In the theory behind gambling psychology, it was found that the largest emotional response occurs on 'near-miss' outcomes-when, for instance, you get two 7's in a row but the third reel stops on a cherry. They weight certain symbols to come up more frequently, and they will often adjust the last reel to have a different statistical likelihood as to the first two reels. It uses an internal random number generator to dictate which symbol each of usually three reels comes to rest. In real life, a slot machine is programmed by the vendor.
