Are people susceptible to prevalence-induced concept change? To answer this question, we showed participants in seven studies a series of stimuli and asked them to determine whether each stimulus was or was not an instance of a concept. The concepts ranged from simple (“Is this dot blue?”) to complex (“Is this research proposal ethical?”). After participants did this for a while, we changed the prevalence of the concept’s instances and then measured whether the concept had expanded—that is, whether it had come to include instances that it had previously excluded.
…When blue dots became rare, purple dots began to look blue; when threatening faces became rare, neutral faces began to appear threatening; and when unethical research proposals became rare, ambiguous research proposals began to seem unethical. This happened even when the change in the prevalence of instances was abrupt, even when participants were explicitly told that the prevalence of instances would change, and even when participants were instructed and paid to ignore these changes.
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