The Hawthorne effect is a form of reactivity whereby subjects improve or modify an aspect of their behavior being experimentally measured simply in response to the fact that they know they are being studied, not in response to any particular experimental manipulation.

The term was coined in 1950 by Henry A. Landsberger when analysing older experiments from 1924-1932 at the Hawthorne Works

Hawthorne Works

The Hawthorne Works, in Cicero, Illinois, was a large factory complex built by Western Electric starting in 1905 and operating until 1983. It had 45,000 employees at the height of its operations. Besides telephone equipment, the factory produced a wide variety of consumer products, including...

 (a Western Electric

Western Electric

Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

 factory outside Chicago).