B.F. Skinner developed the theory of operant conditioning through experiments on rats. He found that if a rat presses a lever and receives a reward of food, over time the rat will learn to press the lever to get more food (positive reinforcement). Skinner developed the "Skinner Box" apparatus to systematically study operant conditioning. His work showed that behaviors can be learned or unlearned through consequences like reinforcement or punishment and influenced theories of behaviorism in psychology.