Clark Leonard Hull (May 24, 1884 – May 10, 1952) was an influential American psychologist and learning theorist in behaviorism. He sought to explain learning and motivation by scientific laws of behavior. Clark Hull's most important contribution to psychology lies in his theory of learning, considered one of the most important learning theories of the twentieth century. He also conducted research demonstrating that his theories could predict and control behavior, and inspired many of his students to continue to develop his theories and maintain his high standards of scientific method in the field of psychology.
Clark Leonard Hull was born in Akron, New York on May 24, 1884. He was raised in Michigan, attending a one-room school for many years. Hull suffered from health problems, had poor eyesight, and was crippled from polio. His education was interrupted at various times due to illness and financial problems. He qualified as a teacher and spent some time teaching in a small school, but soon realized his education was insufficient even to answer questions posed by high school students.
After pursuing mining engineering at the University of Michigan, where he obtained bachelor's and master's degrees, he turned to psychology, receiving his Ph.D. in psychology in 1918 from the University of Wisconsin, where he stayed for ten years as an instructor. His doctoral research on "Quantitative Aspects of the Evolution of Concepts" was published in Psychological Monographs.
During that time, Hull studied the effects of tobacco smoking on performance, reviewed the existing literature on testing, and began research on suggestion and hypnosis. In 1929, Clark Hull was named to a research position at Yale University and began the serious development of his behavior theory. During that period of his professional career, Hull also published influential articles on behavior theory in the Psychological Review. Until the end of his career, Hull and his students dominated behavioristic psychology.
Clark Hull died on May 10, 1952, in New Haven, Connecticut.
Clark Hull’s system is intricate and relies heavily on mathematical elaboration, with detailed modifications as the system unfolded over time. Essentially, Hull’s theory of learning is centered on the necessity of reinforcement, defined in terms of "drive reduction." The behaving organism is viewed in the context of homeostatic model seeking equilibrium from "drive forces."
The core level of psychological analysis concerns the notion of "intervening variables," described as "unobservable behavior." Thus, from a purely behavioral perspective Clark Hull extended John B. Watson’s behavioristic conceptualization of behavior in terms of the peripheral stimulus–response (S–R) events to a consideration of central, organismic factors, stimulus–organism–response (S–O–R), or intervening variables.
Clark Hull's theory was systematic and generated a great deal of research. Hull insisted on strict adherence to the scientific method, requiring well-controlled experiments and the quantification of the resulting data. The formulation of the deductive theory of learning involved a series of postulates which should eventually be tested by experimentation. The final formulation of the theory consisted of 18 postulates and 12 corollaries, stated in both mathematical and verbal forms. Hull's theory also includes intervening variables, constructs which are assumed but never really subject to experimental verification.
One aspect of Hull's work on aptitude testing would prove instrumental in the development of his behaviorism. To facilitate the computation of correlations between various tests, he constructed a machine to perform the calculations, completing the project in 1925 with support from the National Research Council. Aside from the machine's practical benefit, the success of the project convinced Hull that a purely physical device with the right arrangement of material components was capable of performing operations characteristic of high-level mental processes.
Hull's full conversion to behaviorism came in the ensuing years when he taught seminars on behaviorism and studied Anrep's 1927 translation of Ivan Pavlov's Conditioned Reflexes. Long sympathetic to British associationism, Hull regarded Thomas Hobbes and David Hume as the philosophical ancestors of behaviorism and saw in Pavlov's conditioned reflexes the physical analogues of Hume's simple "impressions" and laws of association.
For Clark Hull, the design of machines that could exhibit intelligent behavior was equivalent to the formulation of a theory of that behavior. Viewing the machines as a vindication of a materialist psychology, Hull used them in his rhetoric forays against such "subjectivists" as the vitalist Hans Driesch and the Gestalt psychologist Kurt Koffka, both of whom taught at Wisconsin during his years there.
Clark Hull is also known for his debates with Edward C. Tolman on the principles of behaviorism. Tolman believed that learning could occur in the absence of a goal (identifying this as "latent learning"), whereas Clark Hull stressed that the goal should be thought of as a "reward" or "reinforcement" and was necessary for learning to occur.
