Placebo effect
From RationalWiki
The placebo effect is the phenomenon that merely being exposed to trappings of a treatment for a disease can make the symptoms of the disease less or cure them completely. The effect is probably due to a combination of feeling less stress, thinking positively, and other mental and physical effects of believing that you are being cured. These improvements appear whether or not the treatment itself actually has any effect. Research has shown that expensive placebos are more effective than cheaper ones.[1]
Because the placebo effect is so powerful, efficacy claims for medical treatments must be tested in an experiment that controls for this effect. This is usually achieved by running a double-blind test, where some patients receive the treatment being examined, and others get a placebo. Neither the patients or the administrators of the treatment know who is in which group. In order for the treatment to be proved useful, it must produce results that are better than the placebo at a statistically significant level.
Quacks who push woo treatments rely on the placebo effect when arguing that their cures have an effect.
