Clever Hans Effect
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[edit] Definition
The Clever Hans Effect is a phenomenon in behavioral experimentation on animals where the human participants or researchers subconsciously communicate to the animal the behavior desired to prove the hypothesis.
The effect can invalidate experimental results tending to demonstrate
- a different perceptive or cognitive ability
- supernatural clairvoyance,
- intelligence (rather than operant conditioning)
[edit] History
In 1907, the psychologist Oskar Pfungst investigated claims of mathematical ability in a horse known as “Der Kluge Hans”. This horse apparently indicated the answers to mathematical questions by stamping his foot a certain number of times.
Pfungst discovered that the horse’s ability depended upon these factors:
- The presence of spectators,
- The relationship of the questioner to the horse,
- Whether the horse could see the questioner
- Whether the questioner knew the correct answer.
Since true mathematical ability would not depend on any of these things, Pfungst concluded that Hans’s apparent ability was due to communication from the people present to the horse. This communication was almost certainly unconscious on the part of the humans involved. Pfungst identified slight variations in posture and facial expression that occurred as the horse reached the correct number of stamps. It is now known that horses can also hear the heartbeats of humans standing nearby.[1]
[edit] Animal Sensation
If one assumes that the animal’s senses are similar to those of humans, then one may also assume that channels of communication unavailable to humans are likewise inaccessible to the animal.
Dogs in particular have an extremely powerful sense of smell, and are also extremely sensitive to slight changes in the timbre of the human voice. Since human scent and voice are affected by emotion, there is a physical mechanism by which dogs can “sense” human emotion, and respond appropriately. Such an ability may, in fact, have evolved specifically as an adaptation to inclusion of the dog in human society.
From a cognitive standpoint, horses and dogs are far more alert to physical gestures than humans, to the extent that Hans could easily “read” the humans’ intentions, by means of cues which were nearly imperceptible to other humans.
[edit] Experimental Protocols
The Clever Hans Effect demonstrates the need for strict controls on animal experiments, including double-blinding.
Often, it is not possible to study animal behavior in the absence of human interaction. For example an experiment might study a behavior that must be commanded by a trainer. Or, an experiment might study a behavior that specifically relates to human social interaction. In these cases, it is necessary that everyone interacting with the animal, (and everyone interacting with them) be blinded.
Consider testing the ability of a dog to find narcotics in a pile of suitcases. To simulate the real situation, it is necessary that many people be around. The dog’s handler will obviously be present to command the dog. If any one of these people knows where the drugs are, they have only to stare at it for the dog to realize where to point. In such a case, the experiment is worthless for testing the dog’s ability. Thus, it is necessary that no one involved know where the drugs are. Since there is the possibility of the researchers unconsciously communicating this information to the participants, not even the researchers should know.
In 1994, a dog named Rico was tested for the claimed ability to understand 200 different spoken words, and, more interestingly, to form temporary associations to new words with a single use. With proper experimental controls in place, researchers verified that Rico did, in fact, have the claimed abilities. [2]
A major criticism of experiments concerning the linguistic abilities of non-human primates (NHP) is that they take place in socially rich settings, in which the human researchers cannot possibly be blinded to the goal of teaching language. [3]
[edit] Psychic Animals
Most dog owners are familiar with the phenomenon where a dog “senses” that its master is soon to arrive home. This phenomenon is mysterious if one assumes that a dog’s senses are similar to humans. In fact, the dog’s superior hearing can detect the approach of the master’s car from a great distance, and like the horse, it is also well-adapted to reading subconscious cues from humans that also anticipate the arrival.
In ignorance of the physical senses of the animal, some have concluded that such an ability must have a supernatural cause.
It is often claimed that animals can sense impending disaster, such as an earthquake. While these claims may often be dismissed as selective memory or shoehorning, it is entirely plausible that domestic animals may sense earth movement several seconds before it becomes obvious to humans.
Following the Indian Ocean tsunami of 2004, there were many claims that animals had sensed the arrival of the tsunami, and fled to higher ground. While this turned out not to be true[4], it is possible that Asian elephants can sense low-frequency vibration through their extremely sensitive feet.[5]
[edit] Legal Issues
In the case of drug-sniffing dogs, there is currently some legal controversy. If the dog “alerts” the possible presence of drugs, this may be considered evidence supporting probable cause for a search. However, the human handler may have a (possibly unwarranted) suspicion, which the dog interprets as a signal that it ought to alert.
In this case, the cause to search is based on nothing but the handler’s suspicion, which might normally be considered a civil rights violation. [6] The handler can still claim innocence, since they may not have consciously prompted the dog.
