The most literal way of defining something as abnormal is according to how often it occurs.
Anything that occurs relatively rarely can be seen as 'abnormal'.
The approach is most useful when dealing with human characteristics that can be reliably measured e.g intelligence. The majority of people will cluster around the average and as we move away from this average, fewer people will attain that score.
The average IQ according to tests on a cross section of the population is set at 100. If someone scores below 70 they fall in the bottom 2% of the population meaning they meet one of the criteria for mental retardation.
Limitations of the statistical infrequency are obvious and serious.
IQ scores of 130+ are just as infrequent as those below 70 but we would not consider calling someone abnormal because they are clever.
Statistical infrequency is not a sufficient criterion for defining behaviour as abnormal, although statistical measures, for example of depression, may form part of the process of diagnosis.
Deviation from Social Norms
A social norm is a behaviour or belief that most people stick to within a society.
When someone simply follows the social norms of their culture it is very hard to use their behaviour to make judgements about them. When someone defies the social norms it attracts our attention.
In some cases where behaviour runs counter to the social norm but is clearly harmless, we might think of people as eccentric or rebellious rather than abnormal. However, there are cases where bahviour runs counter to the moral values shared by whole societies.
In these cases we can use deviation from social norms as a basis for defining someone as abnormal.
Deviation from Social Norms
People with a diagnosis of antisocial personality are commonly known as psychopaths.
Part of one of the defining symptoms is 'failure to conform to social norms'.
Other diagnoses such as paedophilia are based largely on the social unacceptability of the individuals behaviour.
Although deviation from social norms is important in defining some mental disorders, it can lead to some problems;
Personal liberty and social control
Cultural variations in social norms
Personal Liberty and Social Control
Nymphomania- Middle class women were attracted to working class men
Moral insanity- Women inherited money and irrationally wished to keep it
These are example of historical diagnoses made on the basis of deviation from social norms.
These diagnoses have been aimed at groups that have always experienced discrimination. In these cases, the main motivation for applying labels of mental disorder was probably a desire to maintain social control over women, black people and gay people.
For example, getting a women diagnosed with moral insanity allowed men to maintain strict control over women.
Cultural Variations in Social Norms
Social norms are culturally specific.
They differ between any 2 groups within a different culture, including different ethnic, regional and socio-economic groups and different generations within the same community.
This can cause problems when a mental health professional from one cultural group assesses someone from another.