Hawthorne effect

From Research Methods in Psychology (p. 347):

One specific novelty effect has been labeled the Hawthorne effect. This refers to changes in people's behavior brought about by the interest that "significant others" show in them. The effect was named after events occurring at the Hawthorne plant of the Western Electric Company in Cicero, Illinois, near Chicago, between 1924 and 1932. Studies were conducted to examine the relationship between productivity and conditions of the workplace. In one experiment, the amount of lighting in the plant was varied and worker performance was examined. Results reveal that both experimental and control groups increased their productivity during the study. Although there is some controversy surrounding the exact factors responsible for this effect, a Hawthorne effect generally refers to a change in behavior that results from participant's' awareness that someone is interested in them.