Data on the nature of the relation between depressive personality and depressive disorders are consistent with most of the causal models described above, illustrating the complexity of the associations between personality and depression. The strongest support for the common cause and continuum/spectrum models derives from twin and family studies. In a large twin study, Ørstavik et al. (2007) found that depressive personality and major depressive disorder (MDD) shared substantial genetic variance, although there was evidence for unique genetic factors as well. Family studies indicate that individuals with depressive personality have an increased rate of mood disorders in their first-degree relatives (e.g., Klein & Miller 1993). In addition, patients with MDD, particularly those with chronic forms of depression, have elevated levels of depressive personality traits in their first-degree relatives (Klein 1999).
