The best analogy I have ever heard goes something like: Everybody knows that one piece of trash won't make the streets dirty. So what does it matter if

The best analogy I have ever heard goes something like: Everybody knows that one piece of trash won't make the streets dirty. So what does it matter if I litter? On the other hand, if everyone littered, the streets would certainly be dirty. If I recall correctly, I heard this in a discussion on the role of social stigma in optimising game theory situations for the common good.