The cognitive developmental cascade at different timescales: Relationships among intrasubject variability, processing speed, working memory, and fluid intelligence in children aged 8 to 14 years

According to the developmental cascade model (Fry & Hale, 2000), accelerations in processing speed (PS) throughout childhood lead to improvements in working memory (WM) which in turn result in increased fluid intelligence (gf). Decreasing intrasubject variability (ISV) of reaction times has been proposed to play a crucial role within this cascade. While most of the evidence supporting the developmental cascade is derived from cross-sectional studies, longitudinal studies are scarce, with heterogeneous findings. Moreover, longitudinal analysis of correlational patterns may produce biased results, if within-person changes (development within the studied timeframe) are not separated from between-person variance (stable differences and long-term changes beyond the studied time frame). To overcome these limitations, N = 173 children aged 8-12 years at baseline (M ± SD: 10.01 ± 1.11, 51% female) were tested five times within 2.5 years using a computer-based testing battery including choice reaction time tasks to measure PS and ISV as well as span, operation span, and coordination tasks to measure WM. Cattell's Culture Fair Test was administered to measure gf. Separating between-person variance from within-person developmental changes using a cross-lagged panel model with random intercepts, the postulated close relationships between PS, ISV, WM, and gf were observed at the level of between-person differences on a long-term timescale alongside no evidence of the expected associations of within-person changes for a time frame of 2.5 years. Thus, associations consistent with the developmental cascade model may reflect stable between-person differences rather than short-term within-person changes. This has important implications for developmental theory and methodology. (PsycInfo Database Record (c) 2026 APA, all rights reserved).