A subject may be shown sets of three figures (say, two round and one triangular; next, two square and one round, and so on). With proper rewards, the subject may learn to distinguish any “odd” member of any set from those that are similar. Animals as low in the evolutionary scale as the pigeon can master the principle of this so-called oddity problem.

Problem solving

Examples of human problem solving are familiar: finding the roots of a quadratic equation, solving a mechanical puzzle, and navigating by the stars. Among other animals, chimpanzees have been observed to solve problems requiring toolmaking.

This list only samples from the remarkable array of animal activities categorized as learning. Beginning with habituation, they range from the simple adjustments of single-celled animals up to the highest intellectual accomplishments of mankind. It would be wonderful indeed if a single theory of learning were enough to account for all this diversity. So far, however, no theory of learning adequately covers more than a small fraction of these phenomena.

Important earlier theorists

Beginning in the 1930s a number of general theories were advanced in attempts to organize most or all of the psychology of learning. The most influential of the contributing theorists are noted below.

E.R. Guthrie (1886–1959) wrote that learning requires only that a response be made in a changing situation. Any response was held to be linked specifically to the situation in which it was learned. Guthrie argued that learning is complete in one trial, that the most recent response in a situation is the one that is learned, and that responses (rather than perceptions or psychological states) provide the raw materials for the learning process.

For E.C. Tolman (1886–1959) the essence of learning was the acquisition by the organism of a set of what he called Sign-Gestalt-Expectations. These referred to propositions said to be made by the learner that his own specific response to given signs (or stimuli) would result in such and such circumstances later on. Tolman seemed to be saying that what the learner acquires is a specific knowledge of “what leads to what.” In brief, his theory was that the learner develops expectations based on experience and that learning depends entirely on successions of events. Although less vocal on the point than others, Tolman implied that learning was a gradual process.

The theory offered by Clark L. Hull (1884–1952), over the period between 1929 and his death, was the most detailed and complex of the great theories of learning. The basic concept for Hull was “habit strength,” which was said to develop as a function of practice. Habits were depicted as stimulus-response connections based on reward. According to Hull, responses (rather than perceptions or expectancies) participate in habit formation, the process is gradual, and reward is an essential condition.

Comparison of these theories yields major questions for empirical investigation. Is learning continuous or discontinuous; is it a gradual or sudden (one-trial) process? Is learning a matter of establishing stimulus–response (S–R) connections or does it depend on the learner’s understanding of perceptual relationships? Is reward necessary for learning?

Are theories of learning necessary?

Such major investigators of learning as B.F. Skinner and J.A. McGeoch maintained in the 1930s and 1940s that preoccupation with theory was misguided. For them the approach simply was to discover the conditions that produce and control learned behaviour. Beyond this, their interests diverged. Skinner studied instrumental conditioning (operant conditioning, as he called it) among rats; McGeoch specialized in human rote memory. Although study of rote verbal learning had become heavily theoretical by the 1970s, Skinner and his associates stuck to their empirical guns, guiding a variety of programs for the practical control of behaviour. Teaching machines and computer-aided instruction, behaviour modification (e.g., the use of tokens to reward desired behaviour among psychiatric patients), and planned utopian societies (Walden II) all found scientific origins in Skinner’s rejection of theory in favour of direct efforts to produce results.

Intervening variables and hypothetical constructs

Learning is a concept and not a thing, and the activity called learning is inferred only through behavioral symptoms. The distinction implicit here between behaviour and inferred process is one of Tolman’s major contributions and serves to reconcile influential views that might seem completely at odds. Classical behaviourism, as developed by John B. Watson (1878–1958), rejected every mentalistic account and sought to limit analysis to such physiological mechanisms as reflexes. Watson argued that these are objective in a way that so-called thoughts, hopes, expectancies, and images cannot be. The opposing view holds that experiential (introspective) activity (exactly what Watson sought to dismiss) does require discussion.

Tolman called himself a behaviourist and ostensibly was bound by Watson’s insistence on objectivity. But he also was interested in thinking, expectancy, and consciousness. Tolman found his solution to this problem of incompatible theories after his association with the Vienna Circle of Logical Positivists, whose deterministic teachings he brought to the attention of U.S. psychologists about 1920. He maintained that learning is inexorably produced (determined) by such independent (directly manipulable) variables as the organism’s previous training and physiological condition and by the response the environment requires. According to Tolman, the development of learning is revealed through the changing probability that given behaviour (the dependent variable) will result. He held that learning itself is not directly observable; it is an intervening variable, one that is inferred as a connecting process between antecedent (independent) variables and consequent (dependent) behaviour.

An attractive possibility is that intervening variables may have discoverable physiological bases. Psychologists Paul E. Meehl and Kenneth MacCorquodale proposed a distinction between the abstractions advocated by some and the physiological mechanisms sought by others. Meehl and MacCorquodale recommended using the term intervening variable for the abstraction and hypothetical construct for the physiological foundation. To illustrate: Hull treated habit strength as an intervening variable, defining it as an abstract mathematical function of the number of times a given response is rewarded. By contrast, Edward L. Thorndike (1874–1949) handled learning as a hypothetical construct, positing a physiological mechanism: improved conduction of nerve impulses.

Intervening variables and hypothetical constructs need not be incompatible; Thorndike’s hypothetical neural process could empirically be found to be the mechanism through which Hull’s abstraction operates.

Miniature theories

With growing realization of the complexity of learning, the grand theories of Guthrie, Hull, and Tolman generally have been abandoned except as historic landmarks. Hope for any impending, comprehensive theory was almost dead in the 1970s. More modest miniature theories remain, many likely to be of temporary value. An account of their major themes and issues, however, should have more enduring interest.