Validating ‘hard’ and ‘soft’ phenotypes within the bipolar spectrum: continuity or discontinuity?

The unitary Kraepelian concept of manic-depressive illness which incorporated attenuated forms, personal dispositions to mood instability, as well as much of the terrain of remitting depressions, may be considered by many to be too broad. On the other hand, the presently preferred unipolar–bipolar dichotomy in official nosology fails to account for the very common occurrence of clinical and subclinical conditions in the interface of major depressive disorders and bipolarity. The emerging concept of the bipolar spectrum represents a provocative working hypothesis to account for these conditions.