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date: 20 November 2018

conjunction fallacy

A Dictionary of Psychology

Andrew M. Colman

conjunction fallacy n. 

A widespread error of judgement according to which a combination of two or more attributes is judged to be more probable or likely than either attribute on its own. It was identified and named in 1982 by the Israeli‐American psychologists Amos Tversky (1937–96) and Daniel Kahneman (born 1934) who presented undergraduate students with personality sketches of a hypothetical person called Linda (young, single, deeply concerned about social issues, and involved in anti-nuclear activity) and asked them whether it was more probable that (a) Linda is a bank teller, or (b) Linda is a bank teller who is active in the feminist movement; 86 per cent of the students judged (b) to be more probable than (a). This is a fallacy, because it is an elementary principle of probability theory that the probability of the conjunction (2) A and B can never exceed the probability of A or the probability of B. The fallacy arises from the use of the representativeness heuristic, because Linda seems more typical of a feminist bank teller than of a bank teller. See also Kolmogorov’s axioms.