Behaviorism is one of the major branches of psychology, explaining behavior in terms of stimuli and responses.
At the turn of the 20th century, Ivan Pavlov started his work on the gastric reflex in dogs. He discovered that a salivary response could be activated by a cue such as a bell ringing if that cue was linked to the presentation of food. This concept was developed into the idea of "classical conditioning." This work was absolutely pivotal in the history of psychology. It combined with the concepts developed in functionalism, and the field of behavioral psychology was born. John B. Watson is generally regarded as the founder of behaviorism. Behaviorism was, in many ways, a reaction to the introspective, subjective-based methods of structuralists and their goal of understanding the elements of the mind. Behaviorists abandoned the concept of mind as a scientific concept altogether. Instead they attempted to describe all behavior as merely the product of inputs from the environment and outputs from the organism. The mind was viewed as a 'black box' and never examined.
Pavlov, and later B.F. Skinner, as well as many others, were able to formulate precise mathematical descriptions of training regimes and reactions. It was hoped that psychology could became a hard science like physics, with mathematically rigorous theories. Skinner is most known for his work with operant conditioning or instrumental learning, where small changes in behavior are rewarded in order to create complex behavior. This is perhaps epitomized by the Skinner box, in which he trained pigeons to perform many complex actions for food rewards.
Skinner also thought that all of human behavior was a function of conditioning and instrumental learning. He wrote several non-fiction books such as Beyond Freedom and Dignity and several fiction books, including Walden 2, advocating the creation of Utopian societies based on instrumental learning.