Volunteers in a study often try to guess what you want, then do that instead

Demand characteristics are the subtle cues in an experiment, the setup, the instructions, the experimenter's tone, that hint at what a study is 'really' testing, nudging participants to act helpfully, or sometimes contrarily, instead of naturally. Psychologist Martin Orne coined the term in 1962 after finding volunteers would keep doing a tedious, pointless task for hours simply because they trusted the experiment must have a good reason. It's a central reason experiments use cover stories.

— Martin T. Orne, On the Social Psychology of the Psychological Experiment — American Psychologist, 1962