Watch for
Letting a compelling story override statistical facts.
Belief & ConfidenceBias #99
Base Rate Fallacy
We ignore general probabilities in favour of specific details.
The tendency to ignore background statistical probabilities in favour of specific vivid or anecdotal information about a single case.
Why it matters: Classic Kahneman and Tversky demonstration with the engineer-lawyer problem.
Try this
Start with base rates and adjust only if specific evidence is strong and reliable.
Real-world example
Assuming a person who fits the description of a terrorist is likely a terrorist even though the base rate of terrorism is extremely low.
Key researchers
Daniel Kahneman, Amos Tversky
First described in 1973
Psychological mechanism
Representativeness heuristic override. Specific individuating information is vivid and story-like engaging System 1 thinking. Base rates are abstract statistical abstractions that require System 2 effort to integrate.
Seminal research
Amos Tversky and Daniel Kahneman (1973), "On the psychology of prediction."
Related Biases
Below is a realistic scenario. Read it, then choose what you would do. The feedback will show whether a cognitive bias influenced your choice — not to judge, but to reveal the pattern in action.
Thought Experiment
This experiment places you in a realistic decision. Your instinctive choice will reveal whether bias is at work.
STEP 1 → ReadSTEP 2 → ChooseSTEP 3 → Reveal
Debrief
The base rate fallacy shows why medical test results are so often misinterpreted. A 99% accurate test for a rare condition still produces far more false positives than true positives because the base rate is so low. Always ask: what is the base rate before updating with new evidence?
Sources
Original ResearchTversky and Kahneman (1973) — Base rate fallacy
EducationalAPA Dictionary entry