The majority of the world’s children grow up learning two or more languages. The study of early bilingualism is central to current psycholinguistics, offering insights into issues such as transfer and interference in development. From an applied perspective, it poses a universal challenge to language assessment practices throughout childhood, as typically developing bilingual children usually underperform relative to monolingual norms when assessed in one language only. We measured vocabulary with Communicative Development Inventories for 372 24-month-old toddlers learning British English and one Additional Language out of a diverse set of 13 (Bengali, Cantonese, Dutch, French, German, Greek, Hindi-Urdu, Italian, Mandarin, Polish, Portuguese, Spanish and Welsh). We furthered theoretical understanding of bilingual development by showing, for the first time, that linguistic distance between the child’s two languages predicts vocabulary outcome, with phonological overlap related to expressive vocabulary, and word order typology and morphological complexity related to receptive vocabulary, in the Additional Language. Our study also has crucial clinical implications: we have developed the first bilingual norms for expressive and receptive vocabulary for 24-month-olds learning British English and an Additional Language. These norms were derived from factors identified as uniquely predicting CDI vocabulary measures: the relative amount of English versus the Additional Language in child-directed input and parental overheard speech, and infant gender. The resulting UKBTAT tool was able to accurately predict the English vocabulary of an additional group of 58 bilinguals learning an Additional Language outside our target range. This offers a pragmatic method for the assessment of children in the majority language when no tool exists in the Additional Language. Our findings also suggest that the effect of linguistic distance might extend beyond bilinguals’ acquisition of early vocabulary to encompass broader cognitive processes, and could constitute a key factor in the study of the debated bilingual advantage.

Dr. Caroline Floccia is a Reader in Developmental Psychology at the University of Plymouth. She is primarily interested in early lexical and phonological development. She completed her Ph.D. at the Ecole des Hautes Etudes en Sciences Sociales in Paris and was a lecturer at the Universitede Franche-Comte (France) until 2006.

Dr. Tom Sambrook holds doctorates in primatology from the University of St. Andrews and neuroscience from the University of Plymouth. He is Lecturer in Psychology at the University of East Anglia.

After a Ph.D. in cognitive science on referential processing and attachment ambiguities, Dr. Claire Delle Luche has been working on the acquisition of the child’s lexicon, focusing on variability in the input and phonological aspects. She is currently a Lecturer at the University of Essex.

Dr. Rosa Kwok is a Research Associate at the Centre for Research in Psychology, Behaviour and Achievement at Coventry University. Rosa’s research focuses on the interface between cognitive development and language impairment. She obtained her DPhil in Psychology from the University of York in 2014.

Dr. Jeremy Goslin holds a Ph.D. in Computer Science from the University in Shefﬁeld and is currently an Associate Professor in Psychology at the University of Plymouth. His research interests are computational and neuroimaging approaches to psycholinguistics, and behavioral economics.

Dr. Laurence White has been a Lecturer in Psychology at the University of Plymouth since 2011. He completed his Ph.D. in linguistics (Edinburgh, 2002) and carried out postdoctoral research on speech perception and production at the University of Bristol and SISSA, Trieste.

Dr. Allegra Cattani obtained her Ph.D. in Experimental Psychology at the University of Genova (Italy) in 1999. She is currently a Senior Research Fellow at the University of Plymouth and manager of the Plymouth Babylab. Her main research interests focus on early gestural and lexical development, and on the early screening of young children with language and communication needs.

Emily Sullivan has a Masters in Behavioural Psychology from the University of Plymouth. Her research interests include the effects of infant-directed speech in young infants and homeopathic interventions on tantrum behavior in toddlers.

Dr. Kirsten Abbot-Smith is Co-Director of the Kent Child Development Unit, University of Kent. She is currently primarily interested in pragmatic language development, language in individuals with autism, and how education professionals can accurately contribute to child language screening.

Dr. Andrea Krott is a psycholinguist who conducted her Ph.D. project at the Max Planck Institute for Psycholinguistics and was awarded her Ph.D. from the Radboud University in 2001. She is currently a Senior Lecturer in the School of Psychology at the University of Birmingham. Her current research interests are lexical development in children and cognitive advantages of bilingualism.

Dr. Debra Mills, Ph.D., is a Professor of Cognitive Neuroscience, Director of the Brain and Cognitive Development Lab, and Research Director of Tir na n-Og Child Development Centre and Nursery, in the School of Psychology at Bangor University. Her research interests are in the effects of language experience on cerebral organization for language and nonlanguage cognitive processes across the lifespan as well as links across genes, brain, and behavior in neurodevelopmental disorders such as Williams syndrome.

Prof. Caroline Rowland is the recently appointed Director of the Language Development Department at the Max Planck Institute for Psycholinguistics in Nijmegen, and a co-Director of the ESRC International Centre for Language and Communicative Development (www.lucid.ac.uk), a multi-million pound collaboration between the Universities of Manchester, Liverpool, Lancaster. Her research focuses on how children learn to communicate with language, how the developing brain supports this process, and how it is affected by cross-linguistic, cultural, and individual variation.

Dr. Judit Gervain is a Senior Research Scientist at the CNRS, working at the Laboratoire Psychologie de la Perception (CNRS & Universite Paris Descartes). Her research focuses on early speech perception and language acquisition.

Prof. Kim Plunkett is Professor of Cognitive Science at Oxford University and Tutorial Fellow in Experimental Psychology at St Hugh’s College, Oxford. His main interest is in early lexical and cognitive development using experimental, neuro-imaging, and computational methods. He obtained his DPhil in Experimental Psychology from Sussex University in 1979 and was welcomed into the Academy of Social Sciences in 2013.

Erika Hoff is Professor of Psychology at Florida Atlantic University. She is a developmental psychologist with interests in how language experience inﬂuences monolingual and bilingual development. She is currently the Principal Investigator on an ongoing longitudinal study of simultaneous Spanish-English bilingual development in children living in the United States.

Cynthia Core is Associate Professor of Speech, Language, and Hearing Sciences at The George Washington University. She is an investigator (with Erika Hoff) on a longitudinal study of simultaneous bilingual language development of Spanish-English speaking children in the United States. She is a certiﬁed Speech-language Pathologist and Early Intervention Professional.

CONTENTS

7 I: INTRODUCTION