Bipolar Disorder (BD) and Major Depressive Disorder (MDD) are two of the most common mental illnesses causing disability in the world. Bipolar depression has been viewed as more endogenous than unipolar depression, based on significant evidence that mania is biologically driven. As a result of this difference in diagnostic categories, mood disorder research tends to focus on either bipolar disorder in general or unipolar depression in particular. Only a few studies have compared unipolar and bipolar depression. If the hypothesis of bipolar depression as distinct is correct, first, biological evidence should be more evident in bipolar depression than in unipolar depression. For example, bipolar depression may have a larger genetic contribution than unipolar depression. Second, one could expect bipolar and unipolar depression to progress in different ways. Third, psychosocial depression triggers should be less