Nonlinear development of speaking rate in child-directed speech

This study investigates the role of child-directed speech (CDS) in language acquisition by re-evaluating the old claim that mothers adapt their speech over the course of child language development. The specific hypothesis tested is that there may be quantal changes in certain properties of CDS such as speaking rate around the time children reach major linguistic milestones. The developmental path of CDS speaking rate was analyzed in 25 mother-child pairs from longitudinal corpora in CHILDES database (MacWhinney, 2000). A parallel analysis was also made on the development of speaking rate in the child as well as the mean length of utterance (MLU) in mother and child. The total number of utterances analyzed approximates one million. The findings reveal that CDS speaking rate changes nonlinearly with a shift occurring early in the multiword stage. There is also some indication that another breakpoint might be present around the onset of child speech production. A parallel pattern of nonlinearity is also observed in the speaking rate of the child and the MLU of both mother and child. The results support the notion that CDS is adapted to the changing needs of the language-learning child, which could reflect its facilitative role in child language acquisition.