Psychology, 2018, 9, 655-678 http://www.scirp.org/journal/psych ISSN Online: 2152-7199 ISSN Print: 2152-7180 Studying Differential Invariants in Developmental Variations Jacques Juhel Univ Rennes, LP3C (Psychology Laboratory: Cognition, Behavior, Communication), Rennes, France Abstract The differential study of intraindividual variability and change is currently reaping the benefits of important methodological advances in longitudinal data modeling. Two major research strategies for detecting invariants in indi- vidual differences in developmental change are considered. The most com- mon approach involves operating at a population level to study differences of behavior throughout development. Though rarely used, another approach is the differential study of developmental change based on an individual model- ing of the dynamics of intraindividual variability. The potential of these two approaches for improving our understanding of the development of human beings is illustrated. Keywords Individual Differences, Intraindividual Variability, Longitudinal Data Modeling 1. Introduction Broadly defined, psychology aims to understand the psychological mechanisms underlying behavioral invariance in its various manifestations. The study of in- variance first requires an “abbreviated” description of the diversity of psycho- logical phenomena (Reuchlin, 1962). This description can be a simple summary of phenomena to which a property is then assigned. An invariant can also be de- fined using abstract principles designed to account for specific phenomena (e.g. a learning rule, a structure of correlations supporting a hypothesis, or a schema specifying the operative invariance of a form of reasoning at a given age). The aim is to specify the (invariant) principles linking variations in behavior in dif- ferent situations, either in different individuals or at different moments in time, with a view to determining whether a hypothesis is confirmed by data or making How to cite this paper: Juhel, J. (2018). Stud- ying Differential Invariants in Developmental Variations. Psychology, 9, 655-678. https://doi.org/10.4236/psych.2018.94041 Received: March 2, 2018 Accepted: April 15, 2018 Published: April 18, 2018 Copyright © 2018 by author and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access DOI: 10.4236/psych.2018.94041 Apr. 18, 2018 655 Psychology