An article in Sociological Images describes researchers who created a Halloween field experiment with the following setup: On Halloween, a woman answers the door and invites the trick-or-treaters in. She tells them to "take one," and then she exits the room leaving the bowl of treats. The researchers repeated the experiment at 27 houses with a total of 1,300 kids.
Overall, most kids (69 percent) took one. But not always. In one experimental manipulation, the woman either asked the kids who they were and where they lived or she allowed them to be anonymous. Experimenters also noted whether the kids were trick-or-treating alone or in groups.
Significantly more stealing was observed under conditions of anonymity and in the presence of a group. Individuals whose identity was known were least likely to cheat.