Definition
The conjunction fallacy is the mistake of believing that a specific combination of conditions is more likely than one of those conditions alone. It cannot be: adding a detail can only narrow the set of matching outcomes, so a conjunction is always at most as probable as either part.
The classic illustration asks whether a described person is more likely to be a bank teller, or a bank teller who is also active in a particular cause. The richer description feels more probable because it tells a better story — but it is strictly less probable, since it is a subset of the simpler claim.
Why it matters
How it works
The fallacy arises because the mind judges probability by representativeness: how well a scenario matches a familiar pattern. A detailed scenario fits a story better and so feels more plausible, even though each added condition is one more thing that must be true.
The corrective is to count conditions. Every extra clause in a prediction is a multiplier below one. The more specific and satisfying the scenario, the more skeptical of its probability you should be.