The conjunction fallacy is a logical error where a specific, detailed scenario is judged more probable than a broader category that necessarily includes it — even though, by basic probability, a conjunction of two events can never be more likely than either event on its own. The classic demonstration describes a woman named Linda as outspoken, socially conscious, and politically active as a student, then asks whether it's more likely she is now 'a bank teller' or 'a bank teller and active in the feminist movement.' A large majority pick the second, more specific option, even though it's a logical subset of the first.
The error happens because the added detail makes the scenario feel more coherent and representative of the character sketch, and that felt coherence gets mistaken for probability. This isn't limited to hypothetical quizzes — it shows up whenever a specific, well-told story about how something might happen feels more convincing than the blunter, broader possibility it's actually a piece of.
In the Linda experiment, participants overwhelmingly ranked 'bank teller and active in the feminist movement' as more probable than plain 'bank teller,' violating basic probability logic in favor of the description that felt more representative of the character sketch they'd read. Kahneman describes a parallel effect with tennis: participants asked to rank outcomes of a hypothetical Wimbledon match involving Björn Borg rated 'Borg will lose the first set but win the match' as more probable than the strictly broader outcome 'Borg will lose the first set' — even though the first outcome is a subset of the second and therefore can't be more likely.