Essentials of Behavior

The book is a restatement and revision of the formal system Hull published in Principles of Behavior (1943). Its purpose is to present the essentials of that system more economically, incorporating experimental findings and theoretical corrections that had accumulated in the intervening eight years. The most consequential change is the abandonment of the postulate that reinforcement necessarily involves the reduction of a primary drive. Hull had come to accept that contiguity of stimulus, response, and reinforcement is sufficient for habit formation. This revision loosened the tight connection between learning and motivation that had defined the earlier system and made room for a more flexible account of how behavior is acquired and performed.

The major revision concerns the reinforcement postulate. In 1943, Hull held that drive reduction was the necessary condition for reinforcement; in Essentials, he replaced this with a contiguity principle, arguing that habit strength grows when stimulus, response, and reinforcement occur together, regardless of whether a drive is reduced. He also separated incentive motivation (K) more sharply from drive (D), treating them as independent determinants of reaction potential. The number of postulates was reduced and their verbal statements tightened. These changes responded to empirical challenges, particularly from studies showing that reinforcement can occur without drive reduction and that incentive effects are not reducible to drive. The revised system was leaner but also less ambitious in its physiological claims.

Skinner's review of Essentials of Behavior, published in Psychological Bulletin in 1952, continued the argument he had made against Principles of Behavior in 1944. Skinner objected to Hull's continued reliance on intervening variables and his alignment with what Skinner called semi-neurological speculation. He argued that the revised postulates, though leaner, still substituted hypothetical constructs for a direct functional analysis of behavior-environment relations. Skinner contended that the proper task of a science of behavior is to establish empirical laws relating observable events, not to construct deductive systems whose postulates are only indirectly testable. The review reinforced the methodological divide between Hull's hypothetico-deductive approach and Skinner's descriptive radical behaviorism, a divide that shaped learning theory for the following decade.

Kenneth Spence was Hull's most influential student and collaborator, and he became the leading figure in the neo-Hullian tradition after Hull's death in 1952. Spence had warned Hull against supplementing formal mathematical definitions with informal mechanistic interpretations, and he emphasized the importance of testing Hullian postulates through rigorous experimentation. After Essentials of Behavior appeared, Spence continued to develop and defend the system, particularly the distinction between learning (habit strength) and performance (reaction potential). Spence's own work on discrimination learning and incentive motivation extended Hull's framework into new domains. The Spence-Hull tradition remained a major force in experimental psychology through the 1950s, though its influence declined as cognitive approaches gained prominence.

Essentials of Behavior was received as an important but less influential sequel to Principles of Behavior. Reviewers acknowledged the value of the revisions, particularly the abandonment of drive reduction as a necessary condition for reinforcement, but noted that the system remained complex and that its postulates were still difficult to test directly. The book did not generate the same volume of experimental work as the 1943 volume, partly because the broader theoretical landscape was shifting. Hull himself regarded it as a more accurate statement of his position, and it is essential for understanding the final form of his system. It was published one year before his death, making it his last major theoretical statement.

Hull's revised system influenced the development of neo-Hullian theories, particularly those of Kenneth Spence and Neal Miller. The separation of learning from performance, which Essentials sharpened, became a central issue in the decades that followed, as researchers investigated the conditions under which acquired behavior is actually performed. The abandonment of drive reduction as a necessary condition for reinforcement anticipated the shift toward cognitive and informational accounts of reinforcement. However, Hull's deductive method and his confidence in quantitative postulates were increasingly rejected as the field moved toward more modest, data-driven theorizing. The book is now read primarily by historians seeking to understand the trajectory of behaviorism's most ambitious attempt to become a formal science.