Manipulated functionality.

What is Deception in Psychology?
A general definition of deception in psychology is the act of misleading research participants about an experiment's purpose, conditions, or procedures or otherwise manipulating an experiment to control the behavior of the participants with the goal of producing better research results.

A person
There are two basic reasons that deception might be used in psychological research. Primarily, deception allows researchers to observe the genuine reactions of participants. Participants who have knowledge of the purpose or expected results of an experiment are more likely to act in a way that favors the expected result. Using deception allows researchers to keep participants from understanding the actual purpose of the experiment, which prevents participants from changing their behavior based on their perceptions of the experiment's purpose.

A secondary purpose for using deception is to artificially create circumstances or situations that are difficult to study in the real world. If a research team wanted to learn more about the behavior of bystanders who film strangers in distressing situations rather than attempting to help, the researchers may create a fictional situation in order to observe behavior and interview bystanders.

A few well-known cases of deception in psychological research are: