In the late 1980s in Columbus, Ohio, an armored car spilled $2,000,000 on the freeway. Only $400,000 was ever recovered, the rest disappeared with the throngs of people who stopped and scooped up the cash. Some folks were honest enough to return what wasn't theirs: Melvin Kaiser gave back $57,000.
Those who have studied human personality say that if we know the people who lost the money, we'll generally give it back. However, if we don't know them, 75% of the time we'll keep the cash.