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Instrumental Conditioning: Thorndike and the Law of Effect

The experimental study of instrumental conditioning began a century ago and was sparked by the debate over Darwin’s theory of evolution by natural selection.

Thorndike and the Law of Effect

The experimental study of instrumental conditioning began a century ago and was sparked by the debate over Darwin’s theory of evolution by natural selection. Supporters of Darwin’s theory emphasized the continuity among species, both living and extinct: Despite their apparent differences, a bird’s wing, a whale’s fin, and a human arm, for example, all have the same basic bone structure; this similarity makes it plausi-ble that these diverse organisms all descended, by a series of incremental steps, from common ancestors. But opponents of Darwin’s theory pointed to something they per-ceived as the crucial discontinuity among species: the human ability to think and reason—an ability they claimed animals did not share. Didn’t this ability, unique to our species, require an altogether different (non-Darwinian) type of explanation?

In response, Darwin and his colleagues argued that there is, in fact, considerable continuity of mental prowess across the animal kingdom. Yes, humans are smarter in some ways than other species; but the differences might be smaller than they initially


seem. In support of this idea, Darwinian naturalists collected stories about the intellec-tual achievements of various animals (Darwin, 1871). These stories painted a flattering picture, as in the reports of cunning cats that scattered breadcrumbs on the lawn to lure birds into their reach (Romanes, 1882). In many cases, however, it was hard to tell whether these reports were genuine or just bits of folklore. Even if they were genuine, it was unclear whether the reports had been polished by the loving touch of a proud pet owner. What was needed, therefore, was more objective and better documented research—research that was made possible by a method described in 1898 by Edward L. Thorndike (1874–1949; Figure 7.16).

 

CATS  IN APUZZLE BOX

Thorndike’s method was to set up a problem for an animal to solve. In his classic exper-iments, he placed a hungry cat inside a box with a latched door. The cat could open the door—and escape from the box—only by performing some simple action such as pulling a loop of wire or pressing a lever (Figure 7.17); and once outside the box, the cat was rewarded with a small portion of food. Then the cat was placed back into the box for another trial so that the procedure could be repeated over and over until the task of escaping the box was mastered.


On the first trial, the cats had no notion of how to escape—and so they meowed loudly and clawed and bit at their surroundings. This continued for several minutes until finally, purely by accident, the animal hit upon the correct response.

Subsequent trials brought gradual improvement, and the animal took less and less time to produce the response that unlocked the door. By the time the train-ing sessions were completed, the cats’ behavior was almost unrecognizable from what it had been at the start. When placed in the box, they immediately approached the wire loop or the lever, yanked it or pressed it with businesslike dispatch, and hurried through the open door to enjoy the well-deserved reward.

If you observed only the final performance of these cats, you might well credit the animals with reason or understanding. But Thorndike argued that the cats solved the problem in a very different way. As proof, he recorded how much time the cats required on each trial to escape from the puzzle box, and he charted how these times changed over the course of learning. Thorndike found that the resulting curves declined quite gradually as the learning proceeded (Figure 7.18). This isn’t the pattern we would expect if the cats had achieved some understanding of how to solve the problem. If they had, their curves would show a sud-den drop at some point in the training, when they finally got the point. (“Aha!” mut-tered the insightful cat, “it’s the lever that lets me out,” and henceforth howled and bit


no more.) Instead, these learning curves suggest that the cats learned to escape in small increments; they displayed no evidence at all of understanding and certainly no evi-dence of any sudden insight into the problem’s solution.

THE LAW OF EFFECT

In Thorndike’s procedure, the cats’ initial responses in the puzzle box—biting at the latch, clawing at the walls—all led to failure. As the trials proceeded, though, the cats’ tendency to produce these responses gradually weakened. At the same time, the animals’ tendency to produce the correct response was weak at first; but, over the trials, this response gradually grew stronger. In Thorndike’s terms, the correct response was gradually “stamped in,” while futile ones were “stamped out.”

But what causes this stamping in or stamping out? Thorndike’s answer was the lawof effect. Its key proposition is that if a response is followed by a reward, that responsewill be strengthened. If a response is followed by no reward (or, worse yet, by punish-ment), it will be weakened. In general, the strength of a response is adjusted according to the response’s consequences (Figure 7.19). In this view, we do not need to suppose that the cat’s performance required any sophisticated intellectual processes. We likewise do not need to assume that the animal noticed a connection between its acts and the consequences of those acts. All we need to assert is that, if the animal made a response and a reward followed soon after, that response was more likely to be performed later.

Notice that Thorndike’s proposal suggests a clear parallel between how an organism learns during its lifetime and how species evolve, thanks to the forces of natural selection. In both cases, variations that “work”—behaviors that lead to successful outcomes, or individuals with successful adaptations—are kept on. In both cases, variations that are less successful are weakened or dropped. And, crucially, in both cases


the selection involves no guide or supervisor to steer the process forward. Instead, selection depends only on the consequences of actions or adaptations and on whether these serve the organism’s biological needs or not.

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