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The observer-expectancy effect (also called the experimenter-expectancy effect, expectancy bias, observer effect, or experimenter effect) is a form of reactivity in which a researcher's cognitive bias causes them to unconsciously influence the participants of an experiment. This can be through the incorrect interpretation of results, due to the expectancy of certain results, or through the attitude and personality of the observer affecting the participant’s responses.

Observer-Expectancy Effect of Observer
Confirmation bias can lead to the experimenter interpreting results incorrectly because of the tendency to look for information that conforms to their hypothesis, and overlook information that argues against it. 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 this was shown when groups of observers were given different information as to what to expect from the behaviour of planaria. Higher rates of movement were recorded by observers who were told to expect a large amount of movement. Similarly, observers who were told to expect a low rate of movement recorded fewer moves and turns of the planaria. The observers’ expectations skewed their observation and report of the movements. Another study found that expectancy through practice changed the vigilance of participants when performing a visual detection task. Participants who were lead to expect high signal probablity, were more vigilant and therefore better at the task. This effect has also been found in different types of research including, avian observation, testimonial evidence , and gross motor performance.

Observer-Expectancy Effect on Subject
The subject of the observation can be affected by the attitude or expectancy of the observer. Evidence for the effect of the observer’s expectations has been found by giving researchers an expectation about the results they would gain from their participants. In this particular study, participants were asked (with identical instructions) to rate pictures of faces using a scale that ran between success and failure. The experimenters were told before the research took place that they would probably obtain a high mean rating or they were told they would probably obtain a low mean rating. Results found that the researchers did record the numbers that they were told they should expect, in fact, the lowest mean rating from the experimenters expecting higher ratings were higher than the highest mean ratings from the experimenters expecting low ratings. Something about the expectancy that the researchers were holding, affected the participants’ responses enough to skew the data. The attitude of the observer has also been found to have an effect on the participant. An independent group of people, watched researchers who had been filmed carrying out their experiment, and then described them using words like ‘cold’ or ‘friendly’. The experiments were then investigated to see whether there was a difference between researchers. In fact there was researchers who had been described as ‘cold’ were found to record lower scores from their participants than those who had been described as ‘friendly’. This shows a profound effect on the data that can be caused simply by the attitude or personality of the observer.

Richard Wiseman
A particularly unusual case of observer-expectancy bias was produced by Richard Wiseman whilst researching the ability to detect an unseen gaze. Wiseman and his colleague Marilyn Schiltz each attempted to replicate studies relating to participants being able to detect an unseen gaze even when the observer was in a different room, however, whilst Schiltz’s results were positive, Wiseman failed to discover any statistically significant results. In an attempt to discover why the results were so different, Wiseman and Schiltz ran simultaneous studies using the same location, equipment, procedures, and subject pool. The only difference being the experimenter themselves. The study involved participants’ electrodermal activity being measured during ‘gaze’ and ‘non-gaze’ trials, where the experimenter directed their attention to the participant through a closed-circuit television, or directed their attention away from the participant. Despite the efforts to remove any extraneous variables, Schiltz’s results were found to be significant, whereas Wiseman’s were not. Wiseman and Schiltz suggested several suppositions regarding what may have caused the observer-expectancy effect even when the participant was unable to see them; including, participant cheating, methodological error, fraud, Schiltz being more skilled at provoking her participants’ psi ability, or Schiltz having a more ‘psychically’ gifted participant group. Though they did not come to any specific conclusions as to what could be causing this, it was clear that the observer affected the participants’ ability to detect unseen gaze.

Clever Hans
Clever Hans was an Orlov Trotter horse, who was supposedly able to perform arithmetic by tapping the answer on the floor. After an investigation, psychologist Oskar Pfungst discovered that the horse was picking up cues from the person asking the question. When the horse’s trainer knew the answer to the mathematic question, the horse got 89% right, but when the trainer didn’t know the answer, he only got 6% right. By watching to body language of the questioner, Pfungst worked out how the horse was completing these sums. He eventually found that the horse was following unintentional physical movements from the questioner, it would begin tapping and observe the posture and facial expression of questioner. When the horse made the final tap correctly, the questioner would show an unconscious release of tension, which alerted the horse that he had guessed correctly and that he should stop tapping. This explained why when the questioner did not know the answer to the question, the horse wasn’t capable of tapping the correct answer every time. This phenomenon was actually an example of the observer-expectancy effect, the expectation of the questioner gave the horse the cues for a certain behaviour, leading to it appearing to have mastered arithmetic.

Backmasking
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. In fact, in 1985 Vokey and Read concluded their research into backmasking and subliminal messages by saying; "the apparent presence of backward messages in popular music is a function more of active construction on the part of the perceiver than of the existence of the messages themselves.”

Facilitated Communication
Another example of the observer-expectancy effect, is facilitated communication. This is a way for severely autistic or disabled children, who sometimes cannot read, write or speak to communicate with others. It involves the use of a keyboard and a ‘facilitator’ who holds the child’s arm or hand as they ‘type’. The idea is that the children have the mental capacity to understand and answer questions, but are not physically able to do so. However, after scientific testing, it became apparent that the subjects were not capable of communication but that the facilitator was unconsciously guiding the child’s answers. It was found that the children were unable to name stimuli that were unseen by the facilitator, if the facilitator was shown a different image to the child, the child would ‘type’ the name to the object the facilitator had seen, and that often when ‘typing’ the child wasn’t even looking at the keyboard. The studies that refuted Facilitated Communication used both single blind and double blind procedures in order to prove that the child was not communicating, but that the facilitator was unconsciously directing them. Another prominent example of the observer-expectancy effect is dowsing.