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.

During his early years at Yale, Hull began to formulate his global theory of behaviour, which he based on principles drawn from a variety of sources. He took certain ideas on conditioning from the Russian physiologist Ivan Pavlov and also borrowed from American psychologists, including John B. Watson, who emphasized the objective study of behaviour, and Edward L. Thorndike, who asserted the importance of reinforcement in learning.

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.

What is drive reduction theory?

Like other behaviorists, Hull was only interested in overt behavior, yet he did not deny existence of cognitive factors like ideas, prior knowledge, intelligence, insight and values. He excluded this factors from his theory since they could not be scientifically measured.

In his theory, Hull tried to explain behavior and learning through drive reduction. “Drive” presents a stimulus in form of a biological need like hunger, thirst, cold or sexual interest. It is a state of need, when a living organism feels the needs to behave in certain way to reduce the need and restore the optimal biological state. The drive therefore results in behavior in order to achieve certain goal or satisfy the need. If the goal of the drive is achieved, the drive is reduced and an optimal state is restored. The drive reduction here is a reinforcer, strengthening the connection between the drive and behavior.

This reinforced stimulus-response (S-R) learning through the reduction of a biological drive was the only kind of learning according to Hull. Because of his beliefs that a living organism would repeat a behavior that would reduce a drive, his theory was also called a drive-reduction theory of motivation.

Hull's theory was mostly orientated on S-R relationship and reinforcement. If a S-R relationship is followed by a reduction of the drive, the probability of same prior response on similar situations in the future increases. S-R relationship (habit strength) becomes stronger through the number of reinforcements. Biological needs were according to Hull primary drives, but he also believed there are secondary drives (learned drives), which refer to situations associated with reduction of primary drives. That means a neutral stimulus can have primary drive characteristics, because it is capable of eliciting responses similar to those caused by primary drive.

In order to offer a full, scientific explanation of learning, Hull developed a formula in which he tried to mathematically explain and predict a likelihood of behavior. This formula measures habit strength defined as the strength of the S-R bond. This bond represents learning. Hull's equation claims:

Hull believed that this formula could account for all behavior in humans and animals.

Hull's theory was further developed by one of his students, Kenneth Spence. Spence disagreed with Hull's assumption that improvement in performance comes only due to habit factors. In his opinion, this was the influence of motivation. He also believed reinforcement does not have a role in learning itself. Reinforcement can serve as motivator for learning and enhance a response, yet it does not necessarily enhance learning of a response. This idea was later known as the Hull-Spence hypothesis of conditioning and learning.

It was Spence's idea that performance in learned behavior cannot be attributed to habituation, but rather to motivational factors behind it. Learning can also occur through “latent learning”. This idea would explain the fact that organisms do not always perform in accordance with what they have learned.

During his life Spence conducted most of experiments on animals. When he finally tried to experiment on human subjects, unlike his forerunners, Spence suggested humans are far more complex than other living beings. Learning theories derived from observations of non-human organisms therefore can not be directly applied to them, and additional components, especially cognitive factors need to be taken into account.

Practical meaning of drive reduction theory

Hull's equation suggests performance is affected by the following variables:

§  sHr - reinforced trainings improve the S-R connection resulting in learning (of a motor skill).

§  D - deprivation of drive results in improved performance. Experiments have shown that if two rats had the same amount of training, the one who had been deprived of food for a greater period of time would be more likely to find a solution to a maze in order to obtain food.

§  K - incentive motivation refers to the size of the prize and directly affects motivation to achieve certain goal. This explains why athletes perform better in playoffs than during regular season play since the incentive motivation of each game has increased.

§  V - clarity of the stimulus can affect the performance. If an sportsman is trying to catch the ball, blinding sunlight can affect clarity of the stimulus.

§  sIr - non-reinforced trainings (trainings that don't result in drive reduction) cause reduction of the probability of displaying certain behavior (extinction).


Hull’s final system summarized

There are three kinds of variable in hull’s theory:

1.      Independent variable –which are stimulus events systematically manipulated by the experimenter.

2.      Intervening variables – which are process thought to be taking place within the organism but directly observable.

3.      Dependent variables – which are some aspect of behavior that is measured by  the experimenter in order to determine whether the independent variables had any effect.



The development of curriculum

In this reference hull emphasized the importance of needs in learning process and accordingly the needs of all categories of children should be incorporated in the curriculum learning becomes meaningful only when it satisfies the needs of children.

The know actual needs of the students by teacher and parents

Hull is fells that teachers and parents of the student should also share their responsibility in teaching the actual needs of the student through various means proper guidance is must for their attitude and aptitudes.

Emphasized anxiety as a drive in human learning

From this line of reasoning, it follows that encouraging some anxiety in students that could subsequently be reduced by success is a necessary condition for classroom learning. Too little anxiety results in no learning (because there is no drive to be reduced), and too much anxiety is disruptive. Therefore, students who are mildly anxious are in the best position to learn and are therefore easiest to teach.

Hull’s system of learning advocated the following chain sequence for improved results in the teaching-learning process:

a.       Drive – This is something which is needed by the learner in order to behave or respond.

b.      Cue – There must be something to which the learner must respond.

c.       Response – The learner must be made to respond in order to learn some act.

d.      Reward – The learner’s response must be reinforced or rewarded, thus enabling him to learn what he wants to learn.


Contemporary Criticism

While Hull's theory was popular during the middle part of the 20th century, it began to fall out of favour 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 reinforcers reduce drives.

Unlike primary drives such as hunger and thirst, secondary reinforcers do nothing to directly reduce physiological and biological needs. Take money, for example. While money does allow you to purchase primary reinforcers, 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.


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.