Many experiments require that participants have no knowledge about the study hypothesis; this translates into deceiving research participants about the true purpose of the research. For example, the famous Asch study asked participants to judge the length of lines drawn on a series of cards in the presence of "confederates" who either gave a correct or incorrect answer. Knowing that the study was examining conformity to group pressure may alter the response to the task, so participants were told that they were participating in a visual perception experiment. This type of deception is not "outlawed" in psychological research.

Ethical standards, however, acknowledge that not revealing the true nature of the research at the outset incurs special ethical obligations. The APA ethics code requires that we use deception only when study risks are mild and there are no feasible alternatives to the deception. In addition, we must provide an explanation of the deception immediately following the conclusion of the experiment and give the research participants the choice to withdraw from the study at that time.

Finally, we may not deceive participants about aspects of the research that might influence their willingness to participate, such as physical or emotional discomforts or time commitment involved. Deceptions in psychological research can range from setting up false expectations about the purpose of the study to giving incorrect feedback about study performance.