When gesture-speech combinations do and do not index linguistic change.

At the one-word stage children use gesture to supplement their speech ('eat'+point at cookie), and the onset of such supplementary gesture-speech combinations predicts the onset of two-word speech ('eat cookie'). Gesture thus signals a child's readiness to produce two-word constructions. The question we ask here is what happens when the child begins to flesh out these early skeletal two-word constructions with additional arguments. One possibility is that gesture continues to be a forerunner of linguistic change as children flesh out their skeletal constructions by adding arguments. Alternatively, after serving as an opening wedge into language, gesture could cease its role as a forerunner of linguistic change. Our analysis of 40 children--from 14 to 34 months--showed that children relied on gesture to produce the first instance of a variety of constructions. However, once each construction was established in their repertoire, the children did not use gesture to flesh out the construction. Gesture thus acts as a harbinger of linguistic steps only when those steps involve new constructions, not when the steps merely flesh out existing constructions.