“One of the main concerns […] is how to account for quantitative growth in development and how to relate this quantitative growth to qualitative analyses and to the parameters in the model. The definition of such quantitative growth (or development) is an autocatalytic quantitative increase in a growth variable following the emergence of a specific structural possibility in the cognitive system. In other words, language development (or cognitive growth in general) consists of growth which is expressed in numbers (quantitative) and which is not entirely caused by some external factor (autocatalytic). This growth is caused by the system itself, i.e. change is induced by the cognitive system itself.”
From: Going the distance: A Non-Linear Approach To Change In Language Development. H.G. Ruhland. Groningen, 1998.