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.

Shead says the original tests were to do with changing light levels. You can read Shead’s story at Hawthorne Effect : Productivity501.

Wikipedia's entry on the Hawthorne effect is also worth reading.

There’s also a good definition 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 invented industrial psychology.

My common sense management experience says you should pay attention to workers as a matter of course.

Sadly this isn’t obvious to everyone and it certainly wasn’t back in the 1920s and 1930s when these ideas were fresh. My view is if you see the Hawthorne effect at your workplace, take it as a sign you aren’t managing correctly.

See also: Taylor’s scientific management doesn’t apply to knowledge work.