The concept of drives is very important to the theories of Clark L. Hull, a neobehaviorist. According to Hull, a drive has at least two distinct functions as far as behavioral activation is concerned. Without drives there could be no reinforcement and thus no learning, because drive reduction is the reinforcement. Without drives there could be no responses, for drives activate behavioral potentials into performance. Drive theory maintains that a state named "drive" (or D) is a necessary condition for behavior to occur; however, D is not the same as the bodily need. D determines how strongly and persistently a behavior will occur; it connects the need and behavior. This distinction between need and drive is necessary because, while the state of need serves as the source of behavior, the intensity of behavior is not always related to the intensity of need. Need can be defined as a state of an organism attributable to deprivation of a biological or psychological requirement, related to a disturbance in the homeostatic state.
There are cases in which the need increases but behavior does not, or in which the need remains but behavior is no longer manifested. Prolonged deprivation, for example, may not result in a linear or proportional increase in behavior. A water-deprived animal may stop drinking even before cellular dehydration is restored to the normal state; the behavior is changing independently of homeostatic imbalance. Cessation of behavior is seen as being attributable to drive reduction.
Hull uses D to symbolize drive and sHr (H is commonly used to denote this, for convenience) to symbolize a habit which consists of an acquired relationship between stimulus (S) and response (R). It represents a memory of experience in which certain environmental stimuli and responses were followed by a reward. An effective reward establishes an S-R relationship; the effect is termed reinforcement. One example of an H would be an experience of maze stimuli and running that led to food. H is a behavioral potential, not a behavior. Food deprivation induces a need state that can be physiologically defined; then D will energize H into behavior. The need increases monotonically with hours of deprivation, but D increases only up to three days without food. A simplified version of the Hullian formula for a behavior would be "behavior = HD," or "performance = behavioral potential ener-gizer." The formula indicates that learning, via establishing behavioral potential, and D, via energizing the potential, are both necessary for performance to occur. This is a multiplicative relationship; that is, when either H or D is zero, a specific performance cannot occur.
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