Conjunction Fallacy

What is it?

The conjunction fallacy is the error of judging that two events together are more likely than one of them alone, when a combination can never be more probable than either of its parts. In Amos Tversky and Daniel Kahneman's well-known "Linda" problem, a woman is described as outspoken and concerned with social justice; many people then rate "Linda is a bank teller and active in the feminist movement" as more probable than "Linda is a bank teller". The detailed version fits the description better, so it feels more likely, even though each added detail can only lower the odds. In decisions, this makes rich, coherent stories seem more credible than plain ones: a business plan with many specific steps, a vivid threat scenario or a precise forecast. It tends to get worse when the details are plausible and consistent with each other, and when you judge a story by how well it hangs together rather than by how many things must all go right.