*Chemical imbalance — Sloppy thinking in psychiatry, part 1*
http://blog.stevenreidbordmd.com/?p=561

“Chemical imbalance” is a phrase used by psychiatrists and laypeople alike.
When a mental problem seems to arise from within instead of without, it is
said to be due to a chemical imbalance. In truth, however, no chemical
imbalance, nor any structural abnormality in the brain, has ever been found
to account for anything we currently consider a psychiatric disorder.
Historically, whenever chemical or structural abnormalities *were* found
to account for abnormal mental functioning, those conditions were no longer
considered psychiatric and were
adopted by another branch of medicine. If this trend continues, psychiatry
will never include pathophysiology in the usual medical sense. It
certainly does not at present.

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