From Noun Bias to Verb Sensitivity: Developmental Shifts in Japanese Children’s Word Learning from 1 to 4 Years

This paper examines whether noun bias serves as an early default mechanism in lexical acquisition among Japanese toddlers, despite Japanese being categorized as a “verb-friendly” language. In Experiment 1, using two complementary methods—the Japanese MacArthur–Bates Communicative Development Inventory and an Intermodal Preferential Looking (IPL) study—we investigated developmental changes in word-mapping tendencies among two age groups of Japanese toddlers (18 months and 26 months). Specifically, we explored whether younger toddlers (1- and 2-year-olds) exhibit a strong preference for mapping nonce words to objects rather than actions and whether individual differences in noun dominance influence word learning in the IPL task. In Experiment 2, we explored older children (3- and 4-year-olds) to examine whether an early verb shift takes place during that age group and compared whether there is any correlation between matured syntax and verb bias. Our findings reveal a robust noun bias as early as 18 months, with older toddlers displaying a shift that suggests an emerging sensitivity to verbs, aligning with the typological characteristics of Japanese. These results provide new insights into the developmental trajectory of verb acquisition and the interaction between lexical and syntactic learning in early childhood.