Instead, he focused on continuously revising his original formula as exceptions showed up. 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. Behaviors were influenced by goals that sought to satisfy primary drives—such as hunger, thirst, sex, and the avoidance of pain. His systematic behavior theory, also known as drive theory, is that of a reinforcement system, which means that in learning, habits are initially formed by reinforcing certain behaviors. Reinforcement of a response to a behavior supplies an effect that satisfies a need. In other words, this satisfaction of needs helps create habits out of behaviors. Specifically, Hull's theory posits that behaviors that satisfy needs, later described by Hull as cravings rather than needs, reduce these cravings. He called this concept drive-reduction, or drive-stimulus reduction. Other behaviorists found Hull's theories to be too cumbersome for practical use, leading to his work to be eclipsed by Skinner. Influence In 1936 Hull worked with students and associates and together they started a series of evening seminars that became known as “Monday Night Meetings”. They would discuss topics such as conditioned reflexes, behavior laws, and Freud's psychoanalysis. These meetings became popular with many kinds of people, such as psychologists, sociologists, and anthropologists and sometimes as many as seventy people would attend. Later in life when Hull was in poor health, he had the help of his research assistants and volunteers to conduct his experiments. He also relied on people to keep him up to date on current discussions on current psychological experiments and theories that he was unable to attend and participate in. Hull was one of the most frequently cited psychologists during the 1940s and 1950s. Aptitude Testing (1928) was a widely quoted textbook and his work Hypnosis and Suggestibility: An Experimental Approach (1933) was widely studied. Hull's Principles of Behavior (1943) was one of the most widely cited books in psychology. In an old Handbook of Experimental Psychology, his work was mentioned on over eighty pages, which was more than all other scientists at the time. In previous issues of the Journal of Experimental Psychology and the Psychological Review over forty percent of the bibliographies included one or more of his writings. Hull advised and inspired a number of graduate students and psychologists that went on to revise his theories and make contributions