Clark Leonard Hull (May 24, 1884 – May 10, 1952) was an American psychologist who sought to explain learning and motivation by scientific laws of behavior.

Hull spent the mature part of his career at Yale University, where he was recruited by the president and former psychologist, James Rowland Angell.

He is perhaps best known for the "goal gradient" effect or hypothesis, wherein organisms spend disproportionate amounts of effort in the final stages of attainment of the object of drives.

[1] Nonetheless, a Review of General Psychology survey, published in 2002, ranked Hull as the 21st most cited psychologist of the 20th century.

[3] As he was about to graduate from the academy, he attended a banquet where the food was contaminated, and contracted a near deadly case of typhoid, which delayed his return to college.

[3] As his health improved, he returned to Alma College, where he studied math, physics, and chemistry, intending to become an engineer.

[3][5] After two years at Alma College, Hull moved to Hibbing, Minnesota to work as an apprentice mining engineer.

[3][4] However, after two months he was afflicted by polio, which left him paralyzed in one leg, causing him to recover at his parents home for a year and forcing him to reconsider his life path.

"[3][4][5] After graduating, he spent some time working as a half-time assistant and eventually teaching as a full-time instructor at the University of Wisconsin before moving on to Yale.

Because he loved the mathematical portion of the course, he changed the class to aptitude testing, which focused on scientific basis of vocational guidance.

Hull had a great desire to teach this course and felt as though this specific type of science was the foundation of true psychology.

Also, in his work he emphasized quantitative data so everything could be analyzed more precisely, and less open to interpretation than previous studies on the topics.

[4] Though no longer doing active research in the field, he retained an interest, debating Karl Lashley's beliefs on the heritability of intelligence.

[3][4][9] After successfully putting a disturbed student into a trance with a so-called "hypnotic crystal",[10] he began to research the phenomenon and its medical applications.

[3][9] Dissatisfied with the unscientific nature of the field, Hull sought to bring greater academic rigor through measuring behavior instead of relying on self-reports.

The main question of Hull's study was to examine the veracity of the apparently extravagant claims of hypnotists, especially regarding extraordinary improvements of cognition or the senses by hypnosis.

The only other notable difference is that Hull believed that those in hypnotic states were better able to remember events that had happened far in the subject's past.

Hull's experiments showed the reality of some classical phenomena such as mentally induced pain reduction and apparent inhibition of memory recall.

Similarly, moderate increases of certain physical capacities and changes to the threshold of sensory stimulation could be induced psychologically; attenuation effects could be especially dramatic.

), V is stimulus intensity dynamism (some stimuli will have greater influences than others, such as the lighting of a situation), and K is incentive (how appealing the result of the action is).

[5] Hull's emphasis was on experimentation, an organized theory of learning, and the nature of habits, which he argued were associations between a stimulus and a response.

[6] Behaviors were influenced by goals that sought to satisfy primary drives—such as hunger, thirst, sex, and the avoidance of pain.

[5] In 1936 Hull worked with students and associates and together they started a series of evening seminars that became known as "Monday Night Meetings".

[7] Hull advised and inspired a number of graduate students and psychologists that went on to revise his theories and make contributions to the field of psychology.

[17] John Dollard taught anthropology, psychology and sociology at Yale and was interested in studying social class and specific learning experiences.

[18] Miller and Dollard collaborated and developed a social learning theory that was successfully applied to psychotherapy and understanding.

[19] They used a similar construct to Hull's theory, however, they proposed that any strong stimulus could have motivating or drive properties without essentially being tied to the need of that particular organism.

[21]: 1687  Spence contributed to the study of incentive motivation and developing mathematical formulation and equations to describe learning acquisition.

[22] His discrimination theory suggests that there are gradients of excitatory and inhibitory potential that are generated around the values of the stimulus that are either reinforced or not.

He followed the acceptable understanding of psychology at that time, and was influenced by the work and the conclusions of the pioneers of Behaviorism (Edward Thorndike, John B. Watson, and Ivan Pavlov).