Traditionally, developmental psychology and psychiatry have constituted very separate enterprises with distinctive and different styles of research. For the most part, the focus of developmental psychology has been on the universals of developmental processes. It is no accident that most of the ‘big’ theories of development have been expressed in stage terms. The content of the stages has, of course, varied across theories: psychosexual stages in psychoanalytic theory; cognitive structures in Piagetian theory; psychosocial tasks in Erikson's theory; and age periods (such as the supposed mid-life crisis) in some life-span theories. Nevertheless, in each case, the emphasis has been on a consistent age-dependent progression followed by all individuals. Research findings have failed to support many of the tenets that are central to stage theories, and modern concepts of development have come to include a much greater attention to individual differences, to the effects of social context, to the interplay between persons and their environments, and to people's overall pattern of functioning, as well as continuities and discontinuities in the expression of particular psychological traits (Rutter & Rutter, 1993). As a consequence, developmentalists have necessarily been forced to consider possible continuities and discontinuities between normality and psychopathology, and to recognize that individuals may follow developmental paths that diverge from each other in certain important respects. These considerations have also led to an extension of developmental research to age periods after childhood and thus to a consideration of life-span concepts.

At the same time, psychiatric research (especially that from an epidemiological perspective) has indicated that so-called traditional models of unitary diseases with single causes have a quite limited application in the field of mental disorders.