Base rate fallacy
Also known as Base rate neglect
The error of ignoring how common something is in general and focusing on specific details about a single case. It is a reasoning fallacy that leads us to overrate vivid details and underrate plain statistics.
In everyday life
A person who tests positive on a fairly accurate test for a very rare disease assumes they almost certainly have it, overlooking that when a disease is very rare, a large share of positive results can be false alarms.
Told only that a stranger is quiet, bookish, and loves poetry, you bet he's a classics professor instead of a salesman, though salesmen vastly outnumber classics professors.
Why your mind does this
We take in far more than we can handle, so the brain uses what is already active in memory as a filter. Things we met recently, often, or with strong feeling are likely to matter again, so pushing them forward saves time and effort.
How to spot it
Ask yourself how common this outcome or type of person is overall before weighing the details of this one case.
How to counter it
Start every estimate from the general rate, then adjust it up or down based on the specific evidence.
Habits that catch it
- Start from the base rate
Ask how things like this usually turn out before you look at what makes this case special.