Speech rate development in Japanese-speaking children and proficiency in mora-timed rhythm

Development of speech rate is often reported as children exhibiting reduced speech rates until they reach adolescence. Previous studies have investigated the developmental process of speech rate using global measures (syllables per second, syllables per minute, or words per minute) and revealed that development continues up to around 13 years of age in several languages. However, the global measures fail to capture language-specific characteristics of phonological/prosodic structure within a word. The current study attempted to examine the developmental process of speech rate and language-specific rhythm in an elicited production task. We recorded the speech of Japanese-speaking monolingual participants (18 participants each in child [5-, 7-, 9-, 11-, and 13-year-old] and adult groups), who pronounced three types of target words: two-mora, two-syllable words (CV.CV); three-mora, two-syllable words (CVV.CV); and three-mora, three-syllable words (CV.CV.CV), where C is consonant and V is vowel. We analyzed total word duration and differences in two pairs of word types: a pair of three-mora words (to show the effect of syllables) and a pair of two-syllable words (to show the effect of moras). The results revealed that Japanese-speaking children have acquired adult-like word duration before 11 years of age, whereas the development of rhythmical timing control continues until approximately 13 years of age. The results also suggest that the effect of syllables for Japanese-speaking children aged 9 years or under was stronger than that of moras, whereas the effect of moras was stronger after 9 years of age, indicating that the default unit for children in speech rhythm may be the syllable even when the language is mora-based.