This article compares two paradigms used to describe, explain, and optimize human development, the Cartesian split, essentialist, and reductionist paradigm and the process-relational one, and the metatheories associated with them, the reductionist metamodel and the dynamic, relational developmental systems (RDS) metamodel, respectively. We provide an RDS-based rationale for rejecting the splitting of quantitative and qualitative changes in depicting human development, discuss past attempts to address issues surrounding quantitative versus qualitative changes, argue that these approaches have failed to adequately provide a theory of the x-axis (time) and, as such, obscure the dynamic integration of quantitative fluctuations and qualitative transformation across the life span. We present a dynamic, RDS-based means to conceptualize and identify ontogenetic tipping points between quantitative fluctuations and qualitative transformations in intraindividual change. Using RDS-based concepts, we recommend methodological means to design, measure, and analyze the integration of quantitative and qualitative change across the human life span.
Lerner et al. (2026) studied this question.