How logically rigorous is the claim that neurochemical changes in the brain 'cause' mood or emotional disorders? Does a running nose cause a cold? In any case, before prescribing powerful chemicals to emotionally distressed patients shouldn't doctors use some sort of machine to test the chemical levels of their brains?

You're right: we shouldn't throw the word "causes" around too casually. Let's fix on depression as our example, and let's keep in mind that simply being sad isn't the same as being clinically depressed. On the one hand, neurochemicals probably aren't just symptoms of depression; they probably have something to do with causing the symptoms -- the listlessness or anxiety, or excessive rumination or protacted feelings of sadness. Perhaps it would be more accurate to say that clinical depression is, at bottom, a malfunction in the neurochemical system, though this may be too reductionistic, and it also may turn out not to get the biology right. But perhaps what you're pointing to is that it still makes sense to ask what causes this malfunction in the first place. That's obviously a very good question. My impression is that sometimes life circumstances can trigger depression, but sometimes there's no clear external cause. The right answer here is likely to be very complicated.

At the moment, far as I know, we have no good way of testing the functioning of the neurochemical system itself. Perhaps we will some day; perhaps we'll develop a blood test or scanning technique that will tell us when someone's neurochemical system is out of whack and allow for a biological diagnosis of mental illnesses. Until then, we have to make trade-offs. Some of the chemicals we use to treat psychiatric conditions have serious side effects. Unfortunately, however, untreated mental illness has serious side effects of its own, including death.

Psychiatric medications are tested for safety. We have reasonably good but imperfect information about what percentage of people taking them are likely to develop which side effects. And so we have a basis for making a trade-off: if a patient has serious symptoms, if we have evidence that a certain medication can help alleviate the symptoms, and if the risk of side-effects is not too great, it might well make sense to try the medication even if we aren't sure what's really going on in the brain. All of this should be monitored carefully, of course, and physicians shouldn't be too quick to give out medications when other approaches (cognitive behavioral therapy, for example) might do the job with lower risk of side effects. But I think that what we might call "chemophobia" -- fear of medications -- is potentially just as dangerous as overprescription.

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