People have been mailing stolen rocks back to Petrified Forest with apology letters since 1934. Visitors take pieces of petrified wood home, then sometimes years later mail them back after a run of bad luck: car problems, sickness, deaths in the family, or just the feeling that something has been off ever since. The park has collected more than 1,200 of these letters. Staff have called the returned rocks the “conscience pile,” because they cannot simply put them back in random places without disrupting the scientific record. The stories became so well known that artists Ryan Thompson and Phil Orr eventually turned part of the archive into a book called Bad Luck, Hot Rocks. But there is another side to the story. For decades, signs at Petrified Forest claimed visitors were stealing around 14 tons of petrified wood every year. Park officials later acknowledged that figure was never actually measured. Then ASU psychologist Robert Cialdini studied theft inside the park and found some...