From Neurologica Blog:
Expectation bias cuts both ways, for positive and negative expectations. Expectation bias, the tendency to perceive and accept data that reinforces your expectation, is one of the many contributors to placebo effects (the illusion of a positive benefit that derive from something other than an active treatment). It is also, however, part of nocebo effects (the illusion of negative side effects from something other than active treatment).
Expectation bias can be powerful enough in some people to lead not only to the perception of a benefit or side effect but to a frank delusion. When this happens on a large scale, that can lead to a mass delusion. There are many episode that demonstrate this effect, but now there is also a controlled experiment that also confirms it.
A recent study looked at sham exposure to wifi signals in 147 subjects. They were first exposed to either a documentary about the dangers of wifi, or to a documentary about internet security. A total of 54% of the subjects experienced
“…agitation and anxiety, loss of concentration or tingling in their fingers, arms, legs, and feet. Two participants left the study prematurely because their symptoms were so severe that they no longer wanted to be exposed to the assumed radiation.”