Conjunction fallacy

Estimation biases biases

The conjunction fallacy is when you think a specific combination of two things is more likely than one of those things alone. In reality, "A and B" can never be more likely than "A" by itself—it can only be equally likely or less. The bias appears when a detailed story feels more plausible than a vaguer one.

Examples

  • "Linda is a bank teller and active in the feminist movement" is judged more likely than "Linda is a bank teller," even though the first is a subset of possibilities that include the second.

  • You think it's more likely that your team will win the league and the cup than that they will win the league, even though winning both is a narrower outcome.

  • A detailed forecast—"there will be a recession and unemployment will rise and house prices will fall"—feels more likely than the single claim "there will be a recession," though the latter must be at least as likely.

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