Hawthorne effect

Meaning: The Hawthorne effect is a change in people's behavior or performance because they know they are being observed or are receiving special attention, not because of the variable being tested. It is named for studies at the Hawthorne Works factory, where worker output rose after changes to conditions such as lighting, and the attention itself was proposed as the cause. It is a form of reactivity and a threat to a study's validity.

On the MCAT: A passage describes participants who improve or change once they are enrolled in a study, and you must explain why the result may not reflect the treatment. Typical fixes are a control group that gets the same amount of attention, observing people unobtrusively or covertly, or letting participants habituate to the observer. You may also be asked to name this effect as a source of confounding in a design or in a field observation.

Common confusion: Students mix it up with the placebo effect, which comes from expecting a treatment to work, and with experimenter bias, which comes from the researcher's expectations influencing the results. The Hawthorne effect depends on awareness of being observed or singled out. The change also does not have to be an improvement; behavior can shift in either direction.