This reminds me of the princess alice experiment.
In the experiment children are encouraged to play a game without cheating.
A test group is told that 'princess alice', an invisible princess, is watching them and will punish them if they cheat.
A control group is not told about princess alice.
The rate of cheating was higher amongst the control group, although this situation is not entirely approxible to real life situations, but it serves to show why a lot of parents encourage their children to believe in santa claus I suppose.