In my post yesterday on whether stories about honesty make children more truthful, I mentioned a classic experimental setup: An adult asks a child not to peek at a toy and then leaves the room while other adults look on via one-way mirror or video camera. Then the adult comes back and asks the kid if he or she peeked. Up to a certain age, most kids peek and most kids lie about having done so — it's a normal part of the developmental process.
Here's some video evidence:
I feel bad for the first kid. Look at that excrutiating expression on his face — he wants so badly not to peek, but he just can't help it. The toy sounds too freaking awesome. Then he lies right to that nice woman's face, as do a couple of others. Those little devils!