Habituation and classical conditioning are both general forms of learning that are rel-evant to many species (including, of course, humans) and many different responses (including a variety of overt behaviors, a range of subjective feelings, and a broad set of bodily responses). An equally important type of learning is instrumental conditioning (also called operant conditioning). This form of learning involves behaviors that aren’t triggered automatically by some stimulus. Instead, instrumental conditioning is con-cerned with behaviors initiated by—and presumably under the control of—the organ-ism itself. In other words, while classical conditioning essentially involves the creation of new reflexes, instrumental conditioning involves the learning of new voluntary behaviors.
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).