Conjunction Fallacy
🇳🇴KonjunksjonsfeilslutningDefinition
The conjunction fallacy occurs when we believe a specific combination of events (A and B) is more likely than one of them alone (A). Mathematically this is impossible: the probability of A and B can never exceed the probability of A. But because the specific story feels more 'plausible,' intuition ranks it higher.
Real-world example
Kahneman and Tversky (1983) presented the classic 'Linda problem': Linda is 31, single, outspoken, with a background in philosophy and activism against discrimination. Which is more likely? (a) Linda is a bank teller, or (b) Linda is a bank teller and active in the feminist movement. 85% of participants chose (b) – which logically must be less likely than (a), since all Lindas-who-are-both-bank-tellers-and-feminists are also Lindas-who-are-bank-tellers.
The effect appears in insurance ('terrorism insurance' sells more easily than general 'travel insurance' even though the latter covers more), political risk analysis (specific scenarios are rated more probable than abstract ones), and intelligence work (detailed threat hypotheses get higher probability than broad categories).
Supplementary perspective
The fallacy is a symptom of using the representativeness heuristic instead of probability reasoning: We ask 'how typical does this sound?' rather than 'how many scenarios fit this description?' The more detailed a scenario, the more 'real' it feels – even though each detail adds a condition that must be met.
Practical advice
Recognize
- —Notice when a detailed scenario feels more credible than a broader one that includes it.
- —Be skeptical of 'stories' that explain everything – the more specific, the less probable.
- —Notice if you choose 'A and B' because the combination is easy to picture.
Counteract
- —Pause and ask: 'Is this really more likely than A alone?'
- —Count the conditions the scenario requires – each condition lowers the probability.
- —Use frequencies instead of percentages: 'Out of 100 similar cases, how many meet A? How many meet A and B?'
Ethical use
- —In risk communication: Avoid using vivid detail to raise perceived probability unreasonably.
- —In sales: Don't present specific scare scenarios as more likely than general categories.
- —In intelligence and planning: Cross-check detailed threat hypotheses against broad categories with base rates.