Steelman
Conjunction fallacy
Also called linda problem, conjunction effect.
Judging a detailed combination of events as more likely than one of those events alone.
What it is
Rating “A and B” as more probable than “A” alone. Because both A and B must happen for the combination to happen, “A and B” can never be more likely than A.
The pattern
P(A and B) is judged greater than P(A). (impossible)
An example
Linda, 31, is outspoken and was deeply concerned with social justice as a student. Which is more likely: (a) Linda is a bank teller, or (b) Linda is a bank teller and active in the feminist movement? Many people choose (b).
(b) can’t be more likely than (a): every feminist bank teller is a bank teller. The description matches a stereotype, which makes (b) feel more plausible.
Why it fails
Every case in which A and B are both true is also a case in which A is true. Adding detail makes a story more vivid and more believable, but never more probable.
When it’s not a fallacy
Most lists skip this part. Calling a fair point a fallacy is a mistake too.
It can be fair when
- People reasonably read option (a) as implying “and not the other thing” (“a bank teller who isn’t a feminist”). Then they’re comparing two different combinations, not a combination with its part.
Try one
A question from the app. The wrong answers are its usual look-alikes.
Group chat, $10 bets on the Finals. Theo: “I’m taking the Celtics to win it. That’s my whole bet.” Ava: “Boring. I’m taking the Celtics in six, with Tatum as Finals MVP. That’s exactly how this series plays out, so mine’s the safer bet.”
What’s the flaw?
Every way Ava’s bet can win is also a way Theo’s wins, since a Celtics title in six with Tatum as MVP is still a Celtics title. Adding details can only make a bet less likely to come in, never safer.
A fair reply
“If yours comes in, mine comes in too, so yours can’t be the safer one.”
Often confused with
- Base rate fallacy
Both come from judging by resemblance to a stereotype rather than by probability, but base-rate neglect ignores how common something is, while the conjunction fallacy overrates added detail.
Where the name comes from
In logic, a conjunction is an “and” statement: A and B together. The fallacy rates the “and” version as likelier than A alone. The “Linda problem” is the famous test question: is Linda more likely a bank teller, or a feminist bank teller? “Conjunction effect” is psychologists’ name for the same pull.
The Linda problem comes from experiments by psychologists Amos Tversky and Daniel Kahneman in the early 1980s.
Sources
- The Conjunction Fallacy, The Fallacy Files
- Conjunction fallacy, Wikipedia
In the app
Practice spotting it on arguments that sound real.
Steelman has more examples of conjunction fallacy, how to answer it in a real conversation, and practice rounds that mix it with its look-alikes, so you learn the difference, not just the name. Coming soon to iPhone and Android.
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