Though I don't recall the bibliography off the top of my head, there's been more than one study demonstrating that children who are told to, say, avoid playing with a car, and offered a cookie if they refrain, will go ahead and play with the car when they think no one is watching, or if no cookie is offered. If no reward or punishment is offered, and the child is simply told not to play with the car, the child will refrain even if no adult is around.

This and other similar studies are, I think, reviewed in Alfie Kohn's Punished by Rewards.