Here in germany there is a chain that connects each cart to the next which can be seperated by inserting a coin.
Re-inserting the chain is how the coin is retrieved in turn.

Aside from the very occasional drunk, nobody leaves their cart unreturned.

But what of people that:

- Return the shopping cart just to avoid potential judging glares from people around them..? Would that be just fear of social pressure..?

- Return the shopping cart because they think there could be negative consquences for them if they do not do it since they are not fully acquainted with laws and market regulations..? Would that be just ignorance..?

- Return the shopping cart because some family member that is with them told them to do so..? Would that be just blind obedience..? Or;

- Return the shopping cart because that one nearby person that looks like one of the market's employee is on line of sight and would notice you leaving the cart there..? Would that be just playing it safe..?

There are many layers and different scenarios that could easily throw a wrench on this theory grounded purely on morality once you really think about it. A lot of external factors that alter the experiment's natural outcome.