A person’s reaction to “empty” (zero) influences corresponding to the reaction in the presence of a real influence. Discovered by Feldman in 1956: Patients felt relief when they believed in the treatment. and not its use; it happens with the release of beta-endorphins – natural painkillers and antidepressants. In experimental psychology a “placebo” is a “null” effect. which is the absence of an effect that the individual is not warned about.