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Research Methods (9th edition)

Clark L. Hull (1884-1952) was an American psychologist, who developed one of the most comprehensive behavioral learning theories. Hull was strongly influenced by Pavlov’s conditioning model and Thorndike’s learning principles. Like other behaviorists, such as Watson, Guthrie, and Skinner, Hull believed that all complex behavior can be explained by basic principles of conditioning. As a behaviorist, he espoused objective psychology, viewed human functioning in mechanistic terms, and had no provision for concepts of consciousness or other mentalistic ideas. 

Hull had been impressed by Isaac Newton’s Principia and Newton’s view of the universe as a vast, complex machine that could be understood by constructing an inclusive hypothetico-deductive theory. That theory-building task was what Hull attempted to do. He created a set of postulates (statements) about behavior, logically deduced from them specific theorems, developed hypotheses, and tested those hypotheses in experiments. The experimental results would support or not support the theorems and, in time, the entire theory would be developed and confirmed or disconfirmed. 

Hull believed that human functioning, including thinking, was a machine process, and that organisms are, after all, machines. So sure was he of the essential mechanical nature of humans that he predicted in 1926 that machines would someday be built that will do everything that a thinking person can do. What would Hull say today, were he alive to see how far computers have progressed?

Like Tolman, Hull postulated intervening variables that operate between stimulus and response (S-O-R rather than S-R). Unlike Tolman, those intervening variables did not include concepts about consciousness. Rather, Hull argued that they were physical, neurological, and physiological events. They could not be directly observed, but could be systematically inferred by relating them to carefully controlled stimulus conditions and to carefully observed behavior.

Hull’s model was a drive-reduction model. He postulated that primary drives are tied to innate biological needs, such as food, water, of air. They are internal stimuli that compel the organism to action. Secondary drives, also called learned drives, are those that develop through the association of stimuli with the reduction of primary drives. Once developed, secondary drives can also impel the organism to action. An obvious example of a powerful secondary or learned drive is the need to acquire money. For some people, this even becomes one of their dominant needs and drives. Primary and secondary drives constitute motivation, and motivation is a major factor of behavior.

Hull believed that learning is the strengthening or weakening of stimulus-response associations. These occur as follows: a drive state, such as hunger in a laboratory animal, motivates the animal to behave. The behavior might involve exploring the cage or pressing a bar. If the behavior results in successfully obtaining and ingesting food, the drive is reduced. That drive-reduction then increases the probability of that behavior occurring when that drive occurs again. 

In Hull’s model, the reduction in drive constitutes reinforcement of the S-R association. Through repetition of the reinforcement, the probability of the occurrence of the response--that is, its habit strength--increases. Through conditioning, habit strength can be increased or decreased.

In the food example, the behavior was successful, or instrumental, in obtaining food and reducing the drive. Thus, Hull’s model is also referred to as instrumental conditioning to distinguish it from Pavlov’s respondent conditioning and Skinner’s operant conditioning models. Two critical concepts in Hull’s model are contiguity and association. That is, stimuli and responses must occur contiguously (together) in order for the S-R association to form. These concepts are included in all of the behavioral models.

Thorndike’s law of effect appears in Hull’s model as the law of primary reinforcement. The reinforcement is the reduction of the drive state, and any behavior associated with such reinforcement, such as a rat’s bar-press, will gain strength and will be more likely to occur under future drive conditions. 

Secondary reinforcement operates in a similar manner. In the money example above, whatever behavior is instrumental in acquiring more money, and thus temporarily reducing the learned drive, will gain habit strength. Likewise, when drive reduction does not occur--that is, when the behavior is not instrumental--the habit strength will gradually decrease. Such decrement in habit strength is called extinction.

Hull developed and refined his model over many years. Its completeness and precision in both verbal and mathematical terms made it the dominant learning model in psychology from about 1940 to the early 1950s. It stimulated an enormous amount of research and influenced all of psychology. 

The ideas of learned drives and secondary reinforcement had obvious implications for complex human behavior and were readily applied to education and mental health treatment. For example, an influential textbook by Dollard and Miller, Personality and Psychotherapy (1950), integrated Hullian learning theory and Freudian psychoanalysis, defining neuroses as learned behavior that can be understood and successfully treated by behavioral methods. This book was a major influence in the development of behavior modification from the late 1950s.

Hull’s major books were Principles of Behavior (1943) and A Behavior System (1952). In these two works, Hull presented his elegant, mathematico-deductive theory of behavior. 

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