Diagnostic models of illness shape the directions of research, treatment, and theory. There are three such models in psychiatry: typological, dimensional, and mixed typological-dimensional. Of these, the typological model is by far the most popular.
A review of pertinent studies suggests, however, that this popularity is not based on clinical validity or the nature of psychiatric variables. On the other hand, the mixed diagnostic models appear to be too complex for general use. There is increasing evidence that the dimensional diagnostic model best fits the nature of psychiatric data and might therefore provide the most fruitful basis for progress in psychiatric research and clinical practice.