Martin Seligman (of both "learned optimism" and "learned helplessness" fame) showed that rats' immune systems are influenced by their psychological state, speci

Martin Seligman (of both "learned optimism" and "learned helplessness" fame) showed that rats' immune systems are influenced by their psychological state, specifically, the extent of control they had over unpleasant stimuli. This included tumour reduction. In the light of this, the placebo effect does not seem so far-fetched. My personal theory is that when an animal feels safe and secure in a non-threatening environment, the immune system is stimulated, because the immune system requires resources, and the body can spare these when it is safe. When volunteers are given placebos, in a controlled and professional setting, it is clearly a safe and stress-free environment. (This could be tested by putting the subjects in the same safe setting, but not giving them the placebo).