Integration/differentiation of mental processes is major mechanism of development. Developmental theories ascribe intellectual development to it. In psychometric theory, Spearman’s law of diminishing returns postulates that increasing g allows increasing differentiation of cognitive abilities, because increased mental power allows variable investment in domain-specific learning. Empirical evidence has been inconsistent so far, with some studies supporting and others contradicting this mechanism. This state of affairs is due to a developmental phenomenon: Both differentiation and strengthening of relations between specific processes and g may happen but these changes are phase-specific and ability-specific, depending upon the developmental priorities in the formation of g in each phase. We present eight studies covering the age span from 4 to 85 years in support of this phenomenon. Using new powerful modeling methods we showed that differentiation and binding of mental processes in g occurs in cycles. Specific processes intertwine with g at the beginning of cycles when they are integrated into it; when well established, these processes may vary with increasing g, reflecting its higher flexibility. Representational knowledge, inductive inference and awareness of it, and grasp of logical constraints framing inference are the major markers of g, first intertwining with in their respective cycles and differentiating later during the periods of 2–6, 7–11, and 11–20 years, respectively. The implications of these findings for an overarching cognitive developmental/differential theory of human mind are discussed.

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.