Japanese has traditionally been called "mora-timed", but studies have shown that this intuition is based not on durational tendencies but rather on phonological, structural factors in the language. Meanwhile, infant-directed speech (IDS) is said to "exaggerate" certain properties of adult-directed speech (ADS), including rhythm. If so, then it is possible that the mora rhythm of Japanese is more strongly observed in IDS than ADS. To investigate this possibility, the present study utilized the RIKEN Japanese Mother-Infant Conversation Corpus, which contains approximately 11 hours of IDS by 22 mothers talking with their 18-to-24-month-old infants, and 3 hours of ADS by the same mothers. Results from durational analyses showed that aspects of mora rhythm, such as the distinction between phonemically short and long vowels and singleton and geminate consonants, and the tendency toward isochrony of moras, were not greater in IDS than ADS. Mora duration in IDS was highly variable, partly stemming from greater phrase-final lengthening and non-phonemic, emphatic lengthening. Results from structural analysis, however, showed that non-CV moras such as nasal moras that characterize Japanese rhythm occurred more frequently in IDS than ADS. These results suggest that even in IDS, Japanese rhythm is manifested structurally, not durationally. [Work supported by JSPS]

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