From Wikipedia:

Behaviorism (or behaviourism) is a systematic approach to understanding the behavior of humans and other animals. It assumes that all behaviors are either reflexes produced by a response to certain stimuli in the environment, or a consequence of that individual’s history, including especially reinforcement and punishment, together with the individual’s current motivational state and controlling stimuli. Although behaviorists generally accept the important role of inheritance in determining behavior, they focus primarily on environmental factors.

Behaviorism combines elements of philosophy, methodology, and psychological theory. It emerged in the late nineteenth century as a reaction to depth psychology and other traditional forms of psychology, which often had difficulty making predictions that could be tested experimentally. The earliest derivatives of Behaviorism can be traced back to the late 19th century where Edward Thorndike pioneered the law of effect, a process that involved strengthening behavior through the use of reinforcement.

During the first half of the twentieth century, John B. Watson devised methodological behaviorism, which rejected introspective methods and sought to understand behavior by only measuring observable behaviors and events. It was not until the 1930s that B. F. Skinner suggested that private events—including thoughts and feelings—should be subjected to the same controlling variables as observable behavior, which became the basis for his philosophy called “radical behaviorism.”[1][2] While Watson and Ivan Pavlov investigated the stimulus-response procedures of classical conditioning, Skinner assessed the controlling nature of consequences and also its potential effect on the antecedents (or discriminative stimuli) that strengthens behavior; the technique became known as operant conditioning.

Skinner’s radical behaviorism has been highly successful experimentally, revealing new phenomena with new methods, but Skinner’s dismissal of theory limited its development. Theoretical behaviorism[3] recognized that a historical system, an organism, has a state as well as sensitivity to stimuli and the ability to emit responses. Indeed, Skinner himself acknowledged the possibility of what he called “latent” responses in humans, even though he neglected to extend this idea to rats and pigeons[4]. Latent responses constitute a repertoire, from which operant reinforcement can select.

The application of radical behaviorism—known as applied behavior analysis—is used in a variety of settings, including, for example, organizational behavior management, to the treatment of mental disorders, such as autism and substance abuse.[5][6][7] In addition, while behaviorism and cognitive schools of psychological thought may not agree theoretically, they have complemented each other in cognitive-behavior therapies, which have demonstrated utility in treating certain pathologies, including simple phobiasPTSD, and mood disorders.