I had cause to refer to this recently, and struggled to remember the name. Hopefully a summary here will help me remember in future.
Studies carried out at the Hawthorne Works (outside Chicago) between 1924-1932 showed that changes in the working environment could improve productivity. But the improvement was only short lived, leading to the conclusion that people were responding because something had changed, and not to the change to the environment itself.
There is an article on Wikipedia