A blog at Discover Magazine's site has a summary of John Trinkaus' limited observational research about the behavior of supermarket shoppers, who apparently can't be bothered to throw away garbage, put their carts where they belong, or care about item limits in express lanes:

An informal enquiry of the behavior of 500 supermarket shoppers clearing carts of litter prior to entering the store showed that 69% dumped the rubbish into another cart, 26% dropped it on the sidewalk, and 5% deposited it in a trash container.

An informal enquiry suggested that only about one out of five shoppers, when finished using shopping carts, returned them to a designated depository.

A total of 68 15-min. observations of customers’ behavior at a food supermarket suggests that only about 7% of shoppers observe the item limit of the express lane. The averages tended to be about four pieces.