Sugiyama, H. , Kobayashi, T. & Minami, Y.
NTT Communication Science Labs
Previous studies have argued that infant's vocabulary growth differs among individual infants due to variation of parents' inputs (e.g., Tardif 1996). However, some words may be acquired at particular timings according to infants' livelihood and/or physical development since their parents often use such words (e.g., body parts and games/routines) at the similar timings. This predicts that individual variations of word acquisition age relates to the degree to universal properties beyond individual, cultural, and linguistic factors; i.e., word acquisition age correlates with individual variation of word acquisition age. We examine this prediction using Japanese and English vocabulary growth data. Using our Japanese MacArthur CDI database (1,699 participants) with English Lex2005 CDI database (Dale et al., 1996), we analyzed 154 words that had a clear correspondence between Japanese and English, and acquisition proportion at 30 month was over 60%. We calculated two parameters for each word: 'acquisition age' defined as a date when acquisition proportion is 50%, and 'individual variation' of acquisition age defined as the inverse of highest derivation value of the logistic function. Our analysis shows that the acquisition age strongly correlates with the individual variation in the both languages (J: r=0.83*, E: r=0.74*; *=p<.01); in contrast, the individual variation between the languages are weakly correlated (r=0.28*). A more precise analysis with between-language comparisons shows little difference in individual variation of words involving 'body parts' and 'games/routines' category, suggesting that the individual variation may be due to culture-general livelihood and/or infants' physical development. Our analysis also provides cross-linguistic inconsistency: in Japanese, the words with small individual variation include personal belongings such as 'furniture and rooms' and 'outside things and places to go' categories; in English, such words include common nouns like 'car' and 'ball'. This may reflect a cultural difference in parents' educational policy (e.g., enjoying vs. teaching).