Some investigators claim they have taught monkeys to understand spoken English, but another scientist argues that the results might be due to a Clever Hans effect. What does this mean?

Question

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The Clever Hans effect refers to an animal appearing to understand human language or perform intellectual tasks, but in reality, it is responding to subtle cues (often unintentional) given by its handler or the experimenter. This term originates from a horse named Clever Hans that was believed to be able to perform arithmetic, but was later found to be responding to the reactions of the human audience.

Some Investigators Claim They Have Taught Monkeys to Understand Spoken