Principles of Behavior: An Introduction to Behavior Theory

Hull's central innovation is the construction of behavior theory as a formal hypothetico-deductive system. He argues that scientific theory consists of a small number of postulates or principles from which secondary principles or theorems are deduced; if the theorems match observation, the postulates are confirmed[citation:2]. This contrasts with the descriptive, inductive approach of Watson's early behaviorism and with Skinner's later radical behaviorism. Hull presents sixteen postulates and five major corollaries, expressed in mathematical form, covering stimulus trace, afferent neural interaction, drive, habit strength, and inhibition[citation:2]. His goal is a behavioral science at the level of development achieved by physics in the age of Galileo and Newton[citation:6].

Intervening variables are hypothetical constructs that mediate between observable antecedent conditions (A) and observable consequent conditions (B). Hull schematizes this as A—f→(X)—f→B, where X is the intervening variable securely anchored to measurable events on both sides[citation:19]. Examples in Hull's system include habit strength (sHr), drive (D), and inhibitory potential (Ir). The key methodological point is that these constructs are defined by their functional relations to observable variables, not by their physiological referents. Hull borrowed the concept from Tolman, though he used it more systematically[citation:4]. This allowed him to theorize about unobservable processes without abandoning the positivist requirement that scientific terms be anchored to observable operations.

The two systems represented opposing poles within behaviorism. Hull embraced intervening variables and formal theory construction; Skinner rejected them as premature and potentially mentalistic. Skinner's 1944 review of Principles of Behavior criticized Hull for abandoning the molar level of behavioral analysis and aligning himself with "semi-neurologists"[citation:4]. Skinner advocated a descriptive, functional analysis of behavior-environment relations without hypothetical mediational constructs. Despite this opposition, some historians have noted that Skinner's early work employed constructs functionally similar to intervening variables, such as reflex strength and reflex reserve[citation:4]. The Hull-Skinner contrast became a defining fault line in mid-twentieth-century learning theory.

The book was enormously influential in the decade following its publication. Approximately 70 percent of studies reported in the Journal of Experimental Psychology and the Journal of Comparative Psychology in the ten years after 1943 made reference to Hull's writings, mostly to Principles of Behavior[citation:6]. However, by the late 1950s Hull's visibility had greatly diminished and his contributions were judged by many to have been misguided[citation:7][citation:17]. A historical study by Coleman and Webster treats this decline as an inevitable outcome of the programmatic nature of Hull's theory: its formal precision made it vulnerable to disconfirmation, and when key postulates failed empirical tests—particularly regarding the necessity of drive reduction for reinforcement—the system lost credibility[citation:7].

Critics from multiple camps attacked the system. Tolman's followers argued that despite Hull's claim that his theory was molar, it revealed a strong molecular bias, citing chapters on physiological mechanisms and postulates that referred directly to neural events[citation:4]. Skinner objected that Hull had abandoned productive molar formulation for semi-neurological speculation[citation:4]. Kenneth Spence, Hull's own student, warned him against supplementing formal mathematical definitions with informal mechanistic interpretations, comparing it to Maxwell insisting on an ether-like medium for electromagnetic phenomena[citation:19]. MacCorquodale and Meehl (1948) argued that Hull's theoretical terms carried surplus meaning beyond their operational definitions, particularly through their everyday-language names like "habit strength" and "drive"[citation:19].

The central equation predicted reaction potential (sEr), the likelihood or intensity of a learned response: sEr = D × V × K × sHr, minus inhibitory factors[citation:9]. Drive (D) was measured by hours of deprivation; stimulus intensity dynamism (V) by signal strength; incentive (K) by reward magnitude; and habit strength (sHr) by number of reinforcements. The equation attempted to integrate motivation, learning, and performance into a single quantitative framework. Hull believed that if all factors could be measured under laboratory conditions, the equation could be checked and behavior predicted. This commitment to formal, quantitative modeling was the most distinctive feature of his neo-behaviorism and its most controversial, as critics questioned whether the constructs were genuinely measurable or merely mathematically convenient[citation:10].