The average person is reasonable and could probably be trusted to do the right thing. It's the 50% below average whom I'm worried about. You'll probably find a Pareto-like distribution on littering, i.e. ~80% of the litter is produced by ~20% of the people.
Another example that comes to mind is graffitis. My city is full of them. It takes seconds to make one, and hours to remove it. A few dozens of people could make a lot of damage within a few months.
