Clark Leonard Hull was born May 24, 1884 near Akron, New York, to a poor, rural family. His was educated in a one-room school house and even taught there one year, when he was only 17. While a student, he had a brush with death from typhoid fever.

He went on to Alma College in Michigan to study mining engineering. He worked for a mining company for two months when he developed polio. This forced him to look for a less strenuous career. For two years, he was principal of the same school he had gone to as a child -- now consisting of two rooms! He read William James and saved up his money to go to the University of Michigan.

After graduating, he taught for a while, then went on to the University of Wisconsin. He got his PhD there in 1918, and stayed to teach until 1929. This was where his ideas on a behavioristic psychology were formed.

In 1929, he became a professor of psychology at Yale. In 1936, he was elected president of the APA. He published his masterwork, Principles of Behavior, in 1943. In 1948, he had a massive heart attack. Nevertheless, he managed to finish a second book, A Behavior System, in that same year. He died of a second heart attack May 10, 1952.

Hull’s theory is characterized by very strict operationalization of variables and a notoriously mathematical presentation. Here are the variables Hull looked at when conditioning rats:

Independent variables:

S, the physical stimulus.

Time of deprivation or the period and intensity of painful stimuli.

G, the size and quality of the reinforcer.

The time delay between the response and the reinforcer.

The time between the conditioned and unconditioned stimulus.

N, the number of trials.

The amount of time the rat had been active.

The intervening variables:

s, the stimulus trace.

V, the stimulus intensity dynamism.

D, the drive or primary motivation or need (dependent on deprivation, etc.).

K, incentive motivation (dependent on the amount or quality of reinforcer).

J, the incentive based on delay of reinforcement.

sHr, habit strength, based on N, G (or K), J, and time between conditioned and unconditioned stimulus.

Ir, reactive inhibition (e.g. exhaustion because the rat had been active for some time).

sIr, conditioned inhibition (due to other training).

sLr, the reaction threshold (minimum reinforcement required for any learning).

sOr, momentary behavioral oscillation -- i.e. random variables not otherwise accounted for.

And the main intervening variable, sEr, excitatory potential, which is the result of all the above...

sEr = V x D x K x J x sHr - sIr - Ir - sOr - sLr.

The dependent variables:

Latency (speed of the response).

Amplitude (the strength of the response).

Resistance to extinction.

Frequency (the probability of the response.

All of which are measures of R, the response, which is a function of sEr.

Whew!

The essence of the theory can be summarized by saying that the response is a function of the strength of the habit times the strength of the drive.  It is for this reason that Hull’s theory is often referred to as drive theory.

Hull was the most influential behaviorist of the 1940’s and 50’s.  His student, Kenneth W. Spence, maintained that popularity through much of the 1960’s.  But the theory, acceptable in its abbreviated form, was too unwieldy in the opinion of other behaviorists, and could not easily generalize from carefully controlled rat experiments to the complexities of human life.  It is now a matter of historical interest only.