Refers to behaviours which are considered numerically rare and don't appear often within society.
An example of this includes IQ
The average IQ is 100 - 68 % of people
Statistically infrequent IQ is below 70 - only 2 % of people
Those with an IQ below 70 are deemed to have an intellectual disability disorder.
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Those below an IQ of 70 are deemed statistically infrequent and therefore abnormal but also those with an IQ higher than 135 are also deemed abnormal.
The issue with this is that only those with an IQ lower than 70 are an unwanted abnormality. Those with a higher IQ are often idolised and desirable which limits the idea of abnormality.