From early on, infants show a preference for infant-directed speech (IDS) over adult-directed speech (ADS), and exposure to IDS has been correlated with language outcome measures such as vocabulary. The present multi-laboratory study explores this issue by investigating whether there is a link between early preference for IDS and later vocabulary size. Infants’ preference for IDS was tested as part of the ManyBabies 1 project, and follow-up CDI data were collected from a subsample of this dataset at 18 and 24 months. A total of 341 (18 months) and 327 (24 months) infants were tested across 21 laboratories. In neither preregistered analyses with North American and UK English, nor exploratory analyses with a larger sample did we find evidence for a relation between IDS preference and later vocabulary. We discuss implications of this finding in light of recent work suggesting that IDS preference measured in the laboratory has low test-retest reliability.

Idioms play an important role in language; however, little research has examined idioms in children’s natural language settings. This study explored idioms usage in maternal talk during mother-child shared book reading and its relation to children’s vocabulary development. Thirty-three Chinese children in Norway (aged 3;0–5;5) and their mothers participated. We observed shared reading at the onset of the study and assessed children’s receptive and expressive vocabulary in Chinese three times across one year. Results demonstrated that mothers used an average of 1.8 idioms and explained one-third of the idioms. Maternal idiom usage was correlated with their talk amount and lexical diversity. Individual growth modeling revealed that the number of idioms mothers used predicted the growth of children’s receptive vocabulary in Chinese. We speculate that idiom usage could be an effective and understudied marker of parental linguistic sophistication. This study underscores the importance of idiom exposure in children’s language environment.

Limited studies have examined demographic differences in children’s vocabulary in longitudinal samples, while there are questions regarding the duration, direction, and magnitude of these effects across development. In this longitudinal study, we included over 400 Dutch children. Caregivers filled out N-CDIs when children were 9–11 months (measuring word comprehension, word production, and gestures) and around 2–5 years of age (measuring word production). At 2–5 years, we also administered a receptive vocabulary task in the lab. We examined demographic effects on vocabulary size across infancy and toddlerhood. We found a disadvantage for males in infants’ gestures and toddlers’ vocabulary production. We found a negative effect of maternal education on infants’ caregiver-reported vocabulary, but a positive effect on toddlers’ lab-administered receptive vocabulary. Lastly, we found a negative effect of multilingualism – but only for the lab-administered task. Examining predictors in large, longitudinal samples ensures their robustness and generalisability across development.

The vocabulary of bilingual children is determined by various linguistic factors that develop depending on the language input and individual factors of these children. To understand vocabulary development and to be able to support these children accordingly, the assessment instruments essentially need to be adapted to this individual process. The current study examined factors influencing productive vocabulary performance by means of explanatory item-response models on a sample of 126 German monolingual and Turkish—German bilingual preschool children aged between 3 and 6 years. We analyzed item features with respect to lexical category, length, frequency, age of acquisition, and complexity in dependence of the language background of the children. Apart from a general delay in the development and some language specific characteristics, the results indicate a parallel shifted acquisition of vocabulary in bilingual children, which is nonetheless shaped by the same factors as in monolinguals.

This study examines lexical and phonological factors that influence word production and pronunciation. Specifically, we investigate whether phonological production (measured by percent consonants correct) contributes to word production and pronunciation over and above the properties of the target words (e.g., word frequency, neighborhood density, and phonetic complexity). Forty French-speaking monolingual and bilingual children, aged 1;11 to 3;1, participated in a spontaneous language sample and were administered a naming and a nonword repetition task. Their parents filled out the MacArthur Communicative Developmental Inventory (MCDI) and rated their children’s pronunciation on an experimental version of the MCDI. Statistical models indicated that word frequency and the phonetic complexity of the target words influenced whether a word was produced. These factors along with neighborhood density and the children’s production capacities influenced whether a word was pronounced poorly or well. Findings indicate that parents can provide reliable information on the word pronunciation of their children.

Many children grow up hearing multiple languages, learning words in each. How does the number of languages being learned affect multilinguals’ vocabulary development? In a pre-registered study, we compared productive vocabularies of bilingual (n = 170) and trilingual (n = 20) toddlers aged 17–33 months growing up in a bilingual community where both French and English are spoken. We hypothesized that because trilinguals have reduced input in French and English due to time spent hearing their third language, they would have smaller French–English vocabulary sizes than bilinguals. Trilinguals produced on average 2/3 of the number of words in these languages that bilinguals did: however, this difference was not statistically robust due to large levels of variability. Follow-up analyses did, however, indicate a relationship between input quantity and vocabulary size. Our results indicate that similar factors contribute to vocabulary development across toddlers regardless of the number of languages being acquired.

