Gary Greenberg, a psychotherapist, writing about antidepressants in the New Yorker:
The serotonin-imbalance theory, however, has turned out to be just as inaccurate as Schildkraut’s. While S.S.R.I.s surely alter serotonin metabolism, those changes do not explain why the drugs work, nor do they explain why they have proven to be no more effective than placebos in clinical trials. Within a decade of Prozac’s approval by the F.D.A. in 1987, scientists had concluded that serotonin was only a finger pointing at one’s mood—that the causes of depression and the effects of the drugs were far more complex than the chemical-imbalance theory implied. The ensuing research has mostly yielded more evidence that the brain, which has more neurons than the Milky Way has stars and is perhaps one of the most complex objects in the universe, is an elusive target for drugs.
Despite their continued failure to understand how psychiatric drugs work, doctors continue to tell patients that their troubles are the result of chemical imbalances in their brains. As Frank Ayd pointed out, this explanation helps reassure patients even as it encourages them to take their medicine, and it fits in perfectly with our expectation that doctors will seek out and destroy the chemical villains responsible for all of our suffering, both physical and mental. The theory may not work as science, but it is a devastatingly effective myth.
The placebo effect is strong. I imagine a fervent belief in the placebo’s efficacy by the prescriber strengthens the effect.