Are there different genotypes in Bipolar II and Bipolar I disorder and if so, why then do we tend to observe Unipolar Depression converting to Bipolar II and then converting to Bipolar I?

We review the recent literature in order to establish the importance of a spectrum for bipolar affective disorder, and that unipolar depression, bipolar II and bipolar I are discrete entities that may however evolve in sequence. We discuss clinical, genetic and neurobiological data which illustrate the differences between bipolar I and bipolar II. To fit the data we suggest a series of multiple mood disorder genotypes, some of which evolve into other conditions on the bipolar spectrum. Thence we discuss the nature of the bipolar spectrum and demonstrate how this concept can be used as the basis of a staging model for bipolar disorder.