This book is the fifth in a series of volumes, emanating from the workshops organized by the European Network on Longitudinal Studies on Individual Development (ENLS) within the framework of the European Science Foundation.

The normal process in which an individual develops from birth through the life course is of interest in itself as a subject for research. Understanding and explaining that process is also fundamental for understanding what contributes to physical and mental health and for revealing the causes of mental, social and physical problems during the life course. As has been convincingly demonstrated in the earlier content-oriented volumes in this series, the development of individuals cannot be adequately and effectively investigated without using a longitudinal research strategy (see, e.g., Bakes & Baltes, 1990; Magnusson & Bergman, 1990; de Ribaupierre, 1990; Rutter, 1.988; Rutter & Casaer, 1991, as listed on p. iv; see also Magnusson (1988) on p. 27 of this volume).

Individual development can be described as a multidetermined, stochastic process in a lifetime perspective, a process in which psychological and biological factors in the individual and factors in the environment are actively involved. In that perspective, important aspects of the process of individual development cannot be investigated, understood and explained efficaciously without following the same individuals across time; i.e., without conducting longitudinal research. Longitudinal research puts special claim on the researcher in terms of careful planning of all steps in the process from the formulation of a problem to the presentation of the results.