The relationship between the structure of interindividual and intraindividual variability: A theoretical and empirical vindication of developmental systems theory

(from the chapter) Proponents of the developmental systems theory (DST) have questioned the relevance of behavior genetics for the study of developmental processes. In this chapter, the criticism of DST will be reformulated in a way that is consistent with Wohlwill's thesis that the study of developmental processes requires analysis of intraindividual differences, not interindividual differences. The main part of the chapter focuses on defending the notion that the results of an analysis of interindividual differences may not be related at all to the structure of intraindividual differences. In a simulation study, it will be shown that behavior genetical factor analysis of interindividual variation can yield results that are entirely unrelated to the structure of intraindividual variation of each of the subjects making up the sample. The psychometrical and practical consequences of this finding are discussed at some length. In the final part of the chapter, it will be indicated how a more valid analysis of nonergodic intraindividual variation by means of time-series analysis techniques like dynamic factor analysis can be carried out. A simple inductive methodology will be sketched with which lawful relationships generalizing over genuinely homogenous populations of subjects can be derived.