In psychology, the two main hypotheses of the placebo effect are expectancy theory and classical conditioning.[70]

  In 1985, Irving Kirsch hypothesized that placebo effects are produced by the self-fulfilling effects of response expectancies, in which the belief that one will feel different leads a person to actually feel different.[71] According to this theory, the belief that one has received an active treatment can produce the subjective changes thought to be produced by the real treatment. Similarly, the appearance of effect can result from classical conditioning, wherein a placebo and an actual stimulus are used simultaneously until the placebo is associated with the effect from the actual stimulus.[72] Both conditioning and expectations play a role in placebo effect,[70] and make different kinds of contributions. Conditioning has a longer-lasting effect,[73] and can affect earlier stages of information processing.[74] Those who think a treatment will work display a stronger placebo effect than those who do not, as evidenced by a study of acupuncture.[75]

The hypotheses hinge on the beliefs of the recipients. "The placebo effect" has always been largely psychological. That's the realm of belief.

To veer even further off-tangent, isn't it hilarious how the Wikipedia illustration of old Placebo bottles indicate that "Federal Law Prohibits Dispensing without a Prescription". Wouldn't want some placebo fiend to O.D.