March 6, 2026, 2:39 pm | Read time: 3 minutes
Are you familiar with the “Clever Hans Effect”? Behind this phenomenon lies a somewhat quirky story about a horse that could supposedly do math and write. PETBOOK explains what’s behind it and how intelligent horses really are.
How Intelligent Are Horses Really?
Many horse owners attribute a high level of awareness and intelligence to their animals. Anyone who spends regular time with horses knows this feeling: The animals seem to know exactly which “buttons” to press with us. During my time at the stable, I often felt that my mare scrutinized me during our first encounter of the day. If I was sad or tense, she often mirrored my behavior.
Studies actually confirm that horses can perceive human emotions. They react to fear, which they detect through the scent our body emits in stressful situations. The question of whether horses can assess situations and think strategically continues to intrigue researchers. A study from 2024 examined this behavior.1
Who Was “Clever Hans”?
The story sounds almost too incredible to be true: a horse that can do math, read, and even recognize art styles. At the beginning of the 20th century, this caused quite a stir. At the center was a horse named Hans II, better known as “Clever Hans.”
His owner, Berlin teacher Wilhelm von Osten, presented Hans II as exceptionally talented. The horse supposedly answered “multiplication and division problems through hoof taps” correctly, as stated in the introduction of the report on Hans. Hans II was not Wilhelm von Osten’s first horse—his predecessor also bore the name Hans and was said to have similar abilities. When the first Hans died, von Osten bought a new stallion five years later. Hans II was taught by him for several years, as von Osten was convinced of the intelligence of his Orlov Trotter.2, 3
Even Scientists Were Baffled
Von Osten initially demonstrated his horse’s abilities to his neighbors. However, he wanted his stallion’s achievements to be scientifically recognized. A commission observed the horse doing math and spelling in the teacher’s backyard, as Dr. Heike Baranzke explained on the news portal of Bergische Universität Wuppertal.4
The surprising result: Wilhelm von Osten was not cheating; the horse gave correct answers. However, it remained unclear how Hans II could seemingly respond correctly to the tasks. Only through a more detailed report was the hypothesis formed that the horse derived its answers not from understanding but from the reactions of its human counterpart.
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The “Clever Hans Effect”
Hans II relied on minimal, unconscious signals: changes in body posture, facial expressions, or the tension of his audience. From this insight, the term “Clever Hans Effect” was coined. It describes the phenomenon where test subjects unconsciously pick up cues from their environment and thereby produce seemingly influenced performances.
The effect continues to play a central role in animal research and psychology. It also serves as an important reminder in modern AI research: Results can be distorted by expectations and subtle signals without the participants being aware of it.
Intelligent – but Different
“Clever Hans” could neither read nor do math. Nevertheless, his story impressively shows how sensitively horses perceive their environment. Their strength lies not in abstract thinking but in social intelligence, observational skills, and emotional perception—abilities that have made them close companions to humans for centuries.