Wikipedia Deep Dive

Clever Hans

4 min read

The Horse That Could Do Math

Imagine a horse that could solve arithmetic problems, tell time, and even understand German. Sounds like something out of a fairy tale, right? Well, in the early 20th century, a horse named Clever Hans captivated audiences across Germany with these very feats. But was he truly a mathematical genius, or was there something else at play?

The Rise of Clever Hans

Clever Hans was owned by Wilhelm von Osten, a mathematics teacher and amateur horse trainer. Von Osten claimed that Hans could add, subtract, multiply, divide, work with fractions, tell the time, keep track of the calendar, differentiate between musical tones, and read, spell, and understand German. For instance, when asked, "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 to indicate the correct answer.

The public was fascinated by these claims, especially given the recent interest in animal intelligence sparked by Charles Darwin's publications. Newspapers like The New York Times reported on Hans’ abilities, and von Osten exhibited him throughout Germany without charging admission. People flocked to see this extraordinary horse, believing that he possessed an advanced level of number sense.

The German Board of Education Steps In

The public interest in Clever Hans was so great that the German board of education appointed a commission to investigate von Osten's claims. Led by philosopher and psychologist Carl Stumpf, the Hans Commission included a veterinarian, a circus manager, a cavalry officer, schoolteachers, and the director of the Berlin zoological gardens. After careful examination, they concluded that there were no tricks involved in Hans's performance.

Oskar Pfungst Uncovers the Truth

The investigation was then handed over to Oskar Pfungst, an assistant to Carl Stumpf. Pfungst conducted a series of experiments to determine how Hans could perform these intellectual tasks:

Isolating Hans and the questioner from spectators so no cues could come from them.
Using questioners other than von Osten.
Varying whether Hans could see the questioner.
Testing if the questioner knew the answer in advance.

Pfungst discovered that Hans could get the correct answer even when questioned by someone other than von Osten, ruling out fraud. However, the horse only gave the right answer when the questioner knew what the answer was and Hans could see them. When Hans couldn’t see the questioner, his accuracy dropped dramatically.

The Clever Hans Effect

Pfungst observed that as Hans's taps approached the correct answer, the questioner's posture and facial expression changed subtly, reflecting an increase in tension which was released when the final tap was made. These involuntary cues were picked up by Hans, allowing him to stop tapping at the right moment. This phenomenon became known as the Clever Hans effect—where a subject responds to subtle, unintentional signals from an observer or experimenter.

The social communication systems of horses may depend on detecting small postural changes, explaining why Hans so easily picked up on these cues. Pfungst even conducted experiments where he acted as the horse, tapping out answers while observing the subjects’ reactions to confirm his hypothesis.

Legacy and Impact

Pfungst's findings had a significant impact on research methodology, particularly in animal cognition studies. The Clever Hans effect highlighted the need for careful experimental design to avoid observer-expectancy effects. This led to practices such as double-blind experiments, where neither the experimenter nor the subject knows what condition the subject is in, thereby reducing bias.

In the field of artificial intelligence, the Clever Hans effect describes a phenomenon where an algorithm seems to make correct predictions using incorrect reasoning or unknown features. Explainable AI research aims to fully comprehend these decision-making processes to detect such biases and ensure robustness.

Beyond Horses: The Broader Implications

The Clever Hans effect has been observed in various contexts, from drug-sniffing dogs to cognitive psychology experiments with humans. For instance, a study at the University of California, Davis, revealed that handlers can unintentionally cue their dogs, leading to false positives. Similarly, studies on dogs' socio-communicative abilities have shown mixed results regarding the presence of Clever Hans effects in different tasks.

Conclusion: The Enduring Lesson

Clever Hans may not have been a mathematical genius, but he taught us an invaluable lesson about how subtle, unintentional cues can influence behavior and perception. His story continues to inspire researchers across disciplines to design experiments more carefully, ensuring that the results are truly reflective of the phenomena being studied.

In essence, Clever Hans was not just a horse with an impressive act; he was a catalyst for scientific rigor and innovation in experimental methodology. And that, perhaps, is his true genius.