The continuum/spectrum model emphasizes the conceptual overlap between depressive disorders and certain personality traits and argues for a fundamental continuity between them. A depressive diagnosis is thought to simply identify individuals who have the most extreme scores on a relevant trait. Like the common cause model, the continuum/spectrum model assumes that personality and depression arise from a similar, if not identical, set of causal factors. However, the continuum/spectrum model goes further in positing that the association between the trait and disorder should be fairly specific because they are on the same continuum.1 Moreover, this association is expected to be nonlinear, so that almost nobody below the definitional threshold on the trait has the diagnosis but nearly everyone above the threshold meets the criteria. Thus, the continuum/spectrum model would be supported by evidence that the trait and depression are associated with the same etiological influences and that the trait-disorder relationship is fairly specific and nonlinear.
