Conjunction Fallacy

The error of judging a specific, detailed scenario as MORE probable than a general one that contains it—violating the basic law that a subset cannot be more probable than its superset. Tversky and Kahneman's famous example: Linda, described as a social activist, was judged MORE likely to be 'a feminist bank teller' than 'a bank teller'—even though every feminist bank teller IS a bank teller, making the broader category necessarily more probable. The mechanism: detailed scenarios activate the representativeness heuristic (the story fits!), which overrides basic probability logic. More detail makes a scenario feel more plausible while actually making it less probable.

When to use it

When detailed scenarios are being judged as more likely than they should be; when business plans with specific narratives feel more credible than simple projections; when risk scenarios with compelling stories are being overweighted; when any probability assessment involves comparing specific and general categories.

How it can help

When evaluating scenarios: be suspicious of detailed, specific predictions that feel highly plausible. The more specific a scenario, the LESS probable it is—because it requires more conditions to be simultaneously true. In business planning: 'revenue will grow because millennials in urban areas who are health-conscious will adopt our product' is less probable than 'revenue will grow'—even though it feels more convincing. The conjunction rule: never rate a specific scenario as more likely than the general category it belongs to. When detailed stories feel compelling, check whether you're committing the conjunction fallacy.