Look up base rates before forming a resemblance judgment
Before deciding "this looks like X," ask how common X actually is in the relevant population.
Key takeaways
- What it is: Before deciding "this looks like X," ask how common X actually is in the relevant population.
- Why it works: Representativeness substitutes "how typical is this?" for "how probable is this?" — an easier question that feels like the right one. The substitution goes unnoticed because the resemblance judgment is computed automatically and feels like evidence. Anchoring on a base rate first forces the harder question to the front, making it much more likely that probability judgment is correctly calibrated rather than distorted by surface similarity.
- Evidence: Backed by observational / correlational evidence.
- Avoid: Treating resemblance as only one factor while still weighting it heavily — if you thought of the base rate at all but still landed on the representative answer, the heuristic won.
Why it works
Representativeness substitutes "how typical is this?" for "how probable is this?" — an easier question that feels like the right one. The substitution goes unnoticed because the resemblance judgment is computed automatically and feels like evidence. Anchoring on a base rate first forces the harder question to the front, making it much more likely that probability judgment is correctly calibrated rather than distorted by surface similarity.
How to do it
- 1Name the category you’re about to assign something to ("this person is probably a doctor").
- 2Before using resemblance as evidence, ask: "What fraction of people in the context I encountered this person are doctors?"
- 3Write down that base rate as your starting probability.
- 4Update for specific observed features only if those features are genuinely more common in the category than outside it.
What the evidence says
ObservationalKahneman and Tversky’s original 1973 research on the engineer/lawyer problem showed that subjects ignored explicit base rate information (70/30 split) when they had a representative description. Many follow-up studies replicate base-rate neglect, though instruction on base rates can partially correct it.
Honest caveat: Base-rate neglect is stronger when the description is vivid and individuating; it reduces (but doesn’t vanish) when no description is given or when subjects are prompted to think like statisticians.
- — Kahneman & Tversky (1973), On the psychology of prediction, Psychological Review
Common mistake
Treating resemblance as only one factor while still weighting it heavily — if you thought of the base rate at all but still landed on the representative answer, the heuristic won.
IX Coach surfaces the base rate question before you profile a person, situation, or risk, so the resemblance anchor doesn’t do all the work unsupervised.
Practice this with IX Coach →