The principle that states that the use of a reinforcing stimulus increases the strength of operant behavior is known as the Law of Effect. This concept was initially proposed by psychologist Edward Thorndike and later played a significant role in the development of operant conditioning by B.F. Skinner. The Law of Effect essentially states that behaviors that are followed by positive outcomes are more likely to be repeated in the future, while behaviors followed by negative outcomes are less likely to be repeated.