That’s actually a named and researched phenomenon, dubbed the “Hawthorne Effect”. It’s derived from studies done at the Hawthorne Works owned by Western Electric. The whole thing is fairly controversial among the interested academics, many of whom can’t quite bring themselves to believe that their observational efforts are skewing the studies.

It’s an interesting deal… I’ve screwed around with it on a small scale, in leadership roles, and I’m convinced that there’s some element of reality to the whole thing, although I’d hesitate to try to define it. Key thing is, you need to be very, very careful when you’re setting these situations up, in order that the participants don’t have a damn clue what you’re doing, and so that your study efforts don’t tilt the whole thing in an undesired direction.