The future of analysis of intraindividual variation

Modern scientific assessment methods enable the (semi) continuous registration of a wealth of psychologically relevant variables, ranging from vibrations of DNA molecules in the nanosecond range and brain electric field oscillations in the millisecond range up to circadian and annual rhythms in all kinds of psychological and biobehavioral processes. Using these methods, one can characterize each individual subject by means of a very high-dimensional dataset that evolves in time on a wide variety of time scales (cf. de Groot, 1954). The dynamic database thus obtained inspires a theoretical picture of the human subject in psychology as being a high-dimensional dynamic system composed of many integrated subsystems, such as the perceptual, cognitive, and neural subsystems. This theoretical picture is fundamental for the proper conceptualization of intraindividual variation as variation associated with the timedependent evolution of individual subjects (I will not consider the possible extension of this conceptualization with the inclusion of variation in both space and time). Before embarking upon the main discussion, I would like to first try to define as clearly as possible the meaning of intraindividual variation that is at stake in this chapter.