In the 1920s, researchers at the Hawthorne Works factory near Chicago changed workers' lighting. The famous account says workers got more done with brighter lights, dimmer lights, and even in a comparison room where nothing changed.
The popular explanation: people work harder when they know they're being studied, the “Hawthorne effect.”
In a 2011 study of the long-missing original data, two economists found only slight signs of the effect, far weaker than first claimed.