Modeling continuity and discontinuity in utterance length: a quantitative approach to changes, transitions and intra-individual variability in early grammatical development

The confluence of an anomaly such as a growth spurt or a temporary regression on the one hand and a temporary increase in intra-individual variability on the other hand, forms a strong indicator of a major transition in early language development. Data concern one-word (W1), two- and three-word (W2-3), and four-and-more-word (W4+) utterances from two French children during their second and third years. A dynamic growth model was fitted, based on a structure of supportive, conditional and competitive relationships. Using a statistical simulation method, we showed two striking peaks of variability in addition to a temporary regression or rapid growth in the proportions of W1, W2-3 and W4+ utterances. We argue that these phenomena show transitions corresponding to critical points in grammatical development, which could be indicative of the emergence of simple combinatorial and syntactic stages of language successively. Our results emphasize the relevance of time-serial data and of intra-individual variability in the study of developmental transitions in general.