Observer-Expectancy Effect: From The Outside Looking In


Let’s say you’re a researcher. You’ve developed a new energy drink that you claim will improve athletic performance, and you want to test whether this new drink actually has its intended effect. You take a sample of people who have around the same level of starting athletic ability and randomly assign each person to either the experimental group, where they’re given the real energy drink product, or the control group, where they’re given some other, similar-tasting drink that has none of the nutritional or chemical supplements that are in the real thing. Then you put both groups through the same exercise test. As the creator of this energy drink, you obviously hope that the experiment results will show that your product actually improves athletic performance. How do you think this personal bias will affect your experiment?

The Answer

Because of your own expectation that your energy drink will work, you might give a thumbs-up to the experimental group during their exercise tests but shake your head disappointedly at the control group. Your actions can show each group your expectations of how well they should be doing, and the experimental group might respond by pushing themselves extra hard in their exercise tests while the control group might get discouraged and put less effort than normal into their exercise tests. Thus, because you believe in your own product, you will probably either subconsciously misinterpret experiment data so that it backs up your claim about the energy drink improving athletic performance, or you’ll behave in a way that makes the subjects aware of your expectations and conform to those expectations. 

Definition of Observer Expectancy Effect

In an experiment, the observer-expectancy effect is when the researcher (the person conducting the experiment) either subconsciously influences the subjects in the experiment or incorrectly interprets results to line up with the outcome that the researcher originally hoped to see. 

Outside of an experimental setting, the observer-expectancy effect can take place whenever a person’s preconceptions of a given situation impacts their own behavior towards that situation. 


A classic example of an experiment that was tainted by observer-expectancy is the story of “Clever Hans.” Hans was a horse owned by Wilhelm von Osten, a teacher who claimed that Hans had learned to do basic mathematics and could understand German. Osten would ask Hans questions (in German) like "If the eighth day of the month comes on a Tuesday, what is the date of the following Friday?" and Hans would tap his hoof eleven times to signify that the date would be the 11th. 

Word spread through Germany about this super intelligent horse, and a number of psychologists including Carl Stumpf and Oskar Pfungst went to Osten’s home to investigate. Through a series of experiments, Pfungst determined that Hans could only answer questions correctly when in the presence of an audience that knew the answers to the questions that the horse was being asked. When the audience did not know, the horse also could not answer correctly. Eventually, Pfungst and Stumpf concluded that the horse did not truly understand math or German, but instead had learned to recognize certain facial and bodily cues of people in the audience that signaled when Hans approached the right number of hoof taps. At that point, Hans would stop tapping his hoof and appear to have solved a math problem when in reality, the horse just adapted his behavior to fit the expectations of the humans around him. 

Why Care?

The observer-expectancy effect can explain some of the differences in how parents raise their sons versus their daughters. For example, parents are typically less emotionally available and give out tougher punishments to their sons compared to their daughters due to the gender-biased expectation that boys require less emotional support and are capable of withstanding more harsh words or physical punishment. 

Observer-expectancy also frequently comes up in the handling of crime. In ambiguous situations where the culprit of a crime cannot be clearly identified and there are multiple suspects, the public tends to accuse men of color because men are perceived as more aggressive and people of color are often stereotyped as criminals. This can lead to many wrongful convictions of men of color.

Think Further

  1. Name some real-life examples where people experience the observer-expectancy effect.
  2. What are some negative consequences of the observer-expectancy effect?
  3. Can you think of any situations where observer-expectancy can have a positive impact on society? If so, explain the situation and how observer-expectancy will positively impact society.


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Learn More

  1. Bosma, Alice, et al. “Observer Reactions to Emotional Victims of Serious Crimes: Stereotypes and Expectancy Violations.” Psychology Crime & Law, 2018, pp. 1–21.
  2. Briton, Nancy, and Judith Hall. “Gender-Based Expectancies and Observer Judgments of Smiling.” Journal of Nonverbal Behavior, vol. 19, no. 1, 1995, pp. 49–65., doi:10.1007/BF02173412.
  3. Buscombe, Richard, et al. “Expectancy Effects in Tennis: The Impact of Opponents’ Pre-Match Non-Verbal Behaviour on Male Tennis Players.” Journal of Sports Sciences, vol. 24, no. 12, 2006, pp. 1265–1272., doi:10.1080/02640410600598281.
  4. Schwartz, Stephan A., and Larry Dossey. “Nonlocality, Intention, and Observer Effects in Healing Studies: Laying a Foundation for the Future.” EXPLORE, vol. 6, no. 5, Sept. 2010, pp. 295–307. ScienceDirect, doi:10.1016/j.explore.2010.06.011.