Subadditivity Effect
We judge the probability of a whole to be less than the sum of its parts.
The subadditivity effect is the tendency to judge the probability of a whole to be less than the probabilities of its parts. If asked to estimate the probability of dying from various causes, people assign higher probability to subcategories than to the broader category. Unpacking a hypothesis into components inflates its perceived probability. This was documented by Tversky and Koehler through support theory.
What it looks like in the wild.
- Everyday life Cause of Death Estimates People estimate there is a 30% chance of dying from heart disease and a 25% chance of dying from cancer, but only a 40% chance of dying from 'natural causes' — even though heart disease and cancer are natural causes.
Recognising a definition is not the same skill as spotting it.
Two scenarios from the app, with distractors drawn from the same family of biases — which is what makes them hard.
Ben is evaluating a business proposal. He thinks there's a 40% chance the product will sell well and a 40% chance the marketing campaign will be effective. He concludes there's a 60% chance the business will be successful overall.
Mark is taking medicine. He thinks there's a 70% chance it will reduce his symptoms and a 60% chance it won't cause side effects. He believes the overall chance of a positive outcome is 45%.
Biases rarely arrive alone.
Understand more. Assume less.
188 biases, 18,869 scenarios, and a record of how you actually decide.
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