It has long been recognized that there are substantial multifactorial placebo effects that create real and illusory improvements in response to even an inactive treatment. There is a tendency, however (especially in popular discussion), to oversimplify placebo effects – to treat them as one mind-over-matter effect for all outcomes. Meanwhile researchers are elucidating the many mechanisms that go into measured placebo effects, and the differing magnitude of placebo effects for different outcomes.
For example, placebo effects for pain appear to be maximal, while placebo effects for outcomes like cancer survival appear to be minimal.
A recent study sheds additional light on the expectation placebo effect for pain. The effect is, not surprisingly, substantial. However it does not extrapolate to placebo effects for outcomes other than pain, and the results of this very study give some indication why. From the abstract:
The effect of a fixed concentration of the μ-opioid agonist remifentanil on constant heat pain was assessed under three experimental conditions using a within-subject design: with no expectation of analgesia, with expectancy of a positive analgesic effect, and with negative expectancy of analgesia (that is, expectation of hyperalgesia or exacerbation of pain).
What they found was that the positive expectation group reported twice the analgesic effect as the no expectation group, and the negative expectation group reported no analgesic effect. This is a dramatic effect, but not surprising.