The Story of Clever Hans

Posted by Greg MacDonald on

At the end of the 19thCentury, and as the 20thCentury dawned, the German public were spellbound by a horse named Hans. Hans seemed to be able to perform mathematical operations (with whole numbers and with fractions).  He could tell the time, identify calendar dates, and he could differentiate between musical notes.  Hans could apparently read and spell!  His owner, a man named Wilhelm von Osten, had taught the horse to tap its hoof in order to communicate, and he exhibited the animal’s talents (free of charge) in Berlin and around Germany.

If von Osten were to ask Hans, “If the eighth day of the month comes on a Tuesday, what is the date of the following Friday?” Hans would tap his hoof eleven times.  The horse would indicate letters of the alphabet by tapping his hoof the number of times that corresponded to the letter’s numerical place.  (So one tap for “a”, two taps for “b”, etc.)  Hans could answer a wide variety of questions with an accuracy of approximately 90%!  He could do this regardless of whether the question was posed in spoken or written form. It also did not seem to matter whether the question was posed by von Osten, or by someone else.

In 1904, Berlin’s Wonderful Horse was the subject of a New York Times article.  In the article von Osten stated that Hans had received “systematic instruction as he would give to a child”.  The abilities that Hans displayed had so far baffled scientists and psychologists.  Several tests had been conducted, and in September 1904 the Hans Commission (composed of a panel of 13 people, including a veterinarian, a circus manager, a cavalry officer, school teachers, and the Director of the Berlin Zoological Gardens) had concluded that no trickery was involved in Hans’s performances.  The Commission had then passed further evaluation of Hans on to Oskar Pfungst, a student at the Psychological Institute at the University of Berlin.

Pfungst devised a scientific series of tests that involved different people asking questions of Hans. Sometimes questioners knew the answer to the question, and other times they did not.  The questioners were placed at varying distances from Hans, and for some tests, Hans was fitted with “blinkers” that eliminated visual contact with the questioner.

During the series of tests, Pfungst carefully observed Hans and his questioner, noting the most minute details.

This test series found that Hans could only provide a correct answer if the questioner knew the answer.  It was also clear that Hans required visual contact with the questioner in order to answer correctly.

It was concluded that Hans was able to read unconscious facial cues and other body language of his questioners, who were totally unaware that they were providing these cues. In particular, Hans seemed to be able to interpret an increase in the intensity of these cues as his hoof tapping approached the correct answer, and then a release of the cues when he made the final and correct hoof tap.  (Some suspect that the social communication system of horses may rely upon detection of slight postural changes, which would further explain Hans’s unusual ability to pick up cues given by humans even when they were unaware of them themselves.)

Pfungst carried out further studies in his laboratory, using human subjects who responded to him playing the part of the horse.  This work confirmed the conclusions of his original experiments with Hans.  More recently, the University of California observed  the Clever Hans Effect in action when drug-sniffing dogs were observed.  Unconscious cues from their handlers led to a higher incidence of false positives.

Wilhelm von Osten died in 1909, and it is recorded that Hans had several owners subsequently.  There is no record of Clever Hans after 1916, and so the fate of this amazing horse is unknown.

The story of Clever Hans, and the Clever Hans Effect has implications for us as we work with children.  It can have a negative effect upon our efficacy as teachers.

We may, for example, conclude that the children understand all that we are sharing with them, when in fact all that they may be doing is responding to the unconscious cues that we are providing. 

What sort of cues?

The cues that we give children can take many forms:

  • When asking a question for which we expect the children’s answer to be “Yes”, we may unconsciously nod our head up and down slightly. For “no” questions, we may move our head from side to side.
  • Our voice might place added emphasis on the answer we’re expecting: “Is five times seven equal to twenty-five, thirty-five, or forty-five?”
  • Rather than a heavier voice emphasis, we may alter the pitch of our voice to achieve the same cueing effect.
  • Sometimes we may unconsciously point with a finger or use a hand gesture to indicate the “answer”, or the item we’re anticipating will be identified by the children, giving them a physical cue.
  • Or our eyes may fixate on the correct choice, and the children’s eyes follow the direction of our stare to arrive at the correct choice.

... When children respond to our unconscious cues we may erroneously believe that our objectives are being achieved, and that the children understand, when in reality all that is happening is that they are responding to our unconscious cues.  All that they understand is that this answer or that item is the choice that the adult is indicating!

And so we may move blithely ahead, taking our children further along a particular academic path, firm in our belief that this is justified because “So far, so good!  They have understood, and they’re ready for the next step/s”, only at some point to be brought to a puzzling and screeching halt when we suddenly realize that the children are unexpectedly confused, that they’ve “forgotten” what has come before ...

Why has this happened? To whom might you point as the agent of this confusion?


(Well that’s hardly fair. Really, you should point to what is now called the Clever Hans Effect, and to your own excellent, sometimes well-practiced, unconscious cues.)

Children, by the way, can sometimes activate our unconscious cues.  They may not respond immediately, but (for example) gaze at the adult expectantly ... And the adult then dutifully (but unconsciously) responds to the children’s cue by offering a cue to indicate the correct choice!

So important point number one here is that you should self-monitor and self-observe to ensure that you do not unconsciously cue the children.  Adopt a habit of doing this with every interaction.  Be sure not to move your head or your hands when a cue might be initiated.  Keep your voice steady and even, without changing emphasis or pitch.  Consider how else might you be cueing the children.  Observe them – To whom/what do they direct their gaze?  What occurs before they respond?

There is an important upside to the Clever Hans Effect, and this is important point number two.  As we have seen, unconscious cueing can enable children to give the appearance of understanding more, and of being more capable, than is actually the case.  This is detrimental to the pedgogical process. However, consider the case of a defeated child, a child who is sorely lacking in self confidence, a child who has very little self esteem.

By consciously cueing this child, the child’s sense of defeat can be replaced by a sense of victory, based upon a series of successful choices/answers (that you have cued, unbeknownst to the child).  A child’s self confidence and self esteem can be built by providing carefully place cues that enable success.  Your unconscious cues can be reduced and eliminated as it becomes evident that the child no longer needs them.  The cues must be subtle, so that neither the child nor any classmates recognize what is going on.

Important point number three is that other adults in the child's life may also be unconsciously cueing ... And as a result, they receive information that is more a representation of their cueing than of what is actually going on!  So at the appropriate moment, share this information with parents, teacher assistants, etc.  

The morals of this story?

1.  Eliminate unconscious cues. 

2.  Employ conscious cues subtly and judiciously, as a particular child may need them.

3.  And thank Clever Hans, a horse who lived over a century ago, for the lessons that he taught us all.


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  • This is fascinating! I witness unconscious cuing often. I attempt not to do it myself, but have caught myself doing it occasionally when offering a choice of two activities to a child who might need assistance finding what they’d like to do. I try very hard not to change the pitch or emphasis of my voice, but it can creep in at times. As far as building confidence, I like to respond to some questions with a question. This is easy to do of course. When the child questions if something right, I ask them what they think and they seem so pleased when they asses their own work and discover it is correct/accurate. (I teach primary) . I love the idea of using discreet, conscious cuing to aid a child with low self-esteem.

    Suzanne on

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