William James had defined psychology as “the science of mental life.” But in the early 1900s, growing numbers of psychologists voiced criticism of the approach used by scholars to explore conscious and unconscious mental processes. These critics doubted the reliability and usefulness of the method of introspection, in which subjects are asked to describe their own mental processes during various tasks. They were also critical of Freud’s emphasis on unconscious motives. In search of more-scientific methods, psychologists gradually turned away from research on invisible mental processes and began to study only behavior that could be observed directly. This approach, known as behaviorism, ultimately revolutionized psychology and remained the dominant school of thought for nearly 50 years.
Among the first to lay the foundation for the new behaviorism was American psychologist Edward Lee Thorndike. In 1898 Thorndike conducted a series of experiments on animal learning. In one study, he put cats into a cage, put food just outside the cage, and timed how long it took the cats to learn how to open an escape door that led to the food. Placing the animals in the same cage again and again, Thorndike found that the cats would repeat behaviors that worked and would escape more and more quickly with successive trials. Thorndike thereafter proposed the law of effect, which states that behaviors that are followed by a positive outcome are repeated, while those followed by a negative outcome or none at all are extinguished.
In 1906 Russian physiologist Ivan Pavlov—who had won a Nobel Prize two years earlier for his studies of digestion—stumbled onto one of the most important principles of learning and behavior. Pavlov was investigating the digestive process in dogs by putting food in their mouths and measuring the flow of saliva. He found that after repeated testing, the dogs would salivate in anticipation of the food, even before he put it in their mouth. He soon discovered that if he rang a bell just before the food was presented each time, the dogs would eventually salivate at the mere sound of the bell. Pavlov had discovered a basic form of learning called classical conditioning (also referred to as Pavlovian conditioning) in which an organism comes to associate one stimulus with another. Later research showed that this basic process can account for how people form certain preferences and fears. See Learning: Classical Conditioning.
Although Thorndike and Pavlov set the stage for behaviorism, it was not until 1913 that a psychologist set forward a clear vision for behaviorist psychology. In that year John Watson, a well-known animal psychologist at Johns Hopkins University, published a landmark paper entitled “Psychology as the Behaviorist Views It.” Watson’s goal was nothing less than a complete redefinition of psychology. “Psychology as the behaviorist views it,” Watson wrote, “is a purely objective experimental branch of natural science. Its theoretical goal is the prediction and control of behavior.” Watson narrowly defined psychology as the scientific study of behavior. He urged his colleagues to abandon both introspection and speculative theories about the unconscious. Instead he stressed the importance of observing and quantifying behavior. In light of Darwin’s theory of evolution, he also advocated the use of animals in psychological research, convinced that the principles of behavior would generalize across all species.
Many American psychologists were quick to adopt behaviorism, and animal laboratories were set up all over the country. Aiming to predict and control behavior, the behaviorists’ strategy was to vary a stimulus in the environment and observe an organism's response. They saw no need to speculate about mental processes inside the head. For example, Watson argued that thinking was simply talking to oneself silently. He believed that thinking could be studied by recording the movement of certain muscles in the throat.
The most forceful leader of behaviorism was B. F. Skinner, an American psychologist who began studying animal learning in the 1930s. Skinner coined the term reinforcement and invented a new research apparatus called the Skinner box for use in testing animals. Based on his experiments with rats and pigeons, Skinner identified a number of basic principles of learning. He claimed that these principles explained not only the behavior of laboratory animals, but also accounted for how human beings learn new behaviors or change existing behaviors. He concluded that nearly all behavior is shaped by complex patterns of reinforcement in a person’s environment, a process that he called operant conditioning (also referred to as instrumental conditioning). Skinner’s views on the causes of human behavior made him one of the most famous and controversial psychologists of the 20th century. see Learning: Operant Conditioning.
Skinner and others applied his findings to modify behavior in the workplace, the classroom, the clinic, and other settings. In World War II (1939-1945), for example, he worked for the U.S. government on a top-secret project in which he trained pigeons to guide an armed glider plane toward enemy ships. He also invented the first teaching machine, which allowed students to learn at their own pace by solving a series of problems and receiving immediate feedback. In his popular book Walden Two (1948), Skinner presented his vision of a behaviorist utopia, in which socially adaptive behaviors are maintained by rewards, or positive reinforcements. Throughout his career, Skinner held firm to his belief that psychologists should focus on the prediction and control of behavior. See Behaviorism; Behavior Modification.