Background: The extant major psychiatric classifications – DSM-IV and ICD-10 – are purportedly atheoretical and largely descriptive. While this achieves good reliability, the validity of a diagnosis is greatly enhanced by an understanding of aetiology. In an effort to group mental disorders on the basis of aetiology, five clusters have been proposed. The purpose of this paper is to find the extent to which a division between bipolar disorder and unipolar depression can be justified, and to consider whether there is a justification for considering a psychosis cluster containing both bipolar disorder and schizophrenic spectrum disorders.
Method: We reviewed the literature about bipolar disorder and unipolar depression in relation to 11 validating criteria proposed by the DSM-V Task Force Study Group, and then summarised similarities and differences between bipolar disorder and schizophrenia on the one hand, and unipolar depression on the other.
Results: There are differences, often substantial and never trivial, for each of the 11 validators between unipolar and bipolar depression. This is because the mania genes and the four biomarkers (shared with schizophrenia) occur only in bipolar disorder, whereas unipolars have high negative affect and low extraversion, bipolars have high extraverson and lower negative affect.
Conclusion: Bipolar disorder has previously been classified, together with unipolar depression – but this is the least justifiable place for it. If placed in a “psychosis cluster” there are several important respects in which it differs from schizophrenia, so the cluster would have a division within it.