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Review of  The Effects of Bilingualism on Infant Language Development

Reviewer: Jon Clenton
Book Title: The Effects of Bilingualism on Infant Language Development
Book Author: Liquan Liu
Publisher: Netherlands Graduate School of Linguistics / Landelijke (LOT)
Linguistic Field(s): Language Acquisition
Issue Number: 25.4699

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Review's Editor: Helen Aristar-Dry


This book is intended for psycholinguists, phonologists, and researchers on bilingualism and infant studies. It discusses the influence of bilingualism on infant sound and word acquisition in the first two years of life, and examines whether monolingual and bilingual infants follow the same language development trajectory. The book is the author’s PhD dissertation, and begins with a handsome collection of acknowledgements, the number of which this reviewer has never encountered in a single volume before. The eight chapters follow the form of a standard experimental PhD dissertation: Chapter One introduces the dissertation, Chapters Two to Seven consist of the five experiments, and Chapter Eight consists of a summary and conclusion.

Chapter One (“Bilingual infants’ phonological and vocabulary development”) begins by defining the volume’s central themes of ‘bilingual’ and ‘bilingualism’ and raises broad issues related to infant speech development for both mono- as well as bilingual infants. This first chapter considers themes central to the dissertation’s two aims (phonological development: including vowels, and tone; vocabulary development: associative word learning in terms of age of development). There is closing discussion of the advantages and disadvantages of bilingualism in infancy. The author presents balanced discussion related to the relative merits of bilingualism, suggesting that multiple influences such as setting affect this very complex area, and implies that advantages can become disadvantages and vice versa. This first chapter also discusses potential language dominance effects, as well as models of speech perception and language acquisition, in terms of their potential for bilingual language development. The chapter ends with an overview of the remaining sections of the dissertation, as well as a consideration of important questions that fall outside its scope.

Chapter Two (“Monolingual and bilingual infant consonant perception”) presents a study on bilingual infants’ perception of consonants. The study examines whether bilingual and mono-lingual infants (5-15 months of age) follow similar developmental trajectories in their ‘Voice Onset Time’ (VOT) perception of consonants. VOT is defined as the length of time between the release of closure of a stop and the onset of voicing. The author finds that initial sensitivity and language environment shape both monolingual and bilingual infants’ VOT perception. Tt 5-6 months bilingual infants demonstrate an initial sensitivity to both short-lag vs. long-lag as well as short-lag vs. long-lead contrasts just like the monolinguals in the study; however, at 6-12 months, performances vary. This difference, between the monolingual and bilingual infants, is ascribed to degree of exposure (DoE), which the author suggests is crucial for infant speech perception. The chapter closes with suggestions for future studies, including the need for studies with a larger sample size.

Chapter Three (“Monolingual and bilingual infant vowel perception”) examines infant sensitivity to speech sounds. The experiment in this chapter explores infant sensitivity to vowel discrimination. The author presents a study that compares monolinguals and bilingual performance discrimination and perception of the /i-I/ contrast. The findings show that the monolingual infants are not able to discriminate between the /i-I/ contrast until after 9 months of age. Bilingual infants, however, are not able to discriminate the contrast at 5-6 months, but show some sensitivity between 6 and 8 months. The author suggests that the bilingual infants demonstrate an enhanced acoustic sensitivity, but also warns against assuming that this is an advantage because this may not help in categorising sounds. Conversely, the author suggests that the bilingual infants often delayed in speech sound discrimination, which, accordingly, might relate to bilingual speech development. In this specific regard, the author implies that such heightened acoustic sensitivity might have a negative effect on speech sound normalisation for bilinguals. The chapter closes by proposing that future research examine differences between monolingual infants and bilingual infants with regard to acoustic sensitivity.

Chapter Four (“Monolingual and bilingual infant tone perception”) presents an experiment that compares monolingual and bilingual infants’ ability to discriminate non-native tonal contrasts. The comparison appears to show that non-tonal learning infant trajectory is U-shaped, with perceptual toning occurring at an early age, sensitivity deteriorating at 8-9 months, but recovering at around 11-12 months for bilingual infants, and 17-18 months for monolingual infants. The author hypothesises that this recovery is likely caused by the non-tonal learning infants’ failed attempt at native category formation and/ or native intonation acquisition. In short, the study reveals two unique perceptual patterns: tonal perception is continuous and plastic across development, and acoustically salient contrasts undergo perceptual toning (the gradual acquisition of tonal perception) to a lesser extent whereas less salient ones are subject to it. The chapter closes by proposing that future research test more contrasts in order to further clarify claims made within this chapter.

