Why do people always spend more money than planned in supermarkets?

W: Questions 14 to 16 are based on a passage about markets. You now have 15 seconds to read Questions 14 to 16.
    Why is it that whenever you pop into a supermarket for a pound of butter and a tin of baked beans, you end up spending twenty pound —or more? The good news id that it’s not because there’s something wrong with you. Most of us do it. The bad news is that from the moment that you step inside a store, you haven’s got a chance. Shops have spent a great deal of time and money researching the psychology of the shopper. They’re using increasingly sophisticated methods to make us buy. A good supermarket environment is one in which you feel relaxed and comfortable. In the 60’s, when supermarkets first took off in Britain, aisles were deliberately very narrow so that no shelf space was wasted. However, that’s all changed now. The idea is to keep you relaxed—though not too relaxed. Consequently lighting is designed to be relaxing, but not enough to send you to sleep. Similarly, the background music which is there to cover up the threatening sound of the hustle and bustle, should be pleasant and put you at ease but always have a fairly brisk beat to keep you moving.