Developmental psychology is a unique, comprehensive, and important aspect of psychology for at least three reasons. First, developmental psychologists adopt a vital perspective on psychological theory and research. When psychologists conduct experiments in perception, investigate language, or study personality, they usually concentrate on perception, language, or personality in individuals of a particular age, be they children, college students, or the elderly, and in so doing may gain important knowledge about perception, language, or personality. To study psychological phenomena at only one point in the life cycle, however, is to limit knowledge about those phenomena by failing to consider the continuity or change in psychological phenomena that are the province of developmental psychology. Indeed, it could be aruged that when we undertake the psychological study of any phenomenon, we must—wittingly or unwittingly—do so in a developmental context. The chapters in this volume on substantive areas of psychology, like perception, cognition, and language, by Bornstein, Kuhn, and Gleitman and Wanner, and those on personality and social psychology by Lamb, Hoffman, Ruble, Patterson, and Achenbach, all demonstrate that the developmental perspective transcends and enriches any focus on particular points in the life span. One purpose of this volume, then, is to provide the inescapable and valuable developmental perspective on all substantive areas in psychology.
Second, developmental psychology is also a major sub-discipline in its own right. It has its own history and systems, as Dixon and Lerner point out, its own methodologies, as Seitz shows, and its own perspectives, as both Baltes and Reese and Rogoff, Gauvain, and Ellis demonstrate. If studying psychol