Clark Hull was an American psychologist known for developing a theory of learning and behavior based on mathematical formulas. He proposed that all behavior is motivated by biological drives to reduce states of tension. When an action reduces a drive, it is reinforced and becomes more likely in the future. Though influential at the time, Hull's drive-reduction theory has been criticized for failing to explain behaviors not related to biological needs or that increase tension. While no longer dominant, understanding Hull's work provides context for later psychological theories.

1. THEORY
CLARK LEONARD HULL
INTRODUCTION
Clark L. Hull, in full Clark Leonard Hull, (born May 24, 1884, Akron, N.Y., U.S. died May 10,
1952, New Haven, Conn.), American psychologist known for his experimental studies on
learning and for his attempt to give mathematical expression to psychological theory. He applied
a deductive method of reasoning similar to that used in geometry, proposing that a series of
postulates about psychology could be developed, from which logical conclusions could be
deduced and tested. If a test failed, the postulate could be revised, and if the test then succeeded,
the findings would be added to the body of psychological science.
The drive reduction theory of motivation became popular during the 1940s and 1950s as a way to
explain behavior, learning, and motivation. The theory was created by behaviorist Clark Hull and
further developed by his collaborator Kenneth Spence. According to the theory, the reduction of
drives is the primary force behind motivation.
While the drive-reduction theory of motivation was once a dominant force in psychology, it is
largely ignored today. Despite this, it's worthwhile for students to learn more about Hull’s ideas
in order to understand the effect his work had on psychology and to see how other theorists
responded by proposing their own theories.
Overview
Hull was one of the first theorists to attempt to create a grand theory designed to explain all
behavior. He started developing his theory shortly after he began working at Yale University,
drawing on ideas from a number of other thinkers including Charles Darwin, Ivan Pavlov, John.
B. Watson, and Edward L. Thorndike.
Hull based his theory on the concept of homeostasis, the idea that the body actively works to
maintain a certain state of balance or equilibrium. For example, your body regulates its
temperature in order to ensure that you don't become too hot or too cold. Hull believed that
behavior was one of the ways that an organism maintains this balance.
Based on this idea, Hull suggested that all motivation arises as a result of these biological needs.
In his theory, Hull used the term drive to refer to the state of tension or arousal caused by
biological or physiological needs. Thirst, hunger, and the need for warmth are all examples of
drives. A drive creates an unpleasant state, a tension that needs to be reduced.

2. In order to reduce this state of tension, humans and animals seek out ways to fulfill these
biological needs. We get a drink when we are thirsty. We eat when we are hungry. We turn up
the thermostat when we are cold. He suggested that humans and animals will then repeat any
behavior that reduces these drives.
Conditioning and Reinforcement
Hull is considered a neo-behaviorist thinker, but like the other major behaviorists, he believed
that human behavior could be explained by conditioning and reinforcement. The reduction of the
drive acts as a reinforcement for that behavior.
This reinforcement increases the likelihood that the same behavior will occur again in the future
when the same need arises. In order to survive in its environment, an organism must behave in
ways that meet these survival needs.
"When survival is in jeopardy, the organism is in a state of need (when the biological
requirements for survival are not being met) so the organism behaves in a fashion to reduce that
need," Hull explained.
In a stimulus-response (S-R) relationship, when the stimulus and response are followed by a
reduction in the need, it increases the likelihood that the same stimulus will elicit the same
response again in the future.
Mathematical Behavior
Hull's goal was to develop a theory of learning that could be expressed mathematically, to create
a "formula" to explain and understand human behavior.
Mathematical Deductive Theory of Behavior
sEr = V x D x K x J x sHr - sIr - Ir - sOr - sLr
sEr: Excitatory potential, or the likelihood that an organism will produce a response (r) to a
stimulus (s)
V: Stimulus intensity dynamism, meaning some stimuli will have greater influences than others
D: Drive strength, determined by the amount of biological deprivation
K: Incentive motivation, or the size or magnitude of the goal

3. J: The delay before the organism is allowed to seek reinforcement
sHr: Habit strength, established by the amount of previous conditioning
slr: Conditioned inhibition, caused by previous lack of reinforcement
lr: Reactive inhibition, or fatigue
sOr: Random error
sLr: Reaction threshold, or the smallest amount of reinforcement that will produce learning
Hull's approach was viewed by many as overly complex, yet at the same time, critics suggested
that the drive-reduction theory failed to fully explain human motivation. His work did, however,
have an influence on psychology and future theories of motivation.
Contemporary Criticism
While Hull's theory was popular during the middle part of the 20th century, it began to fall out of
favor for a number of reasons. Because of his emphasis on quantifying his variables in such a
narrowly defined way, his theory lacks generalizability. However, his emphasis on rigorous
experimental techniques and scientific methods did have an important influence in the field of
psychology.
One of the biggest problems with Hull's drive reduction theory is that it does not account for how
secondary reinforces reduce drives.
Unlike primary drives such as hunger and thirst, secondary reinforces do nothing to directly
reduce physiological and biological needs. Take money, for example. While money does allow
you to purchase primary reinforces, it does nothing in and of itself to reduce drives. Despite this,
money still acts as a powerful source of reinforcement.
Another major criticism of the drive reduction theory of learning is that it does not explain why
people engage in behaviors that do not reduce drives. For example, people often eat when they’re
not hungry or drink when they’re not thirsty.
In some cases, people actually participate in activities that increase tension such as sky-diving or
bungee jumping. Why would people seek out activities that do nothing to fulfill biological needs
and that actually place them in considerable danger? Drive-reduction theory cannot account for
such behaviors.

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Impact
While Hull's theory has largely fallen out of favor in psychology, it is still worthwhile to
understand the effect it had on other psychologists of the time and how it helped contribute to
later research in psychology.
In order to fully understand the theories that came after it, it's important for students to grasp the
basics of Hull’s theory. For example, many of the motivational theories that emerged during the
1950s and 1960s were either based on Hull's original theory or were focused on providing
alternatives to the drive-reduction theory.
One great example is Abraham Maslow's famous hierarchy of needs, which emerged as an
alternative to Hull's approach.