Psychosomatic cures for ills and other kinds of magical but not 'scientific' treatment for humans' problems are highly controversial. To scientists, who believe in causation via known or at least plausible mechanisms, the mind is one thing, the body another. Perhaps we tend to cling to Descartes' mind-body dualism, despite our materialistic worldview.
In that view the body is material, but the mind isn't. Of course the mind can affect the body, as when you run like hell if someone is chasing your (or run like hell after an object of your affections). So how and where the distinction lies is never clear. Today we don't accept the dualism: even if we know we cannot yet explain what consciousness or 'mind' are, we believe they can in principle be accounted for by the fleshy stuff and the genes that control it. It's a chemical, cellular phenomenon of some sort. But if you're feeling poorly, much less if you have a clogged artery or cancer, how can you possibly affect it with your 'mind'?
We don't believe that seeing is believing: we want to know how, if we are to accept that, something is true. But why then is there so much evidence that believing is being--that one's state of mind can in fact bring about the wished for, imagined, or intended state? We're triggered to write by a CNN story that reports, among other things, a paper showing that the placebo effect works even when the participants know they are being treated with a placebo! This study found this effect for patients with irritable bowel syndrome.
Let's assume, undoubtedly correctly, that there was no fraud or sleight of hand in the study, and no aspects that were not reported or even that the investigators were unaware of. How can their results be? Perhaps just the idea that you were in a study, that a doctor was monitoring your health, and so on, boosts your morale.....and then what?
The biological question is how your body can be instructed by your mind to improve specific symptoms--not just your mood but your actual physical health. Genetically, and evolutionarily, what could be the mechanism? Unless this is restricted to humans, our ancestral species had no way to have the experience of being 'treated' for a problem, unless just tender loving care by relatives, or the companionship of comrades in your troop boosted your morale. But even so, or even if this evolved with humans only, based perhaps on the mood-altering powers of language, how does your mind know what part of your body, what feedback messages from that part, to correct and how to correct it?
One might imagine fitness advantages for individuals who could self-cure, or group advantages for groups with members who could maintain health in this way. But one can imagine almost anything, and it is difficult to get one's head around how genetic mechanisms could exist, or evolve, that can generate the effect.
And what if, like the ability to play a violin or do calculus is a by-product of some other selective advantage in the past? That's a standard, legitimate explanation of many of our abilities that clearly did not even exist in the past when our genomes evolved. Did our ancestors' brains evolve the ability to apply neural feedback mechanisms to and from, say, injured or ill parts in a way that the brain could use trial and error to try to fix the problem? That, at the time, would have been mechanical and abstract--not based on doctors (witch or otherwise) but simply biofeedback. The brain could adjust hormone levels, blood flow, perhaps immune activity to the affected part, and in some cases it may have worked.
That could easily have provided a fitness advantage. And it would involve no mystic or new or secret mechanisms, but just what we already know about. If something like that is what happened, the use of cultural experience (language, symbolic settings like clinics, and so on) could have picked up the same mechanism, and be reflected in the various placebo effects that are by now very well know.