Numerous attempts have been made to demonstrate that species of animals other than man are capable of learning by observation or imitation. An especially important study for our present purposes is that by Crawford and Spence (1939), who, using two opposing cages with a stimulus tray between them, trained a chimpanzee to observe another depress one of three stimulus objects. After several trials by the demonstrator, the observer was given a test trial to determine if the observer would depress the same object. Both demonstration and test trials continued alternately until the observer either learned the problem or evidenced inability to learn. Results were ambiguous, with only one of eight chimpanzees learning a "pure" imitation problem. The ambiguity of the results suggested that although the general procedure was correct, the stimulus-display conditions were not conducive to rapid discrimination learning.Data collected since have defined the conditions necessary for rapid discrimination learning by primates (Harlow, 1944;Jarvik, 1953;Jenkins, 1943; McCIeam & Harlow, 1954), and it was thought that by utilizing the results of these recent researches, unambiguous data might be collected on observational learning. Because of these considerations, we were motivated to try a demonstration of observational learning under more efficient conditions and to learn what we could about factors contributing to successful performance.Two studies were conducted. In the first, described in detail elsewhere, Darby (1956) placed two monkeys that could solve object-1 Supported in part by a grant (M-589) from the
Progressive improvement in the learning of successive visual discrimination habits by rhesus monkeys and by human children (formation of "learning sets") has been described by Haiiow (4). These two primate species had markedly different pre-experimental histories; nevertheless, some of the characteristics of the performance measures were similar. Both showed gradual improvement in performance within each problem, and both showed gradual improvement from problem to problem. Subsequently Hayes, Thompson, and Hayes (7) reported that chimpanzees (Pan satyrus) also form learning sets, and with efficiency about equal to that of rhesus monkeys. The phenomenon has been reported also for three simian species: rhesus (Macaco, mulatto), squirrel monkey (Saimiri stiiirea), and marmoset (Callithrix jacchus) by Miles (12) and by Miles and Meyer (13); for cats by Warren and Baron (20); for rats by Koronakos and Arnold (11); for raccoons (Procyon lotor) by Shell and Riopelle (18) and by Johnson (9). Further data for rhesus monkeys have been reported by Braun (2) and by Riopelle ( 16).These investigations, some of which were conducted under substantially similar conditions, have shown that phyletic status is of importance in determining proficiency of performance despite the fact that any component discrimination problem.in the series would not successfully differentiate the species. Gardner and Nissen (3), for example, tested sheep, cows, horses, goats, chimpanzees, and human aments, and found comparable levels of performance in all species in a single discrimination problem. Clear differences among species were obtained for the multiple-discrimination task.Two factors, the success with which the
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