that experiments have the additional problem of reactivity-the tendency of subjects to change their behavior ill response to the researcher
or to the fact that they know they are being studied. This problem was first noted in a study conducted between 1927 and 1932 by the social psychologist Elton Mayo, who used a series of experiments to determine how worker productivity and morale might be improved at Western Electric's Hawthorne plant.
To identify variables that tend to increase worker productivity, Mayo separated one group of women (the experimental group) from the other workers and then systematically varied factors in that group's work environment while closely observing them. Meanwhile, the working conditions of the other workers (the control group) were not changed. The researchers tested a number of hypotheses, including one stating that an increase in the amount of lighting would raise the workers' productivity. Much to the researchers' surprise, the level of productivity rose not only when the lighting was brightened but also when it was dimmed.
Indeed, all of the changes increased productivity. Mayo concluded that the subjects were trying to please the researchers because of the interest being shown in the subjects (Roethlisberger and Dickson, 1939). Thus, the Hawthorne effect refers to changes ill tile subject's behavior caused by the researcher's presence or by the subject's awareness of being studied. ("Groups and Organlzations").