Mothers do not enhance tonal contrasts in child-directed speech: Perceptual and acoustic evidence from child-directed Mandarin lexical tones

Abstract

Prosodically, child-directed speech typically has a higher pitch and more varied pitch contours. Studies that have examined acoustic differences between child-directed and adult-directed vowels and consonants have reported mixed results and proposed two hypotheses explaining the function of the acoustic modifications in child-directed speech. The hyperarticulation hypothesis suggests that mothers enhance the phonemic contrasts in child-directed speech to facilitate speech and language acquisition in children. The pragmatic hypothesis claims that the acoustic differences between child-directed and adult-directed speech result from mothers' expression of affective emotions towards young children. In tone languages, pitch is used at the syllable level to make lexical contrasts and at the utterance level to serve pragmatic functions. This study compared the perceptual clarity and acoustic characteristics of adult-directed and child-directed Mandarin tones to test the two hypotheses. 1648 child-directed and adult-directed tones produced by 20 mothers in monosyllabic and disyllabic words were low-pass filtered to eliminate segmental information and presented to five judges for tone identification. Child-directed tones were identified with poorer accuracy than adult-directed tones. Acoustic analysis revealed that child-directed tones, regardless of tone type, were produced with higher pitch and more positive slopes than adult-directed tones. The findings did not support the hyperarticulation hypothesis.