“Instead, the researchers theorize, patients seem to improve after taking placebos because most diseases have uneven courses in which their severity waxes and wanes. In studies in which treatments are compared not just with placebos but also with no treatment at all, they said, participants given no treatment improve at about the same rate as participants given placebos.”
http://www.nytimes.com/2001/05/24/us/placebo-effect-is-more-myth-than-science-study-says.html
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