Behaviorism is a psychological theory of human development that posits humans can be trained (conditioned) to respond in specific ways to specific stimuli and that given the correct stimuli, personalities and behaviors of individuals, and even entire civilizations, can be codified and controlled. In other words, behaviorists believe that a positive change in external behavior can be accomplished using reinforcement. The main tenets of behaviorism are methodology, philosophy, and theory. Also called behaviorist approach, this theory is based on empirical data carefully measured through controlled observation of behavior. Behaviorism was a primary theory of psychology from the 1920s to the 1950s.

American psychologist Edward Lee “Ted” Thorndike (1874–1949) initially proposed that humans and animals acquire behaviors through the association of stimuli and responses. He advanced two laws of learning to explain why behaviors occur the way they do: The law of effect specifies that any time a behavior is followed by a pleasant outcome, that behavior is likely to recur. The law of exercise states that the more a stimulus is connected with a response, the stronger the link between the two. The groundbreaking work of Russian physiologist Ivan Petrovich Pavlov (1849– 1936) on classical conditioning also provided an observable way to study behavior. In 1897, Pavlov published his results on conditioning performed on dogs after earlier studying their digestive habits. Although most psychologists agree that neither Thorndike nor Pavlov were strict behaviorists, their work paved the way for the emergence of behaviorism.

The birth of modern behaviorism was championed early in the twentieth century by John Broadus Watson (1878–1958), an American psychologist at Johns Hopkins University. In his 1913 book Psychology as the Behaviorist Views It, Watson summarizes the viewpoint of behaviorists as “a purely objective experimental branch of natural science. Its theoretical goal is the prediction and control of behavior. Introspection forms no essential part of its methods, nor is the scientific value of its data dependent upon the readiness with which they lend themselves to interpretation in terms of consciousness.” Watson added: “The behaviorist, in his efforts to get a unitary scheme of animal response, recognizes no dividing line between man and brute. The behavior of man, with all of its refinement and complexity, forms only a part of the behaviorist's total scheme of investigation.” Further, in his 1924 book Behaviorism, Watson made the notorious claim that, given a dozen healthy infants, he could determine the adult personalities of each one, “regardless of his talents, penchants, tendencies, abilities, vocations, and the race of his ancestors.” While making such a claim seems ridiculous today, at the time, Watson was reacting to emerging Freudian psychoanalytical theories of development (named after Sigmund Freud [1856–1939], an Austrian neurologist), which many people found threatening.

Watson's scheme rejected the hidden, unconscious, and suppressed longings that Freudians attributed to behaviors and posited that humans respond to punishments and rewards. Behavior that elicits positive responses is reinforced and continued whereas behavior that elicits negative responses is eliminated.

Later, the behaviorist approach was taken up by American psychologist and behaviorist B. F. (Burrhus Frederic) Skinner (1904–1990), who deduced the evolution of human behavior by observing the behavior of rats in a maze. Skinner wrote The Behavior of Organisms in 1936, which introduced the two behaviorism concepts: operant conditioning and shaping. Operant conditioning, as opposed to classical conditioning (or Pavlovian conditioning), involves a response that is followed by a reinforcing stimulus, what is called operant behavior (or the modification of voluntary behavior). Shaping is a method that uses a successive series of reinforcements that progressively become closer to a targeted (desired) type of behavior.

Based on behaviorism, Skinner wrote a novel, Walden Two, published initially in 1948, about a fictional Utopian society where human behavior is governed totally by self-interested decisions based on increasing pleasure. The book increased Skinner's renown and led many to believe that behaviorism could indeed produce such a society.

In the 1950s, however, the popularity of behaviorism began to decline. The first sustained attack on its tenets was made by Avram Noam Chomsky (1928–), a renowned linguist, who demonstrated that the behaviorist model simply could not account for the acquisition of language. Other psychologists soon began to question the role of cognition in behavior. Much of the skepticism with behaviorism came about when cognitive-behavioral therapy began to emerge in the latter half of the twentieth century.

Cognitive-behavioral therapy, often abbreviated CBT, is a type of psychotherapy that has successfully been used to treat abnormal behaviors and mental illnesses such as depression, phobias, addictions, and post-traumatic stress disorder (PTSD). Skinner, however, continued to espouse his belief in behaviorism in Beyond Freedom and Dignity, in which he argues that free will is only imagined in humans, for it is an illusion.

In the early 2000s, many psychologists continued to debate the extent to which cognitive learning and behavioral learning affect the development of personality. Over its extent, behaviorism contributed much to the scientific advancement of psychology. Areas that benefited from behaviorism (and that can be explained through conditioning) include gender development, language development, learning, and moral development.

Although the original facets of behaviorism subsided in importance over the years, in the early part of the twenty-first century, a next-generation form of behaviorism was still practiced by some psychologists. Called behavior analysis, it is applied through what is called behavioral technology, or what is more commonly known as applied behavior analysis (ABA). ABA is a form of behaviorism especially useful in the areas of developmental disabilities and autism.