Familial transmission of major affective disorders. Is there evidence supporting the distinction between unipolar and bipolar disorders?

The two-threshold multifactorial polygenic (MFP) model was applied to blind family study data, collected in a long-term follow-up and family study of major affective disorders. This model tested whether bipolar and unipolar disorders are manifestations of the same underlying factors or if they are independently caused disorders. The hypothesis that bipolar and unipolar disorders are, respectively, severe and mild forms of the same disorder was supported. There was little evidence for different familial aetiologies for bipolar and unipolar disorders in our sample.