How should mood disorders be modelled?

Classification of any mental disorder is likely to have clinical utility only if it is based on a valid underlying model. The depressive disorders have long provoked debates as to whether a categorical or a dimensional model is all explanatory. This paper will argue that no single (categorical or dimensional) model is likely to be valid, and that a mix of models is required to classify, diagnose and shape management decisions for the mood disorders. After reviewing limitations to the dimensionally based official classificatory systems (DSM-IV and ICD-10), and noting some of the consequences, a set of alternative strategies is outlined. In essence, identifying syndromal 'fuzzy sets' from phenotypic and aetiological clustering, a model that occurs in the rest of medicine.