At the turn of the 20th century, a horse named Hans captivated the German public—and eventually, the world. Dubbed Clever Hans, this seemingly unremarkable horse was believed to possess astonishing intellectual abilities. Under the guidance of Wilhelm von Osten, a German mathematics instructor and amateur horse trainer, Hans was presented as an animal prodigy capable of solving arithmetic problems and even answering calendar-related questions.
According to von Osten, Hans could add, subtract, multiply, and divide. He could work with fractions, and his understanding extended beyond numbers into concepts related to dates and sequences. Whether questions were posed verbally or in writing, Hans would respond with a series of hoof taps—stopping, remarkably, at the correct number every time.
Von Osten toured with Hans across Germany, drawing large crowds wherever they went. Spectators were eager to witness what seemed like undeniable proof that animals, under the right circumstances, could possess intelligence rivalling that of humans. Hans’s performances were met with awe and curiosity, inspiring admiration as well as scepticism.
Enter Oskar Pfungst, a psychologist with a growing interest in animal behaviour and the scientific method. Sensing that there was more to Hans’s abilities than met the eye, Pfungst set out to investigate. In 1907, he conducted a series of careful and controlled experiments designed to isolate the true source of the horse's apparent intelligence.
Pfungst’s experimental design was ahead of its time. He tested Hans with multiple questioners, sometimes allowing them to know the answers and other times deliberately keeping them in the dark. He varied the presentation of questions and observed the horse under different environmental conditions. Over time, a clear pattern emerged.
Hans only performed well when the questioner knew the answer. When the person posing the question was unaware of the correct response—or if visual cues were blocked—Hans could no longer produce the right number of taps. This discovery led to a breakthrough: Hans wasn’t doing maths at all. Instead, he was reading incredibly subtle signals from humans—tiny changes in posture, shifts in facial expression, even minute muscle movements.
Pfungst’s findings dismantled the myth of Hans the mathematician but revealed something arguably more fascinating: the horse’s incredible sensitivity to human body language. This became known as the Clever Hans Effect, a term that remains relevant in scientific and training communities to this day. It describes the way an animal (or human) can unconsciously respond to involuntary cues provided by another individual, especially during testing or training.
What made Pfungst’s contribution so valuable wasn’t just the outcome—it was the methodology. His meticulous attention to experimental controls set a precedent for behavioural science. By demonstrating how unintentional cues could skew results, he highlighted the critical importance of controlling for observer bias in any investigation involving live subjects.
The Clever Hans Effect has wide-ranging implications. In scientific research, particularly in the study of behaviour and cognition, it has prompted the widespread use of double-blind procedures. These approaches ensure that neither the experimenter nor the subject knows which variables are being tested, helping prevent the kind of unconscious cueing that fooled the world in Hans’s case.
Even outside the lab, the effect plays a significant role. For example, when training drug-sniffing dogs, it’s essential that handlers are unaware of which containers contain contraband. If they know, they might—without realising—give away the location through a glance, a shift in stance, or a change in breathing. The dog, attuned to its human partner, may pick up on that signal and indicate a "find" based on human behaviour rather than scent detection.
This story also holds important lessons for those of us working closely with animals, whether as trainers, behaviour consultants, or curious observers. It reminds us to reflect carefully on what our animals are responding to and whether we might be shaping behaviour unintentionally. It challenges us to be more precise in our training, more thoughtful in our observations, and more humble in our assumptions.
Above all, the tale of Clever Hans is a powerful example of the scientific process in action. What began as a sensation built on anecdotal performance became, through careful investigation, a case study in critical thinking and experimental rigour. It urges us to meet extraordinary claims with healthy scepticism and to ask deeper questions about the mechanisms behind what we see.
So, the next time someone shares a story about an animal with seemingly supernatural abilities, take a moment to think of Hans. Let curiosity lead the way—but don’t forget the value of cautious inquiry and the importance of good experimental design.
To see the full story brought to life, check out the video on my YouTube channel, Train Me Please.
Reference
Bellows, A. (2007, February). Clever Hans the Math Horse. Damn Interesting. https://www.damninteresting.com/clever-hans-the-math-horse/
Photo reference
The “Clever Hans Phenomenon” revisited - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/clever-Hans-an-Orlov-trotter-horse-1895-1916-and-his-owner-and-teacher-Wilhelm-von_fig1_260376462 [accessed 25 Feb, 2024]
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