A horse in Berlin at the beginning of the twentieth century claimed to have mastered human language and arithmetic

Hans was purchased by a retired German schoolteacher, Wilhelm von Osten, in 1900 with the aim of reproducing the remarkable behaviour originally noted by von Osten in an earlier Hans. Soon the feats of Hans II overshadowed those of his predecessor, endowed him with the respectable title 'Clever Hans' (Pfungst 1911), and earned him an honourable position in the history of experimental psychology.

Surely Clever Hans was not the first animal to which human-like mental capabilities have been attributed. Neither was he the first horse to gain eternal fame. Nineteen centuries earlier, the Roman Emperor Caligula invited his beloved Incitatus to banquets and may had even announced his intention to appoint the horse to a Consul, probably in a lurch of doubtful Imperial humour (Barrett 1990). Throughout history individuals of many other species have been regarded as possessing some degree or another of human reason, although they did not attend such a high respect in government. This "anthropomorphism was not always taken in the animals' favour, and in some cases even led them to the gallows (Evans 1906). Clever Hans was, however, different in that he was treated seriously by respected professors, and the investigation of his alleged wit unveiled and singled out some basic issues in behavioural research, which are highly relevant to memory research to date.

Hans was clearly clever, however, in his own equine way. Mr von Osten trained Hans to provide answers to a plethora of questions by either tapping with its front hoof or pointing its snout to symbols on a board. Hans could apparently do arithmetic, including fractions, categorize objects, and even read German (e.g. in response to the question 'What is the woman holding in her hand', he would gallantly reply, letter by letter, 'Schrimm', i.e. parasol). His wisdom was so impressive that a scientific commission was called to exclude any deceit, and indeed, in 1904, the committee declared confidentially that von Osten used no tricks. But the rest remained a puzzle even to the high ranking academics: 'This is a case', they wrote, 'which appears in principle to differ from any hitherto discovered' (cited in Rosenthal 1965). The "enigma led to a signal research programme, executed by Oskar Pfungst in collaboration with Carl Stumpf (Pfungst 1911), which finally shed light on the Clever Hand phenomenon, although leaving some issues unresolved (Hediger 1981). Briefly, Pfungst showed that Hans learned to respond to subtle human bodily movements, unleashed unintentionally when the horse came to a correct answer. The horse came out rather stupid unless someone he could watch had the answers. Hans clearly knew nothing about math, logic, or the German language, yet knew a great deal about human gestures. Pfungst even stepped further and became himself a 'Clever Hans' in a "control experiment, only to find out that with proper attention, he could come up with correct answers to questions merely by observing the experimenter in front of him.

The Clever Hans phenomenon turned attention to critical issues in experimental psychology. These include: (a) the importance of a systematic research "methodology, including appropriate controls, as well as 'blind tests' in which the experimenter could not know the relevant history of the experimental "subject

Fig. 15 Clever Hans, Clever von Osten, and blackboards in an equine classroom.The horse learned to respond to subtle human gestures.

and the expected outcome; (b) the effect of 'demand characteristics', i.e. unintentionally influencing the performance of an experimental subject by expecting a certain outcome (Orne 1962; "bias); (c) the importance of "Ockham's razor, i.e. preferring a simple explanation to a more complex one; (d) the subtlety of innate and acquired communication abilities of animals (e.g. Sebeok and Rosenthal 1981; "a priori); (e) the need to become well versed in the natural behavioural repertoire of animal subjects; (f) the pitfalls of anthropomorphism; (g) the lack of correlation between one's academic title to success in animal training (Hediger 1981); and finally, last but not least (h) the absence of correlation between the academic statue of members of a site visit team to the robustness of their conclusions.

During the early days of experimental psychology Clever Hans occupied a key position in textbooks (Watson 1914), but then dwindled into footnotes (though see Sebeok and Rosenthal 1981). The lesson should, however, be refreshed, as if forgotten, a mouse in a "maze might be deemed smarter than it is, or at least for the wrong reasons. With time, horses ceased to be favourable subjects for learning research, yet not utterly forgotten (e.g. Heird et al. 1981; Houpt et aL 1990).

Selected associations: Anthropomorphism, Artefact, Control, Subject

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