Not too long ago a study was done in various cities across the world to see how honest people were.
Wallets with a modest amount of money were left around the city, as well as information on the "owner's" ID. The study aimed to test which cities were more honest than others - it looked at 1) how many wallets were returned and 2) how many with the money still inside.
I'd be afraid to keep the money in case I skewed the results for my city :D