By the end of their first year, infants can interpret many different types of complex dynamic visual events, such as caused-motion, chasing, and goal-directed action. Infants of this age are also in the early stages of vocabulary development, producing their first words at around 12 months. The present work examined whether there are meaningful individual differences in infants’ ability to represent dynamic causal events in visual scenes, and whether these differences influence vocabulary development. As part of the longitudinal Language 0–5 Project, 78 10-month-old infants were tested on their ability to interpret three dynamic motion events, involving (a) caused-motion, (b) chasing behaviour, and (c) goal-directed movement. Planned analyses found that infants showed evidence of understanding the first two event types, but not the third. Looking behaviour in each task was not meaningfully related to vocabulary development, nor were there any correlations between the tasks. The results of additional exploratory analyses and simulations suggested that the infants’ understanding of each event may not be predictive of their vocabulary development, and that looking times in these tasks may not be reliably capturing any meaningful individual differences in their knowledge. This raises questions about how to convert experimental group designs to individual differences measures, and how to interpret infant looking time behaviour.

To code-switch or not to code-switch? This is a dilemma for many bilingual language teachers. In this study, the influence of teachers’ CS on bilingual children's language and cognitive development is explored within heritage language (HL) classes in Singapore. Specifically, the relationship between children's language output, vocabulary development, and cognitive flexibility to teachers’ classroom CS behavior, is examined within 20 preschool HL classrooms (10 Mandarin, 6 Malay, and 4 Tamil). Teachers’ and children's utterances were recorded, transcribed, and analyzed for CS frequency and type (i.e., inter-sentential, intra-sentential). 173 students were assessed with receptive vocabulary and dimensional card sort tasks, and their vocabulary and cognitive switching scores assessed using correlational and mixed effects analyses. Results show that inter-sentential and intra-sentential CS frequency is positively and significantly related to children's intra-sentential CS frequency. Overall, findings revealed that teachers code-switched habitually more often than for instructional purposes. Neither inter-sentential nor intra-sentential CS was significantly related to children's development in HL vocabulary, and intra-sentential CS was found to positively and significantly relate to children's growth in cognitive flexibility. These findings reveal the multi-faceted impact of teacher's CS on children's early development.

This longitudinal study assessed three acoustic components of maternal infant-directed speech (IDS) – pitch, affect, and vowel hyperarticulation – in relation to infants’ age and their expressive vocabulary size. These three individual components were measured in IDS addressed to infants at 7, 9, 11, 15, and 19 months (N = 18). All three components were exaggerated at all ages in mothers’ IDS compared to their adult-directed speech. Importantly, the only significant predictor of infants’ expressive vocabulary size at 15 and 19 months was vowel hyperarticulation, but only at 9 months and beyond, not at 7 months, and not pitch or affect at any age. These results set apart vowel hyperarticulation in IDS to infants as the critical IDS component for vocabulary development. Thus IDS, specifically the degree of vowel hyperarticulation therein, is a vehicle by which parents can provide the most optimal speech quality for their infants’ linguistic and communicative development.

A longitudinal study of lexical development in very young French–English bilinguals is reported. The Computerized Comprehension Test (CCT) was used to directly assess receptive vocabulary and processing efficiency, and parental report (CDI) was used to measure expressive vocabulary in monolingual and bilingual infants at 16 months, and six months later, at 22 months. All infants increased their comprehension and production of words over the six-month period, and bilingual infants acquired approximately as many new words in each of their languages as the monolinguals did. Speed of online word processing was also equivalent in both groups at each wave of data collection, and increased significantly across waves. Importantly, significant relations emerged between language exposure, vocabulary size, and processing speed, with proportion of language exposure predicting vocabulary size at each time point. This study extends previous findings by utilizing a direct measure of receptive vocabulary development and online word processing.

This study examines the strength and direction of lexical-grammatical associations within and between first and second languages (L1 and L2) in a longitudinal sample of sequential bilinguals. Thirty-three children who spoke Vietnamese (L1) and English (L2) completed picture-naming and story-telling tasks in each language at four yearly intervals. Hierarchical linear modeling across Years 1–4 revealed bidirectional within-language associations and a unidirectional cross-language association from the L1 to L2. Results suggest a conditional relationship between languages in which the L1 supports L2 growth, but not vice versa. Findings contribute to defining pathways for L1 and L2 learning across domains and languages.

Do infants learn their early words in semantic isolation? Or do they integrate new words into an inter-connected semantic system? In an infant-friendly adaptation of the adult lexical priming paradigm, infants at 18 and 24 months-of-age heard two words in quick succession. The noun-pairs were either related or unrelated. Following the onset of the target word, two pictures were presented, one of which depicted the target. Eye movements revealed that both age groups comprehended the target word. In addition, 24-month-olds demonstrated primed picture looking in two measures of comprehension: Named target pictures preceded by a related word pair took longer to disengage from and attracted more looking overall. The finding of enhanced target recognition demonstrates the emergence of semantic organisation by the end of the second year.