Just knowing you're being observed changes how you behave

Researchers studying productivity at Western Electric's Hawthorne Works found that output kept rising almost no matter what change they made to lighting or breaks, simply because workers knew they were part of a study. The pattern, now called the Hawthorne effect, is a standard warning in research design: being watched changes behavior on its own.

— Fritz J. Roethlisberger and William J. Dickson, Management and the Worker — Harvard University Press, 1939
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