<P> In the mid-20th century, researchers theorized that depression was caused by a chemical imbalance in neurotransmitters in the brain, a theory based on observations made in the 1950s of the effects of reserpine and isoniazid in altering monoamine neurotransmitter levels and affecting depressive symptoms . During the 1960s and 70s, manic - depression came to refer to just one type of mood disorder (now most commonly known as bipolar disorder) which was distinguished from (unipolar) depression . The terms unipolar and bipolar had been coined by German psychiatrist Karl Kleist . </P> <P> The term Major depressive disorder was introduced by a group of US clinicians in the mid-1970s as part of proposals for diagnostic criteria based on patterns of symptoms (called the Research Diagnostic Criteria, building on earlier Feighner Criteria), and was incorporated into the DSM - III in 1980 . To maintain consistency the ICD - 10 used the same criteria, with only minor alterations, but using the DSM diagnostic threshold to mark a mild depressive episode, adding higher threshold categories for moderate and severe episodes . </P> <P> The ancient idea of melancholia still survives in the notion of a melancholic subtype . The new definitions of depression were widely accepted, albeit with some conflicting findings and views, and the nomenclature continues in DSM - IV - TR, published in 2000 . </P> <P> There have been some continued empirical arguments for a return to the diagnosis of melancholia . There has been some criticism of the expansion of coverage of the diagnosis, related to the development and promotion of antidepressants and the biological model since the late 1950s . A study found that Afghans, have the highest rate of depression in the world . </P>

Who was the first person diagnosed with depression