Pavlov's experiments in classical conditioning were familiar to Western psychologists from 1906 on and formed the heart of the behaviorist method. In 1913 John B. Watson (1878–1958) systematically and provocatively set out the principles of behaviorism in a manifesto entitled "Psychology As the Behaviorist Views It." Watson was a comparative psychologist interested in making psychology a real science by defining it as the study of outwardly observable behavior, rather than of thought, imagery, consciousness, or mind. In doing so he intended to break psychology's ties with philosophy. As a comparative psychologist with interests also in developmental psychology—the study of how the mind develops in childhood, as well as over the course of evolution—Watson knew that the conventional psychological method of introspection was inapplicable to the subjects of his science. Children and animals could not be asked to introspect, to divulge the contents of their minds, and guessing at what they were thinking Watson judged to be unscientific. Focusing instead on behavior rather than on consciousness was therefore the only way to proceed. Watson also had a practical motivation for his behaviorism—he wanted to make it the science of how people act.
Watson was born in Greenville, South Carolina. He attended college at Furman University and graduate school in the department of philosophy at the University of Chicago, where he was trained in comparative psychology by the neurologist Henry Herbert Donaldson and the psychologist James Rowland Angell. Watson was interested in the work of Jacques Loeb (1859–1924), a German-trained materialist physiologist at Chicago who studied tropisms, or movements in plants and animals, that he interpreted in solely physico-chemical terms. Donaldson and Angell, however, dissuaded Watson from working with the radical Loeb, and Watson instead wrote his dissertation on the correlation between brain growth and learning ability in rats.
In 1908 Watson became professor of psychology at the Johns Hopkins University, but as a comparative psychologist felt marginalized in the department. In his 1913 manifesto he devised a way to bring comparative psychology to center stage. A truly scientific psychology, he wrote, would abandon talk of mental states or conscious content of minds and instead focus on prediction and control of behavior. By focusing on objectively observable behavior, by getting away from mind, consciousness, and introspection and examining physical variables instead, psychology would become a legitimate science. Like Pavlov, Watson believed in observing and training physical responses to stimuli, making no reference to mind, and thereby treating animal and human behavior on the same level. In his 1919 book, Psychology From the Standpoint of a Behaviorist, Watson rejected the concept of mind completely, interpreting even imagery, thought, and language all in terms of behavior accessible to an objective observer.
At Johns Hopkins, Watson was associated with the psychiatrist Adolf Meyer, the head of the Phipps Clinic, where Watson applied behaviorist conditioning methods to children. In a famous series of experiments conducted with his graduate assistant Rosalie Rayner, Watson trained a child by the name of Little Albert (aged 9–13 months) to fear a rat, a response the child then produced in reaction to the sight of any furry creature. Watson's evident success in training even such a deep-seated reaction as fear led him to believe that all of a person's behavior could be altered, any habit could be formed or broken, by the engineering of stimuli—that is, by the control of the person's immediate environment. In his 1924 book Behaviorism Watson expressed this environmentalist view in its most extreme terms. But by then Watson had been forced to resign his position at Hopkins because of his involvement in an extramarital love affair with Rayner. He and Rayner moved to New York City, where Watson joined the John Walter Thompson advertising agency, and where both Watson and Rayner became popular authorities on child-rearing according to behaviorist principles. Their Psychological Care of Infant and Child appeared in 1928.
Watson was not the only psychologist during the 1910s to advocate behaviorism. At Columbia University Teachers College, Edward L. Thorndike (1874–1949), experimenting with cats learning their way around puzzle-boxes, similarly argued that the study of objectively observable changes in behavior, and their correlation with changes in stimuli, formed the heart of a legitimately scientific psychology. Thorndike formulated the law of effect, which held that pleasure or reward will reinforce a certain behavior, while pain will extinguish it: thus the animal's experience has important consequences for its behavior.