The conjunction fallacy. People intuitively choose the vivid option, even if it is obviously less likely.
Perhaps the simplest and most basic qualitative law of probability is the conjunction rule: the probability of A AND B both being true cannot be more likely than the probability of A or B alone.
But the description of a conjunction can be more detailed than one of its constituents and our thinking heuristics are biased towards vivid descriptions and stereotypes, even when they contradict simple logic.
When asked which alternative is more probable?
- Linda is a bank teller.
- Linda is a bank teller and is active in the feminist movement.
The vast majority—roughly 85 percent—of the people asked opted for number 2, even though it is logically obvious that the number of bank tellers in the feminist movement must be less than the number of bank tellers overall.
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