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Comparative psychology began shortly after Darwin published his ideas about natural selection. Darwin himself suggested in The Descent of Man (1874) that human intelligence had evolved into its present form gradually, so it might be interesting to look at a range of different animals to see how intelligence evolved. Romanes, a friend and student of Darwin, pursued this idea in his book Animal Intelligence (1882), comparing the intelligence of animals from snails to humans. Romanes relied upon anecdotes (stories) for information. He assumed that animals had the same sorts of mental experiences as humans and often speculated about their thoughts, feelings, and emotions.
Early comparative psychologists seemed to accept the idea of a scale upon which animals could be arranged from lowest to highest. This idea can be traced back to Aristotle's "Great Scale of Nature." The idea is rejected by modern scientists. Biologists point out that present-day species have evolved separately and are not descended from one another. There is no reason to expect that they would show a smooth progression of increasing intelligence from "lower" to "higher."
In 1894, C. Lloyd Morgan published Introduction to Comparative Psychology. In the book he made a pronouncement that became known as Morgan's Canon, suggesting animal behavior should be interpreted in the simplest way possible. Morgan believed that all animals were conscious, however, because he assumed that consciousness was necessary for learning.
By 1900, some scientists were ready to reject the whole idea of animals having minds or consciousness. Jacques Loeb suggested only humans had minds or consciousness. Loeb portrayed animals as biological machines without mental lives. This is called the mechanistic perspective.
In the early 1900s, the public eagerly believed stories of humanlike intelligence in animals. In 1904 journalists wrote about Clever Hans, a horse that did arithmetic, spelled words in German, and analyzed musical chords. Hans could perform even when his trainer was absent. However, a psychologist named Oskar Pfungst showed that Hans could answer correctly only when the humans observing him knew the answer to a question. Evidently Hans was responding to subtle cues from the humans.
A variety of American psychologists carried on the tradition of comparing different species, using standardized laboratory tasks. Edward Thorndike used an apparatus which he called a "puzzle box." Harry Harlow studied "learning set" or "learning to learn." Most comparative psychologists of the mid-20th Century assumed that laws of learning would apply to all species, so they studied the most convenient, notably rats and pigeons. Now it is less common for undergraduate psychology students to work with live animals at all, and rat psychology is becoming a thing of the past.
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Copyright © 2007 Russ Dewey