Disfluency in speech input to infants? the interaction of mother and child to create error-free speech input for language acquisition

One characteristic of infant-directed speech is that it is highly fluent compared with adult-directed speech. However, the speech that infants hear still contains disfluencies. Such disfluencies might potentially cause problems for infants during language development. We first analyzed samples of spontaneous speech in the presence of infants (both adultand infant-directed) and found that under ideal circumstances the speech infants hear is highly fluent. Under less than ideal circumstances infants hear much more highly disfluent speech however this disfluent speech is almost entirely adultdirected. While grammatically ill-formed, the prosodic structure of these disfluencies might signal their illformedness to the infants. In a preference experiment, 10 month olds listened longer to infant-directed speech samples containing prosodic disfluencies than to equated samples without disfluency. However, this effect was found in only one of two counterbalancing groups. Using adult ratings of low-pass versions of these speech samples, we found that infants’ preferences were correlated with the adults’ perception of the relative disfluency of the samples. A followup experiment using adult-directed disfluencies found that while the 10 month olds showed no differences in their listening preferences, older infants preferred to listen to the fluent speech. These results suggest that younger and older infants attend differently to infant and adult-directed speech, and that older infants may be able to differentiate grammatical adult-directed input from input distorted by disfluency. We discuss implications of these findings for language acquisition.