Conjunction Fallacy
Type: Cognitive Bias
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Definition
Believing that specific conditions are more probable than general ones. Adding details makes a story seem more likely, even though it’s mathematically less probable.
Linda problem (Tversky & Kahneman): “Linda is 31, single, outspoken, majored in philosophy. Which is more probable? A) Linda is a bank teller, or B) Linda is a bank teller and active in feminist movement?” 85% choose B. Can’t be — B is a subset of A.
Why It Matters
Legal: Jurors find detailed stories more credible, even when details reduce probability. Marketing: “This car is safe AND stylish AND fast” — each AND reduces likelihood. Conspiracy theories: Detailed conspiracies feel more true than simple explanations. Planning: “We’ll launch in 6 months AND hit all targets AND beat competitors” — each AND is less likely.
The Math
P(A and B) ≤ P(A)
The probability of two things both happening cannot exceed the probability of either alone.
But narratives with details feel more “representative” — and we confuse representativeness with probability.
Examples
- Linda problem — Feminist bank teller vs bank teller
- Medical symptoms — Specific diagnosis feels more likely than general one
- Predictions — “X will happen AND Y will happen” — less likely than just X
- Conspiracies — Complex plots with many actors feel more true
Fighting It
- Remember the math — Conjunction cannot be more probable
- Strip details — “What’s the core claim?”
- Compare to subset — Is this a special case of something more general?
- Think Venn diagrams — The intersection is smaller than either circle
Related Biases
- [[Base Rate Fallacy** — Both involve probability errors
- [[Representativeness Heuristic** — Matching to prototypes
- [[Narrative Fallacy** — Stories feel true
Audio
Podcast episode: Conjunction Fallacy
Part of the Cognitive Bias Reference