Robert Cialdini, a researcher and an expert on the theory of persuasion, conducted an experiment at the Petrified Forest National Park in Arizona. The park had a problem, as it made clear on a warning sign:

YOUR HERITAGE IS BEING VANDALIZED EVERY DAY BY THEFT LOSSES OF PETRIFIED WOOD OF 14 TONS A YEAR, MOSTLY A SMALL PIECE AT A TIME.

The sign plainly appealed to the visitors' sense of moral outrage. Cialdini wanted to know if this appeal was effective. So he and some colleagues ran an experiment. They seeded various trails throughout the forest with loose pieces of petrified wood, ready for the stealing. On some trails, they posted a sign warning not to steal; other trails got no sign. The result? The trails with the warning sign had nearly three times more theft than the trails with no signs.

How could this be? Cialdini concluded that the park's warning sign, designed to send a moral message, perhaps sent a different message as well. Something like: Wow, the petrified wood is going fast—I'd better get mine now! Or: Fourteen tons a year!? Surely it won't matter f I take a few pieces.