Mark Shead at Productivity 501 writes:
The Hawthorne effect refers to some studies that were done on how training impacts employees’ productivity at work. The studies found that sending someone to training produces employees that work harder. The funny part about it is that you still get the productivity increase even if the training doesn’t teach them how to be better at their jobs. Sending someone to training helps them feel like they are important, like the company is investing in them and they are valuable. Because of this, they work harder.
An explanatory note at the bottom of Shead’s post points out the original tests were to do with changing light levels. You can read Shead’s original story at Hawthorne Effect : Productivity501.
It’s also worth reading the Wikipedia entry on the Hawthorne effect. There’s also a good definition of the effect at Donald Clark’s site: The Hawthorne effect. Clark writes:
The Hawthorne effect – an increase in worker productivity produced by the psychological stimulus of being singled out and made to feel important.
Clarke links The Hawthorne effect to work done by Frederick Taylor who gave birth to the idea of industrial psychology.
My own common sense experience as a manager says you should pay attention to workers as a matter of course. Sadly this isn’t obvious to everyone. It certainly wasn’t back in the 1920s and 1930s when these ideas were fresh and new. If the effect is clear among knowledge workers at your workplace, it’s a sign you aren’t managing people correctly.