The Hawthorne effect

The Hawthorne effect is a form of reactivity whereby subjects improve an aspect of their behavior being experimentally measured simply in response to the fact that they are being studied, not in response to any particular experimental manipulation.
The term was coined in 1955 by Henry A. Landsberger[3] when analyzing older experiments from 1924-1932 at the Hawthorne Works (a Western Electric manufacturing facility outside Chicago). Hawthorne Works had commissioned a study to see if its workers would become more productive in higher or lower levels of light. The workers' productivity seemed to improve when changes were made and slumped when the study was concluded. It was suggested that the productivity gain was due to the motivational effect of the interest being shown in them. Although illumination research of workplace lighting formed the basis of the Hawthorne effect, other changes such as maintaining clean work stations, clearing floors of obstacles, and even relocating workstations resulted in increased productivity for short periods of time. Thus the term is used to identify any type of short-lived increase in productivity.

Via: wikipedia