Beyond Structure and Change: Ergodicity and the Dynamic Organization of Intellectual Development

In developmental research, it is assumed that development is ergodic, i.e., differences between children reveal how individuals change over time. Across four longitudinal studies, we examined ergodicity across cognitive systems—including attention, working memory, reasoning, and cognizance—timescales and developmental phases. We found that development was not globally ergodic. In Study 1, spanning ages 4–17, cross-sectional relations partially resembled patterns of change. Individual development involved overlapping, process-specific periods of acceleration, consolidation, and differentiation rather than one common developmental timetable. Study 2, spanning ages 4–12, showed short-term convergence and partial profile correspondence, with relational integration as the main carrier of change. Study 3, spanning ages 6–13, revealed that psychometric organization was highly stable across three waves, but children changing broadly in the first annual interval were not the same as children changing in the second. In Study 4, 5–7-year-old children were intensively trained on attention control and examined three times on other processes. The trained processes formed a recognizable common organization, but within-person coupling was substantially weaker than between-person coupling, rank correspondence declined with temporal distance between waves, and group dynamics represented individual children only approximately. Across phases, the carriers of change shifted from attention-related control, through relational integration, toward complex reasoning. Psychometric g, although strong across time, was very weakly related to developmental g (i.e., phase-sensitive mechanisms transforming states of psychometric g over time), indicating that population-level cognitive organization does not determine individual development. Implications for psychometric and developmental theories are discussed here.