Is the placebo effect a form of cognitive bias? Recently I’ve been trying to get a better understanding for the cause and mechanism of the placebo effect. A friend suggested that the placebo effect is a form of cognitive bias, but I’ve had trouble finding evidence for this.
The question is not terribly clear since it can be just an issue of categorizing the placebo effect in a philosophical sense as a "cognitive bias" for some broad meaning of the latter. But...


  Some have argued that neuroimaging technologies such as functional magnetic resonance (fMRI) and positron emission tomography (PET) can help determine whether placebo effects are independent of response bias (27). For example, one team of researchers has reported that placebo responses occur in pain-related areas of the brain during the time of stimulation and not only during assessment (28), while other researchers have shown that spinal cord mechanisms are involved with placebo analgesia (29). These experiments seem to indicate that at least some of the observed effect of placebo in an experimental setting is independent of response bias; however, they cannot rule out the hypothesis that some of the observed clinical effect is due to response bias.


Note that response bias, whith which the placebo effect is contrasted above, is a specific [cognitive] bias.