Keith E Rice's Integrated SocioPsychology Blog & Pages

Aligning, integrating and applying the behavioural sciences

What is Mental Illness? #2

Underpinning the attempts to categorise and classify mental illness are 4 broad approaches…

Deviation from statistical norms (statistical infrequency)
This is based on the notion that abnormal thought and behaviour are pretty rare in the general population.

The scores of most human characteristics and behaviours, when measured, tend to fall fairly evenly about the mean, giving the bell curve of normal distribution. Eg: there are a lot of people who are ‘averagely’ tall or aggressive whereas very few people are very small or highly aggressive. Thus, there are as many people/scores above the mean as below it in a normal distribution. The further you travel from the mean, the fewer  people/scores there are in the population.

Graphic copyright © 2018 Guang Jin/Illinois State University

In a normal distribution – see graphic above – the  majority of individuals are clustered  around the mean. In a normal distribution the scores of 95.5% of the population will fall within 2 standard deviations from the mean. Scores beyond 2 standard deviations are considered statistically infrequent and, therefore, abnormal. Thus, thoughts and behaviours which occur in 4.5% or less of the population can be considered abnormal.

As a way of defining what is meant by mental illness, this approach works well for some illnesses – eg: Schizophrenia at 1% in the general population and Bipolar Disorder at 2.8%. However, for Depression it doesn’t. According to the World Health Organisation (2018), up to 25% of females will be diagnosed at some point in their life and up to 12% of men. At any one point in time it is estimated 9% of women are clinically depressed. Such figures are well within 2 standard deviations and thus are not statistically infrequent. Yet the WHO also says: “Depression is the leading cause of disability worldwide, and is a major contributor to the overall global burden of disease.” Clearly, most people would consider Clinical Depression a mental illness!

There are also issues of desirability which confound this approach to mental illness. For example, as per the graph below, the mean for intelligence (in white Caucasians) is 100 (as measured on a Stanford-Binet IQ test). Someone 2 standard deviations below with a score of 70 is classified clinically as ‘retarded’. However, someone 2 standard deviations above the mean, with a score of 130, would be classified as ‘genius’ Many people would consider having ‘genius’ status as desirable while no one would consider ‘retard’ status as desirable.

Graph copyright © 2001 Psychology Press Ltd

The statistical infrequency approach also ignores relativism. Eg: it is considered acceptable for children to be frightened of the dark; but not adults.

Deviation from social norms

Failure to Function Adequately

Deviation from Ideal Mental Health


The Labelling of Mental Illness


Szasz  society uses stigmatising labels to exclude

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