Today's post, Conjunction Fallacy was originally published on 19 September 2007. A summary (taken from the LW wiki):

Elementary probability theory tells us that the probability of one thing (we write P(A)) is necessarily greater than or equal to the conjunction of that thing and another thing (write P(A&B)). However, in the psychology lab, subjects' judgments do not conform to this rule. This is not an isolated artifact of a particular study design. Debiasing won't be as simple as practicing specific questions, it requires certain general habits of thought.

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