Aldi's cart system generally results in all carts being returned. It is free to use, but requires a quarter as insurance against the cart being left in the parking lot. If you have an emergency you can leave it, but it will cost you a quarter & someone else will have a small incentive to use your cart & return it. In practice, people also tend to be fairly generous & leave their carts in the stall with quarters still in them for the next person.

The goal is for society to be structured in such a way that good things are accomplished whether people are good or not.