Hull, Clark Leonard (1884-1952), an American psychologist, expressed psychological theory in mathematical terms.
Hull was nearly 30 when he completed his undergraduate education at the University of Michigan. Although he had originally planned to be a mining engineer, after convalescing from polio and spending two years as a schoolteacher, he decided to become a psychologist. After doing graduate work at the University of Wisconsin, where he earned his doctorate in 1918, he joined the psychology department there. Hull's research in the 1920's on the measurement of intelligence and ability led to his Aptitude Testing (1928). During that decade, he also carried out a 10-year large-scale research project, which culminated in Hypnosis and Suggestibility: An Experimental Approach (1933).
In 1929, Hull left Wisconsin to join the faculty at Yale University, where he became the intellectual leader at the Institute of Human Relations. There he trained several generations of graduate students to carry out experimental work aimed at discovering the fundamental laws of behavior, while he concentrated on theory. According to Hull, psychology had laws that could be discovered and formulated mathematically, just as Newton had done for nature as a whole. His attempt to give mathematical expression to learning theory resulted in Mathematico-Deductive Theory of Rote Learning (1940), a collaborative work, in which the research is described not only verbally but also mathematically.
In Principles of Behavior (1943), Hull developed this concept further. Inspired by Edward Lee Thorndike's reinforcement theory, Hull presented a quantitative account of the mechanisms, including reinforcement, that enable organisms to adjust to their environment. A revised and expanded version, A Behavior System (1952), appeared posthumously.
The Hullian system became the target of criticism after Hull's death in 1952. While admired for its effort to set high scientific standards for psychology, the Hullian theory was criticized for not being able to generalize beyond the laboratory to actual learning settings.