Lexical acceleration coincides with the onset of combinatorial speech

Five children whose ages at the beginning of the study ranged from 12 to 17 months were observed weekly for 8-10 months. For each subject the cumulative numbers of both new words and new word- combinations were plotted as a function of week of observation. An exponential function was fitted to the lexical curves, and lexical acceleration was defined as an upward deviation of the observed curves from the fitted curves. It was found that lexical accelerations occurred at the time the subjects began to produce word- combinations. It is suggested that combinatorial speech, requiring greater specificity of expression (than holophrastic speech), motivates the learning of new words. At the same time, the sensitivity to sentential speech facilitates the mapping of word meanings.