Chapter Five (“Monolingual and bilingual infants’ word learning of a non-native contrast”) examines monolingual and bilingual infants’ lexical development through associative word learning. The study examines the extent to which 14-15 and 17-18 month old monolingual and bilingual infants perform in a non-native tonal word learning task, and whether there is any identifiable difference between monolingual and bilingual perception along the developmental trajectory. The findings appear to demonstrate that monolingual and bilingual infants present similar word learning patterns across age, with no indication of bilingual delay. Both the monolingual and the bilingual infants were able to construct a sound-object association at 14-15 months, but not at 17-18 months which is likely due, according to the author, to acoustics. The author proposes that future studies ‘look deeper into this issue’ (p.120).

Chapter Six (“The development of vocabulary comprehension and production in monolingual and bilingual infants: a Communicative Developmental Inventory (CDI) study”) targets the vocabulary acquisition of monolingual and bilingual infants. The study investigates monolingual and bilingual infants at ages 11-12, 14-15, and 17-18 months in terms of word comprehension and word production with a CDI. The CDI employed was an adapted questionnaire (parents completed responses with their children) with 536 items of which 434 were vocabulary items. The results, in agreement with other literature, found significant differences in comprehension, with larger vocabularies overall in the bilingual infant subjects. The author also suggests that there is no observable delay shown in early bilingual vocabulary development. The chapter closes with five potentially worthwhile avenues of further research, the first of which highlights the need to study the threshold of lexical entries by exploring rapid word learning studies.

Chapter Seven (“Parents’ estimates of degree of language exposure: The bilingual / Multilingual infant questionnaire”) discusses degree of exposure (DoE) from the perspective of the parent along with a comparison with literature, and the results from a (Bilingual/ Multilingual Infant Questionnaire (MIQ)). The results appear to indicate that the parents are broadly aware of the language exposure of their children. The author also compares the parents in terms of their educational history (whether high or average education), and suggests that the parents with ‘average educational backgrounds’ were less accurate in their estimations than their high educational background counterparts. The author suggests that further research explore such differences, and definitely take the parental education level into consideration.

Chapter Eight (“Summary and Conclusion”) summarizes the central findings of the experimental chapters (2-7), integrates these findings into the main theoretical proposals of the dissertation/ volume, and poses questions for future research, as well as considerations related to infant research in general. The chapter ends with an exploration of the findings future research needs to consider and is tempered by reasserting that, as each individual child is unique, research should be cautious in stating general claims, and when dealing with high variation, individual developmental paths should be studied separately.


Liu has clearly contributed to the research on the effects of bilingualism on infant language development. The book represents a substantial overview of work within this area, particularly, as is its aim, work on the acquisition of sounds and words. This is a PhD dissertation and should be read as such; Liu should receive credit for collating what is clearly a considerable amount of experimental research.

For the most part, the text is clearly written, in language that is broadly accessible (there were a few occasions where I had to retrace my steps to remind myself what a particular acronym related to), and provides detailed explanation of the experiments conducted. The volume might provide a useful reference for the procedures Liu adopted in experimentation, and certainly provides a useful grounding for any such future studies. The studies were executed within the Dutch context, and the bilingual subjects were exposed to Dutch plus one other language. It would be revealing for subsequent research to be conducted in other contexts, perhaps beyond Europe even, to determine whether the findings presented here could be extended elsewhere or, indeed, to determine the extent to which conclusions might be applied to a variety of different L1s and L2s. The author is correct to present the caveat that the studies in this volume should be treated with caution, particularly given the variation in dealing with such young learners. It would help if the experiments reported here could somehow inform best practice.

As the book is basically a PhD dissertation, it should not be considered a manual on ‘how to’ raise a child with two languages. I am both a linguist with an interest in bilingualism, and a parent of a child raised as a bilingual. I found the content more relevant to my linguistic role than my parent role. The appeal of the book rests in its academic content rather than in any suggestions about the approaches to adopt in raising a bilingual infant.

Overall, while this is a well written dissertation, it should not be considered a ready resource for the lay person on the effects of bilingualism on infant language development.
Jon Clenton works at the department of English Language and Applied Linguistics, at Reading University in the UK. His current research focuses on vocabulary testing, bilingual vocabulary development, and the testing of young bilingual learners' vocabulary.