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Boysen's Chimps

Psychologist Sarah Boysen of Ohio State University explored the ability of chimps to form concepts such as "more than" and "less than." She found that two of her chimp subjects, Sarah and Sheba, were both capable of learning such discriminations easily.

What experiment did Boysen do with gumdrops?

Boysen then tried a variation of the experiments, using gumdrops as stimuli. Gumdrops were some of the chimps' favorite treats. The chimpanzee was presented with two plates of gumdrops. One had more on it, the other had less. For example, one might have five gumdrops, the other three. While the other chimp watched, the chimp being tested was asked to point to one of the plates. Whichever plate it pointed to was given to the other chimpanzee.

In this situation, the chimp doing the pointing should have learned to point to the plate with fewer gumdrops on it, in order to have that plate given to the other chimpanzee and in order to get the plate with more gumdrops for itself. Instead, something odd happened. The chimps insisted on pointing to the plate that had more gumdrops, even though this meant that more gumdrops went to the other chimpanzee. Boysen says they seemed to know they were making a mistake when they pointed to the plate with more gumdrops, but they could not stop themselves. Often they expressed frustration immediately after pointing at it, even before Boysen removed the plate and gave it to the other chimp. (One student said this was the chimpanzee equivalent of Homer Simpson's "Doh!")

Boysen herself was surprised that the chimps could not learn to maximize their gain. She said, "It was the first task in 20 years that I'd failed to teach a chimpanzee" (Fischman, 1993). Then she tried a simple variation. She replaced the gumdrops with plastic poker chips. Now the chimps had no trouble with the task. They pointed to the plate with fewer poker chips on it. This meant the plate with fewer gumdrops went to the other chimp, and the chimp that did the pointing got the larger number of gumdrops.

Before the poker chips were substituted for the gumdrops, the chimpanzees seemed to be at the mercy of their desire for food. "The chimps understood the rule," Boysen says, "but they couldn't act on it" because of some biological imperative to get more food. Moving into the symbolic realm, by using poker chips in place of the gumdrops, allowed them to transcend that biological imperative and use abstractions like "more" and "less" to maximize their gain (Fischman, 1993).

Why was Byrne enthusiastic about Boysen's findings?

Richard Byrne of the University of St. Andrews in Scotland was enthusiastic about Dr. Boysen's experiment, because it provided a clue about what is special in human intelligence. Suppose, Byrne suggested, the crucial advance was the ability to use symbols, beliefs, language...in short, abstract thinking...to suppress selfish appetitive behavior and plan for the future. Early humans had to store food for hard times; they had to plan for the future. Only creatures capable of deferring immediate gratification and planning for the future could engage in such strategic behavior. Humans have such a capability; chimpanzees apparently do not.

How does Colinvaux's "CPSB" concept relate to this?

Colinvaux (1991) made a similar point. He proposed a model in which the "selectively most important aspect of intelligence is the property of overriding innate behavior, for which the term Conscious Prevention of Stimulated Behavior (CPSB) is proposed." That sounds much like the talent that Boysen's chimps were lacking. They could not prevent themselves from responding to the simulation of a large place full of gumdrops by pointing at it...until they used symbols to substitute for actual gumdrops.


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