This study examined whether singular/plural marking in a language helps children learn the meanings of the words 'one,' 'two,' and 'three.' First, CHILDES data in English, Russian (which marks singular/plural), and Japanese (which does not) were compared for frequency, variability, and contexts of number-word use. Then young children in the USA, Russia, and Japan were tested on Counting and Give-N tasks. More English and Russian learners knew the meaning of each number word than Japanese learners, regardless of whether singular/plural cues appeared in the task itself (e.g., "Give two apples" vs. "Give two"). These results suggest that the learning of "one," "two" and "three" is supported by the conceptual framework of grammatical number, rather than that of integers.

Japanese provides a valuable contrast for crosslinguistic studies of noun and verb dominance in early child language, and the effect of input on the early lexicon. In this study, 31 Japanese children between 1;0 and 2;0 and their caregivers were recorded in two contexts: joint bookreading and play with toys. Context had the largest effect, as nouns were much more frequent in the book context. Noun dominance was constant across development in the book context, but in the toy context there was a shift away as children developed from single words through the presyntactic stage to the syntactic stage. Caregiver language was verb dominant in a number of respects across development in the toy context, and thus was not closely related to child lexical balance. We conclude that in early lexical development, all children have a conceptual disposition to learn nouns. With vocabulary growth and the emergence of grammar, the proportion of verbs increases substantially, and at this stage properties of the input language may influence development.

Temporal correspondences between the attainment of specific milestones in play and language were examined through a longitudinal study of four Japanese children (aged o;7 to o ; n at the beginning of the study). There were developmental correspondences between the onset of six language landmarks (the emergence of first words, naming words, vocabulary spurts, word-chains, nonproductive two-word utterances, productive two-word utterances) and the onset of subcategories of play. Language and play both reflected the development of underlying symbolic ability, and both developed in a parallel manner at the singleword stage. After the emergence of word-chains, language and play developed interdependently. All the children proceeded through the same sequence of stages, but the rate of development was different depending on their environment.

Previous studies indicate that English-learning children acquire the distinction between singular and plural nouns between 22-and 24-months. Also, their use of the distinction is correlated with the capacity to distinguish non-linguistically between singular and plural sets in a manual search paradigm (Barner et al., 2007). Three experiments explored the causal relation between these two capacities. Relative to English, Japanese and Mandarin have impoverished singular-plural marking. Using the manual search task, Experiment 1 found that by around 22-months, Japanese children also distinguish between singular and plural sets. Experiments 2 and 3 extended this finding to Mandarin-learning toddlers. Twenty-to 24-month-old Mandarin-learners did not yet comprehend Mandarin singular-plural marking (i.e., yige vs. yixie, or -men), yet they did distinguish between singular and plural sets in manual search. These experiments suggest that knowledge of singular-plural morphology is not necessary for deploying the non-linguistic distinction between singular and plural sets.

This study compared the sequential structure of mother-child conversation during joint picture book reading in Japanese and American dyads with children between 12 and 27 months. Although there were commonalities such as increased maternal elaborative information-asking in response to children's labelling with children's vocabulary growth, there were substantial differences. Japanese children produced labelling following maternal labelling more than American children, while American children produced labelling following information-asking more than Japanese children. Japanese mothers responded to children's labelling with interpersonal utterances more than American mothers, while American mothers responded to labelling with elaborative informationasking more than Japanese mothers. Interaction in American dyads generally followed an instruction model, while interaction in Japanese dyads reflected aspects of an osmosis model.