Brain's Own Pain Relievers At Work in Placebo Effect,

Scientific American: Brain's Own Pain Relievers At Work in Placebo Effect, Study Suggests:
Sometimes, just thinking you are receiving treatment is enough to make you feel better, a phenomenon known as the placebo effect. Scientists have long wondered what causes this outcome, the magnitude of which is not the same for all people. A new brain imaging study suggests that the body's natural painkillers, endorphins, play a significant role.