The clever Hans effectThe clever Hans effect is named after a horse called Hans that reportedly could solve mathematical problems by tapping the correct response with his hoof. After scientific investigation, however, it was found that Hans could not real

The clever Hans effect
The clever Hans effect is named after a horse called Hans that reportedly could solve mathematical problems by tapping the correct response with his hoof. After scientific investigation, however, it was found that Hans could not really add, subtract, multiply or divide. Instead, he was responding to subtle cues from his trainer. Apparently, the trainer would tense up just prior to the correct answer and then relax with the final and correct tap. Because of the discovery of the Clever Hans Effect, or how an observer can have a definite influence on experimental results, researchers now enforce strict observer controls in experimental design.