Christopher Lane in Psychology Today:
Almost as soon as it was floated in 1965 by Harvard psychiatrist Joseph Schildkraut, the serotonin hypothesis of depression—reduced and simplified by pharma marketing to the “chemical imbalance” theory of depression and anxiety—has been subject to critical research and found wanting.
The poor standing of the hypothesis in the scientific literature, however, barely dented its afterlife in textbooks, across clinical and treatment settings, and on mental health apps and websites. Nor has it dispelled the continued use of the phrase as “shorthand” between doctors and patients and in everyday settings, including for quite different mental states and conditions.
More here.