Your guess
Hans tapped out answers with his hoof. How did he do it?
Eleven taps
His owner Wilhelm von Osten, a maths teacher, would ask: if the eighth of the month is a Tuesday, what is the date of the following Friday? Hans tapped eleven times. Von Osten showed him all over Germany and never charged admission.
The test
89%correct answers when Hans could see the questioner – otherwise 6 %
Pfungst had strangers ask the questions and varied whether the questioner knew the answer and whether Hans could see him, using blinders. It wasn't fraud. But as the taps neared the right number, the questioner grew tense – and relaxed at the final tap.
A horse, recognized by the small print
In 2019 a team led by Sebastian Lapuschkin looked at what image recognizers pay attention to. One model recognized horses by a source tag that appeared in about one in five horse pictures. Remove the tag, no horse. Put it on a photo of a car, and the car became a horse. The authors call this a Clever Hans effect.
Snow instead of wolf
In 2016 researchers at the University of Washington built a bad model on purpose: every wolf picture in training showed snow. Afterwards it said "wolf" whenever there was snow – whatever the animal. Without an explanation, 10 of 27 test subjects trusted it. With an explanation of what it looked at, only 3.
