Variability of lip and jaw movements in the speech of children and adults.

One purpose of the present study was to examine displacement variability of lower lip and jaw movements to determine whether sounds that are generally learned earlier and should, therefore, have been practiced more (e. g., stops and nasals) would be less variable than sounds that tend to develop later (e. g., fricatives). It was also of interest to determine whether individual articulators such as the lower lip and the jaw show any differences in displacement variability, given that lower lip gestures may need to be more precise than jaw movements. Repetitions of several labial-initial, CVC stimuli embedded in short phrases produced by three groups of children (5-, 8-, and 11-year-olds) and a group of adults were examined to determine the variability of articulatory gestures. No evidence was found to suggest that fricatives were more variable than stops or nasals for any of the groups. For the children but not the adults, lower lip gestures tended to be more variable than jaw movements.