A number of research designs can be useful in studying the relation between personality and depressive disorders. The common cause, continuum/spectrum, precursor, and predisposition models would all be supported by family studies demonstrating personality differences between nonaffected relatives of probands with and without a history of depression. The common cause, continuum/spectrum, and precursor models would be supported by twin and genetic association studies demonstrating that the same genes predispose to both personality and depressive disorders. The precursor and predisposition models posit that personality abnormalities are trait markers and hence should be present prior to the onset, and after recovery from, depressive episodes. Hence, these models can be tested by comparing individuals with a history of depression that is currently in remission to persons with no history of depression on relevant personality traits. An even stronger approach to testing the precursor 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
