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Professor David Healy, in his clinic in Bangor, a town in the north of Wales. He has written the most detailed history of antidepressants we have. When it comes to the idea that depression is caused by low serotonin, he told me: "There was never any basis for it, ever. It was just marketing copy. At the time the drugs came out in the early 1990s, you couldn't have got any decent expert to go on a platform and say, 'Look, there's a lowering of serotonin in the brains of people who are depressed' ... There wasn't ever any evidence8 for it." It hasn't been discredited, he said, because "it didn't ever get 'credited,' in a sense. There wasn't ever a point in time when 50 percent of the field actually believed it." In the biggest study of serotonin's effects on humans, it found no direct relationship9 with depression. Professor Andrew Skull of Princeton has said attributing depression to low serotonin is "deeply misleading and unscientific."