and predisposition models is to use prospective longitudinal studies of persons with no prior history of mood disorder to determine whether particular personality traits predict the later onset of depressive disorder. Although no single design can distinguish among these four models, the combination of designs can bolster the case for particular accounts. For example, finding substantial common genetic variance in twin studies, but no evidence of developmental sequencing in longitudinal studies, would support the common cause and continuum/spectrum models. In turn, these two models could be compared by examining the specificity of the association between trait and disorder and whether there is a nonlinear relation between trait level and probability of disorder. On the other hand, if there were evidence of developmental sequencing in longitudinal studies as well as substantial common genetic variance in twin studies (or overlap of other etiological factors in other designs), it would support the precursor model (particularly if the trait was also phenomenologically similar to depression). In contrast, developmental sequencing but less shared genetic (or other etiological) variance would support the predisposition model. Also crucial for the predisposition model is evidence from longitudinal studies demonstrating that other variables (e.g., life stress) moderate or mediate the
