Believe it or not, there was a time when shopping carts weren't so popular. When they were first introduced into grocery stores in the late 1930s they made everyone self-conscious: men thought carts would make them seem weak, women thought the carts were too ugly, and older people didn't want to seem helpless. Sylvan Goldman, a grocery store owner and inventor of the shopping cart, hired models of both sexes and of various ages to push carts around the store and pretend they were shopping. His marketing technique obviously worked, because soon after stores were redesigned with the cart in mind.
Psychology surrounding shopping carts hasn't changed since their invention. Goldman created them because he wanted an easier shopping experience for customers and a cart would prompt them to buy more. Now, shopping carts are strategically placed to greet you on your way into stores and we're likely to buy more when we use a cart just because there's room.