The observer-expectancy effect

(also called the experimenter-expectancy effect, observer effect, or experimenter effect) is a form of reactivity, in which a researcher's cognitive bias causes them to subconsciously influence the participants of an experiment. It is a significant threat to a study's internal validity, and is therefore typically controlled using a double-blind experimental design.

An example of the observer-expectancy effect is demonstrated in music backmasking, in which hidden verbal messages are said to be audible when a recording is played backwards. Some people expect to hear hidden messages when reversing songs, and therefore hear the messages, but to others it sounds like nothing more than random sounds. Often when a song is played backwards, a listener will fail to notice the "hidden" lyrics until they are explicitly pointed out, after which they are obvious. Other prominent examples include facilitated communication, dowsing, and applied kinesiology[1].

Clever Hans was an Orlov Trotter horse that was claimed to have been able to perform arithmetic and other intellectual tasks.
Oskar Pfungst (1874-1933) was a German comparative biologist and psychologist. While working as a volunteer assistant in the laboratory of Carl Stumpf in Berlin, Pfungst was asked to investigate the horse known as Clever Hans, who could apparently solved a wide array of arithmetic problems set to it by its owner. After formal investigation in 1907, Pfungst demonstrated that the horse was not actually performing intellectual tasks, but was watching the reaction of his human observers.
Pfungst discovered this artifact in the research methodology, wherein the horse was responding directly to involuntary cues in the body language of the human trainer, who had the faculties to solve each problem. The trainer was entirely unaware that he was providing such cues.
In honour of Pfungst's study, the anomalous artifact has since been referred to as the Clever Hans effect and has continued to be important knowledge in the observer-expectancy effect and later studies in animal cognition.

[1] Applied kinesiology (AK) is a chiropractic diagnostic and treatment modality using manual muscle-strength testing for medical diagnosis and a subsequent determination of prescribed therapy. Nearly all AK tests are subjective, relying solely on practitioner assessment of muscle response. Specifically, some studies have shown test-retest reliability, inter-tester reliability, and accuracy to have no better than chance correlations.