The Hawthorne effect, also known as Subject Reactivity, can be defined as changes in behavior resulting from attention participants believe they are getting from researchers, and not the variable(s) manipulated by the researchers (in the Hawthorne case, the amount of light in the work environment).
The effect came out of a series of studies conducted at the Hawthorne Plant of Western Electric by Elton Mayo and a team of researchers from Harvard University. The purpose of these studies was to examine the relationship between worker productivity and the environment in which the work was performed. The first study, which led to the Hawthorne Effect, examined the relationship between light intensity and worker productivity. The researchers found that productivity improved not from the lighting, but from the attention the workers received from the researchers themselves.