The placebo effect is a somewhat well known phenomenon. Imagine you take a group of people with a headache, you give them a mint pill but you tell them that it was an aspirin with mint flavour. What would happen? A few people will start feeling better; some may even stop having their headache. That is, even if they do not really take a pain killer, somehow, the fact that they think they are taking it actually changes the way they feel: they feel better. This is the placebo effect.
The nocebo effect is its evil twin and produces the opposite results. If you take a group of people that feel fine, and you give them a mint pill and tell them that it is going to give them nausea and a headache, there will be some people that will actually start feeling ill. Studies on the placebo and nocebo effect suggest that perception is not merely a result of external stimuli, but it is not yet clear how exactly our beliefs or expectations alter our perceptions. So thinking good thoughts and outcomes make you feel better!
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