Clever Hans - The Clever Hans Effect

The Clever Hans Effect

Pfungst made an extremely significant observation. After he had become adept at giving Hans performances himself, and fully aware of the subtle cues which made them possible, he discovered that he would produce these cues involuntarily regardless of whether he wished to exhibit or suppress them. Recognition of this striking phenomenon has had a large effect on experimental design and methodology for all experiments whatsoever involving sentient subjects (including humans).

The risk of Clever Hans effects is one strong reason why comparative psychologists normally test animals in isolated apparatus, without interaction with them. However this creates problems of its own, because many of the most interesting phenomena in animal cognition are only likely to be demonstrated in a social context, and in order to train and demonstrate them, it is necessary to build up a social relationship between trainer and animal. This point of view has been strongly argued by Irene Pepperberg in relation to her studies of parrots (Alex), and by Allen and Beatrix Gardner in their study of the chimpanzee Washoe. If the results of such studies are to gain universal acceptance, it is necessary to find some way of testing the animals' achievements which eliminates the risk of Clever Hans effects. However, simply removing the trainer from the scene may not be an appropriate strategy, because where the social relationship between trainer and subject is strong, the removal of the trainer may produce emotional responses preventing the subject from performing. It is therefore necessary to devise procedures where none of those present knows what the animal's likely response may be.

The Clever Hans Effect has also been observed in drug sniffing dogs. A study at University of California Davis revealed that cues can be telegraphed by the handler to the dogs, resulting in false positives.

For an example of an experimental protocol designed to overcome the Clever Hans effect, see Rico (Border Collie).

As Pfungst's final experiment makes clear, Clever Hans effects are quite as likely to occur in experiments with humans as in experiments with other animals. For this reason, care is often taken in fields such as perception, cognitive psychology, and social psychology to make experiments double-blind, meaning that neither the experimenter nor the subject knows what condition the subject is in, and thus what his or her responses are predicted to be. Another way in which Clever Hans effects are avoided is by replacing the experimenter with a computer, which can deliver standardized instructions and record responses without giving clues.