The Hawthorne effect occurs when people change their behaviour because they know they are being watched. The term was coined from a study done at the Hawthorne Works (an electric company). They had commissioned a study to look at whether people worked more effectively at higher or lower levels of light. Originally, it looked like increased lighting resulted in higher productivity, but productivity rapidly returned to baseline. The surprise was that a subsequent decrease in lighting also resulted in increased productivity. A number of other changes to the workplace were also tested, and in each case productivity increased initially, but ultimately fell back to baseline. Ultimately, it was determined that it was not the environmental changes, but the fact that the workers knew that they were being watched that was driving the increased productivity.
Sedgwick P, Greenwood N. Understanding the Hawthorne effect . BMJ. 2015;351:h4672.