Infant-directed speech: Final syllable lengthening and rate of speech

Abstract

Speech rate has been reported to be slower in infant-directed speech (IDS) than in adult-directed speech (ADS). Studies have also found phrase-final lengthening to be more exaggerated in IDS compared with ADS. In our study we asked whether the observed overall slower rate of IDS is due to exaggerated utterance-final syllable lengthening. Two mothers of preverbal English-learning infants each participated in two recording sessions, one with her child, and another with an adult friend. The results showed an overall slower rate in IDS compared to ADS. However, when utterance-final syllables were excluded from the calculation, the speech rate in IDS and ADS did not differ significantly. The duration of utterance-final syllables differed significantly for IDS versus ADS. Thus, the overall slower rate of IDS was due to the extra-long final syllable occurring in relatively short utterances. The comparable pre-final speech rate for IDS and ADS further accentuates the final syllable lengthening in IDS. As utterances in IDS are typically phrases or clauses, the particularly strong final-lengthening cue could potentially facilitate infants’ segmentation of these syntactic units. These findings are consistent with the existing evidence that pre-boundary lengthening is important in the processing of major syntactic units in English-learning infants.