A phenomenon in which perception and behavior changes as a result of personal expectations or the expectations of others.

The expectation effect refers to ways in which expectations affect perception and behavior. Generally, when people are aware of a probable or desired outcome, their perceptions and behavior are affected in some way. A few examples of this phenomenon include:1

Halo Effect—Employers rate the performance of certain employees more highly than others based on their overall positive impression of those employees.

Hawthorne Effect—Employees are more productive based on their belief that changes made to the environment will increase productivity.

Pygmalion Effect—Students perform better or worse based on the expectations of their teacher.

Placebo Effect—Patients experience treatment effects based on their belief that a treatment will work.

Rosenthal Effect—Teachers treat students differently based on their expectations of how students will perform.

Demand Characteristics—Participants in an experiment or interview provide responses and act in ways that they believe are expected by the experimenter or interviewer.

The expectation effect demonstrates that expectations can greatly influence perceptions and behavior. For example, tell a large group of people that a new product will change their lives, and a significant number will find their lives changed—the belief is simply a device that helps create the change. Once a person believes something will happen, the belief alone creates that possibility. Unfortunately, this can have a negative impact on the ability to accurately measure a design’s success. Since designers are naturally biased toward their designs, they often unintentionally influence test subjects through words or actions, or may omit certain results in order to corroborate their expectations. Test subjects often respond by seeking to meet the expectations communicated to them.

Consider the expectation effect when introducing and promoting a design. When trying to persuade, set expectations in a credible fashion for the target audience rather than letting them form their own unbiased conclusions. When evaluating a design, use proper test procedures to avoid biases resulting from the expectation effect.