How we change what others think, feel, believe and do
The exception fallacy occurs when data about an individual is used to draw conclusions about a group of people.
A first class passenger on a train is arrogant and rude to a person who walks into the first class compartment. The person concludes that all first class passengers are arrogant.
A person in France is helped by a policeman there. They assume all French police are helpful.
A teacher finds that a few students from a particular part of town are difficult. They conclude that that part of town is rough.
We are always in a hurry to classify people and groups and, when we have limited data about a group, we will often use what information we have, even if it is not statistically valid -- and even if it is a single data point.
This data may well be the exception rather than the rule - hence the name of the fallacy.
This effect is particularly common when several factors are taken into account:
This fallacy is an effective reversal of the Ecological fallacy.