I remember a study done in the mid-90's. (Unfortunately I read it in the NY Times, and so would have no chance of tracking it down again.) They "lost" a series of wallets in various large cities by dropping them in the street. Each included a driver's license and $100. They got virtually all of the wallets back, most of them with $100. A significant portion with letters saying things like, "You are lucky that I picked it up, most of the people around here would have taken the money."
The researchers wrote back to those who sent in $100 explaining that it was an experiment and gave them $200. To the researcher's surprise, multiple people refused to be paid $200 for having done what they thought was right!
The conclusion, while everyone "knows" that courtesy and kindness are lost, most of us, push comes to shove, won't wrongly take $100 even if it is literally lying in the street.
There are, of course, exceptions...
Cheers,
Ben