Cycles in speed-working memory-G relations: Towards a developmental–differential theory of the mind

This article presents three studies, two of them longitudinal, which investigated the relations between age, processing speed, working memory (WM), and fluid intelligence (gf) from 4 to 16years of age. Structural equation modeling showed that speed was a powerful covariate of age (~−.6 to −.7) from 4 to 13years, declining thereafter (to ~−.2). WM was stably related to speed through the whole age-span studied (~−.4 to −.5). A large part (59%) of age-related changes in gf (83%) from 4 to 7years and a lower but significant part later on, especially in adolescence (~10–20% out of ~40–50%), were mediated by WM. However, with speed and age controlled, WM was almost fully commensurate with gf (~.9), from about the age of 8–9years onwards. A series of models suggested an ever present efficiency level defined by speed and control and a representational level defined by WM and gf, which are increasingly differentiated with development. All processes develop in cycles concerted by a dynamic G. Change in each process over time originated from within the processes themselves and G, in proportions varying with developmental phase. Overall, speed signified age-associated changes in processing capabilities, partly expressed in WM expansions and gf reconstructions. An overarching model is proposed integrating differential with developmental theories of human intelligence. ► Speed was a powerful covariate of age in childhood but declined thereafter. ► Working memory was related to speed from early childhood through adolescence. ► A large part of age-related changes in gf were mediated by working memory. ► Change over time originated from within the processes themselves and G. ► Any process may be a differential or developmental factor.