What is Base rate neglect?
Base rate neglect is the tendency to ignore the overall frequency of an event (the base rate) when judging the probability of a specific case. People focus on vivid, case-specific information and underweight the statistical background.
How it works
In Kahneman and Tversky’s famous lawyer-engineer problem, participants read personality descriptions and judged whether each person was a lawyer or an engineer. Their judgments were almost entirely driven by the description’s fit with stereotypes and barely influenced by being told that the pool was 70% lawyers or 70% engineers.
Applied example
After a positive result on a medical screening test with a 5% false positive rate, a patient may assume they almost certainly have the disease. But if the disease affects only 1 in 1,000 people, the odds of actually having it are still only about 2%, because the base rate of the disease is so low.
Why it matters
Base rate neglect is a critical concept in medical diagnosis, criminal justice (where jurors overweight forensic evidence relative to prior probabilities), and any domain where case-specific information competes with statistical baselines.
