L2 on stops in sequential bilingual children because L2 typically influences L1 in simultaneous bilingual children. It is not certain why this happens. It may be due to the fact that sequential bilingual children fully develop a stop system of their L1; thus, it may affect

**Figure 2.** Developmental model of phonetic category formation in sequential bilingual children. *Note*: The size of the circle indicates the phoneme development within each language. Arrow direction denotes the influence of one language

on the other. Both unidirectional and bidirectional influences are indicated when both claims are reported.

stops of L2, which is not fully developed yet.

94 Multilingualism and Bilingualism

The developmental model proposed in this chapter is based on current empirical evidence. Some research studies are a single case study without employing rigorous statistical analysis. Thus, this model should continue to develop based on more empirical findings in the future. Future studies should consider following aspects when phonetic category formation is examined in bilingual children. First, more group studies are expected in the future. Among 20 studies examining phonetic category formation in bilingual children, only half of the studies employed group comparisons. In order to lead to a more solid theoretical model of phonetic category formation, findings should be based on group studies. Second, when studies examine phonetic category formation in bilingual children, it is necessary to employ monolingual control groups of each language. Without understanding the phonetic development of monolingual children, it is not certain whether such a pattern shown in bilingual children is a natural developmental consequence or a bilingual effect. For example, several studies reported that bilingual children whose stops are produced with voicing lead often produced voiced stops with short lag VOT instead of voicing lead. It is not certain whether such production is attributed to the fact that these children acquire two languages or one language influences the other. Third, compared to studies examining stops, vowel studies are relatively limited. Only vowels produced by Korean-English and Chinese-English are currently available. In addition, no fricative or other consonantal study has been conducted. Thus, future studies are warranted to examine vowels and other consonants in simultaneous or sequential bilingual children. Fourth, although recent studies examined more diverse bilingual languages, still limited bilingual languages have been studied. Some bilingual languages are similar in that stops are categorized as either voiced or voiceless. Only Koreans, whose stop systems are different from other languages, were examined. Future studies may examine more simple or complex stop or vowel systems in order to fully understand how bilingual children manifest distinctive phonetic categories when they are in different language systems.

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