What is the origin of the idea that mental disorders are caused by chemical imbalances in the brain?
American psychiatrists of the 1960s and 1970s tended to be skeptical of biological theories of mental disorder. Many psychiatrists of that era were trained in the heyday of Freudian psychoanalysis and thought biochemical theories of psychiatric illness to be, at best, overly simplistic and reductionistic explanations of much more complicated psychological phenomena.
It was not until the late 1980s, after the release of the landmark antidepressant Prozac, that the idea of the chemical imbalance hit the psychiatric mainstream. As the psychiatrist Peter Breggin (1991) points out in his book Toxic Psychiatry, the drug company Eli Lilly advanced the chemical imbalance theory as a marketing scheme to sell their new drug Prozac.
There was, of course, no demonstrable evidence showing that depressed patients had any imbalance, but Lilly ran with it. Before long, psychiatrists and psychiatric patients alike came to identify with the idea that mental disorders are caused by chemical imbalances in the brain.