Placebo effects can arise not only from a conscious belief in a drug but also from subconscious associations between recovery and the experience of being treated—from the pinch of a shot to a doctor’s white coat. Such subliminal conditioning can control bodily processes of which we are unaware, such as immune responses and the release of hormones.
Researchers have decoded some of the biology of placebo responses, demonstrating that they stem from active processes in the brain.
And from The Skeptic’s Dictionary:
The body’s neurochemical system affects and is affected by other biochemical systems, including the hormonal and immune systems. Thus, it is consistent with current knowledge that a person’s hopeful attitude and beliefs may be very important to their physical well-being and recovery from injury or illness.
Another popular belief is that a process of treatment that involves showing attention, care, affection, etc., to the patient/subject, a process that is encouraging and hopeful, may itself trigger physical reactions in the body which promote healing.
While it may be unethical to knowingly package, prescribe, or sell placebos as magical cures, complementary and alternative medicine folks seem to think they are ethical because they really believe in their chi, meridians, yin, yang, prana, vata, pitta, kapha, auras, chakras, energies, spirits, succussion, natural herbs, water with precise and selective memory, subluxations, cranial and vertebral manipulations, douches and irrigations, body maps, divinities, and various unobservable processes that allegedly carry out all sorts of magical analgesic and curative functions.
And, uh, Wiki:
Another factor increasing the effectiveness of placebos is the degree to which a person attends to their symptoms, “somatic focus”. Individual variation in response to analgesic placebos has been linked to regional neurochemical differences in the internal affective state of the individuals experiencing pain.
Those with Alzheimer’s disease lose the capacity to be influenced by placebos, and this is attributed to the loss of their prefrontal cortex dependent capacity to have expectations. Children seem to have greater response than adults to placebos.