The Clever Hans effect

The Clever Hans effect occurs when an observer unintentionally cues or signals responses from a subject, influencing the subject’s behavior without realizing it. Named after a horse in the early 1900s believed to perform math, it was later shown that subtle, unintentional signals from the questioner helped the animal give correct answers. In essence, the effect highlights how human expectations or body language can affect outcomes, leading to false assumptions about a subject's abilities. Recognizing this effect is important in experiments and assessments to ensure that results reflect true abilities rather than unintended cues.