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

Abstract

This article presents three studies, two of them longitudinal, which investigated the relations between age, processing speed, working memory (WM), and fluid intelligence (g f) 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 g f (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 g f (~.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 g f reconstructions. An overarching model is proposed integrating differential with developmental theories of human intelligence. © 2012 Elsevier Inc.