The error of judging a conjunction of two events (A and B) as more probable than one of the events alone (A), violating the basic rule that P(A and B) ≤ P(A).
The famous Linda problem demonstrates this: Linda is described as concerned with social justice and discrimination. People judge 'Linda is a bank teller and active in the feminist movement' as more probable than 'Linda is a bank teller,' even though the conjunction must be less probable (all feminist bank tellers are bank tellers, but not all bank tellers are feminists). This occurs because the conjunction seems more representative of the description—it matches the stereotype better. The fallacy reveals how representativeness overrides basic probability rules, and it persists even among statistically sophisticated people when the description is vivid and detailed.
After reading that 'John is a shy, introverted person who likes puzzles,' people judge 'John is a librarian who plays chess' as more probable than 'John is a librarian,' even though adding 'plays chess' can only reduce the probability. The detailed description feels more representative.
More detailed, specific scenarios are more likely than general ones—actually, every added detail reduces probability; specificity increases representativeness but decreases probability.
What basic probability rule does the conjunction fallacy violate?
True or False: The conjunction fallacy only occurs in people without statistical training; those with statistical knowledge avoid this error.
The slow, deliberate, effortful mode of thinking that allocates attention to complex computations, self-control, and conscious reasoning.
Mental ModelThe fast, automatic, intuitive mode of thinking that operates effortlessly and generates impressions, intuitions, and feelings without conscious control.
Mental ModelJudging the frequency or probability of events by how easily examples come to mind, leading to overestimation of vivid or recent events.
Mental ModelJudging probability by how much something resembles a typical case while ignoring base rates, sample size, and statistical principles.
Mental ModelThe tendency to rely too heavily on an initial piece of information (the anchor) when making subsequent judgments, even when the anchor is arbitrary or irrelevant.
Mental ModelThe principle that losses loom psychologically larger than equivalent gains, with losing something feeling roughly twice as bad as gaining the same thing feels good.
PrincipleA descriptive model of decision-making under risk showing that people evaluate outcomes relative to a reference point, are loss-averse, and weight probabilities non-linearly.
FrameworkSystem 1's tendency to construct the most coherent story possible from currently available information without considering what's missing or questions not asked.
PrincipleThe error of judging a conjunction of two events (A and B) as more probable than one of the events alone (A), violating the basic rule that P(A and B) ≤ P(A).
After reading that 'John is a shy, introverted person who likes puzzles,' people judge 'John is a librarian who plays chess' as more probable than 'John is a librarian,' even though adding 'plays chess' can only reduce the probability. The detailed description feels more representative.
More detailed, specific scenarios are more likely than general ones—actually, every added detail reduces probability; specificity increases representativeness but decreases probability.
Conjunction Fallacy is explored in depth in "Thinking, Fast and Slow" by Daniel Kahneman. Distilo provides a deep AI-powered analysis with key insights, audio narration, and practical frameworks.