Although not a "key study," the Serotonin Hypothesis (also known as the 5-HTT Hypothesis) is a key theory used to explain the origins of depression. Coppen"s (1967) original theory argued that a deficit in 5-HTT was the primary cause of depression. His theory was based on finding low levels of metabolites of serotonin in the cerebrospinal fluid of depressed patients. The modern theory argues that dysfunctions of serotonergic...