Clark Hull is often credited with having begun the modern study of hypnosis. His work Hypnosis and Suggestibility (1933) was a rigorous study of the phenomenon, using statistical and experimental analysis. Hull's studies emphatically demonstrated once and for all that hypnosis had no connection with sleep: "hypnosis is not sleep…it has no special relationship to sleep, and the whole concept of sleep when applied to hypnosis obscures the situation" (Hull 1933).
The main result of Hull's study was to rein in the extravagant claims of hypnotists, especially regarding extraordinary improvements in cognition or the senses under hypnosis. Hull's experiments did show the reality of some classical phenomena such as hypnotic anesthesia and post-hypnotic amnesia. Hypnosis could also induce moderate increases in certain physical capacities and change the threshold of sensory stimulation; attenuation effects could be especially dramatic.
In experimental psychology, he created the "hypothetic-deductive" systematic method, after the observation and elaboration of hypotheses. This method brought him precise definitions and conceptualized axioms which helped him develop his theories. He believed that behavior was a set of interactions between an individual and their environment. He analyzed behavior from a perspective of biological adaptation, or an optimization of living conditions through need reduction.
As a behaviorist, Hull centered his psychological views on habit formation, the accumulations of environmental experiences for effective adaptation. His scientific approach was truly systematic. While recognizing the importance of observation and experimentation, Hull advocated a hypothetic-deductive structure to guide research. In this strategy, following the approach of Euclidian geometry, a behavior principle or formulation was first postulated and then rigorously tested. A successful test supported belief in the principle; failure resulted in revision of the principle. Hull’s theory was positive and followed a logical progression, verified through empirical demonstration.
Similar to B. F. Skinner, Clark Hull stressed the importance of reinforcement, if learning was to take place. Reinforcement was successful because it resulted in the reduction of drives. Thus the concept of drives and their reduction became an important aspect of Hull's theory. He considered the environmental influences on the organism as well: these were the input, while the responses the organism made were the output.
Clark Hull's scientific system has been viewed both as a failure that led to the cognitive revolution of the 1960s, and as a natural precursor to cognitive psychology's information processing and artificial intelligence approaches. Hull's work has also been regarded both as a noble effort, still worthy of emulation, to set high standards for psychology as a natural science, and as an object lesson in the futility of natural science models for psychology and the damaging effects of scientific rhetoric. Clearly, Hull's ambitious efforts left a mixed legacy, raising difficult issues that are still being played out in psychology.
In the decades before and after World War II, Clark Hull symbolized psychologists' hope that psychology could be an objective natural science. Hull established a reputation as an eclectic experimental psychologist, then rose to prominence as a learning theorist.
Hull's most significant works were the Mathematico-Deductive Theory of Rote Learning (1940), and Principles of Behavior (1943), which established his analysis of animal learning and conditioning as the dominant learning theory of its time.
Hull's most famous student and later collaborator was Kenneth Spence (1907–1967), who spent his most productive years at the University of Iowa. The research of Spence was characterized by a concern with refining Hull's theory as well as applying those principles to varieties of behavioral processes, including an analysis of anxiety. His major contribution to the theoretical basis of Hullian behaviorism was his explanation of discrimination learning. Spence held that gradients of excitatory potential and inhibitory potential were generated around "stimulus values" that are reinforced and not reinforced, respectively, during discrimination learning.
Another important student of Hull was Neal Miller, whose productive career has involved important studies of a variety of psychological issues (Miller 1969). His early work (Dollard and Miller 1950), attempted to apply a Hullian analysis to behavioral issues derived from psychoanalytic literature. Dollard and Miller's research on frustration and psychological conflict has become classic, leading to direct support for the contemporary behavior modification trend. Turning to physiological substrates, Neal Miller made significant findings concerning the relationship between reinforcement mechanisms and the control of autonomic behavior.
Another influential student of Hull was Hobart O. Mowrer (1907–1982), who said that there should be a distinction between Pavlovian and instrumental conditioning. Mowrer argued that in avoidance learning the fear of the conditioned stimulus was acquired by Pavlovian principles, and the motor response to that fear was instrumentally acquired through the reinforcing effect of fear reduction. The conditioned stimulus then functions as a sign of impeding shock.
Further research in neo-Hullian tradition has also extended to questions concerning the physiological basis of learning. Borrowing from the "psycho-physiological" findings, these investigations focus on such areas as the ontology of learning, consolidation and retrieval processes of memory, and sensory factors of attention. These findings have rapidly expanded the scientific understanding of learning processes.
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