display | more...
The polar opposite of the placebo effect. "Placebo" is Latin for "I will placate;" "Nocebo" means "I will do harm." When a test subject is informed of a danger to himself, he will feel the effects of that danger, regardless of whether such a danger exists. From overlawyered.com:
When a newspaper at which I worked moved to a new building somebody told the librarian that there was formaldehyde in her new library bookshelves. Formaldehyde is a suspected human carcinogen and the librarian knew this. Soon she was suffering from a headache, aching joints, and labored breathing -- all classic psychosomatic symptoms.

But then she heard there was no formaldehyde in the shelves. Suddenly the symptoms disappeared. Apparently the final word on the shelves was that they did contain formaldehyde but the librarian remained blissfully ignorant of this and hence symptom free.