An example of the need for deception in a study might be an experiment in which it is predicted that the viewing of gun magazines (or other materials associated with potential violence) will lead to increased scores on a questionnaire measuring hostility.

• All participants are told that the experiment is one focusing on short-term memory, and they will be completing a memory task on two occasions separated by a 15-minute waiting period during which they will be reading magazine articles. All participants first complete baseline measures (including the hostility questionnaire).
• Next, all (including participants complete a computer-administered memory task.
• During the waiting period, the experimental group is told to read selections from a gun magazine that is made available in the lab; the control group is told to read selections from a nature magazine (neutral with regard to violent imagery). All participants later complete the computer-administered memory task again.
• Finally, all participants complete the battery of self-report instruments a second time. We are not so much interested in the viability of this hypothesis as we are in the need for some deception in the experiment.

As you can see, to tell participants the real purpose of the experiment would likely influence their responses to the questionnaires (especially to the one measuring hostility). Therefore, the investigator might need to introduce the experiment as one that is focusing on short-term memory.