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Another possibility is that people that don't return the cart may not be having their needs met. A person who is tired after walking across the hot parking lot may not return it out of a desire to maintain a modicum of health. Or, perhaps, they may not think about it because their cognition is temporarily hindered by hunger, exhaustion, or some other carnal need.
On Maslow's hierarchy, I'd say if a person meets all of their physiological and safety needs they are more likely to return the cart than those who do not.