In the 1930s B. F. Skinner developed a new methodology for the study of animal learning and behavior. He called it operant behavior, to reflect the fact that the animal "operated" on the environment to produce a reward, or reinforcer. The Behavior of Organisms, published in 1938, was the principal document in which he presented his findings and his conceptual approach to the study of animal learning and behavior.

In the method that Skinner developed, the animal (most often a rat, pigeon, or monkey) emits particular behaviors, called instrumental responses (or behaviors), to gain a reinforcer. Most often, these responses involve an operandum (formerly called manipulandum) that is suited to the subject's motor abilities. Rats, monkeys, and other mammals press a horizontal bar (or lever) in the experimental chamber (often called a Skinner box), while pigeons peck at a vertical disk (or key); fish can be taught...