‘No occurrence is sole and solitary, but is merely a repetition of a thing which has happened before, and perhaps often.’ —Mark Twain
The systematic biases seen in people’s probability judgments are typically taken as evidence that people do not use the rules of probability theory when reasoning about probability but instead use heuristics, which sometimes yield reasonable judgments and sometimes yield systematic biases. This view has had a major impact in economics, law, medicine, and other fields; indeed, the idea that people cannot reason with probabilities has become a truism. We present a simple alternative to this view, where people reason about probability according to probability theory but are subject to random variation or noise in the reasoning process. […] Results suggest that people’s probability judgments embody the rules of probability theory and that biases in those judgments are due to the effects of random noise.
