There is no chemical imbalance, nor has there ever been

The Epidemic of Mental Illness: Why?
Marcia Angell
New York Review of Books

Excerpt from review of
- The Emperor's New Drugs: Exploding the Antidepressant Myth (Irving Kirsch)
- Anatomy of an Epidemic: Magic Bullets, Psychiatric Drugs, and the Astonishing Rise of Mental Illness in America (Robert Whitaker)
- Unhinged: The Trouble with Psychiatry - A Doctor's Revelations about a Profession in Crisis (Daniel Carlat)

Second, none of the three authors subscribes to the popular theory that mental illness is caused by a chemical imbalance in the brain. As Whitaker tells the story, that theory had its genesis shortly after psychoactive drugs were introduced in the 1950s. The first was Thorazine (chlorpromazine), which was launched in 1954 as a “major tranquilizer” and quickly found widespread use in mental hospitals to calm psychotic patients, mainly those with schizophrenia. Thorazine was followed the next year by Miltown (meprobamate), sold as a “minor tranquilizer” to treat anxiety in outpatients. And in 1957, Marsilid (iproniazid) came on the market as a “psychic energizer” to treat depression.

then yada yada yada ..... But over the next decade, researchers found that these drugs, and the newer psychoactive drugs that quickly followed, affected the levels of certain chemicals in the brain.

more yada yada yada ... Thus, instead of developing a drug to treat an abnormality, an abnormality was postulated to fit a drug.....That was a great leap in logic, as all three authors point out. It was entirely possible that drugs that affected neurotransmitter levels could relieve symptoms even if neurotransmitters had nothing to do with the illness in the first place (and even possible that they relieved symptoms through some other mode of action entirely). As Carlat puts it, “By this same logic one could argue that the cause of all pain conditions is a deficiency of opiates, since narcotic pain medications activate opiate receptors in the brain.” Or similarly, one could argue that fevers are caused by too little aspirin.

But the main problem with the theory is that after decades of trying to prove it, researchers have still come up empty-handed.
Psychiatrist and author Carl Jung would not subscribe to the biochemical imbalance theory were he alive today.

These forces did not originate in our patient out of nowhere. They are most emphatically not the result of poisoned brain cells, but are normal constituents of our unconscious psyche. They appeared in numberless dreams, in the same or a similar form, at a time of life when seemingly nothing was wrong. And they appear in dreams of normal people who never get anywhere near a psychosis. (1939)