Mickey, thanks. There's actually been quite a lot of studying of shamanism gone on over the years - particularly by anthropology but not exclusively. Also, the drugs in different traditions often create the very illusions of astral travel, communing with other dimensions or gods, and so on that the beliefs are based upon. And, of course, most shamanic traditions actually use drugs of various kinds - though they usually attribute the healing power to gods rather than biochemistry. Most people who've meditated for a while will also tell you that these kinds of illusions are quite common (experiences that we interpret and give a narrative, often the proscribed one if we're believers). Part of having certain kinds of out-of-body and "supernatural" experiences seems to be an inheritable biological trait, and some people seem more prone to them, while they can also be triggered by drugs or certain practices.

It is well established that faith and also group faith have a rather large psychological power in tribal traditions. Shunning people who have been cursed, essentially shutting them out and averting eyes and so on which is quite common practice in shamanic traditions, is hard on social animals like humans - particularly in extremely interdependent societies where one is likely to die if excluded. I don't see this as being magic though, it's just basic human psychology. One shouldn't underestimate the placebo affect :-)

Having known a couple of Westerners who consider themselves shamen, from my experience so far it seems to be a facet of their mental illness combined with a desire to have power over others (this sense of superiority may also be a facet of the mental illness or perhaps they started out rather narcissistic). I wouldn't think tribal shaman are as likely to be suffering from a mental illness as Westerners since they're just filling a job post in their society - though of course there's lots of speculation about schizophrenia, OCD, and religiosity since it is a feature of certain kinds of mental illnesses. (Aliens seem to have replaced the devil as the Evil Other for schizophrenics, no doubt due to our culture being more secular).

I'm entirely skeptical of there being any actual magic at work, and there seem to be quite reasonable, non-magical explanations for much that is claimed to be magic. That said, it IS a fascinating topic - particularly if one puts aside all the western fantasies (and using shamanism as an excuse to get high, which I just find kinda lame personally).