Definitions of abnormality

The history of abnormality

Ideas about abnormality have changed with each era's knowledge and values.

• Supernatural explanations: unusual behaviour was once blamed on spirits or punishment.
• Asylums: people were confined, often in poor conditions, away from society.
• Physical treatments: in the twentieth century treatments included lobotomy; from the 1950s the first antipsychotic drugs (typical antipsychotics) changed hospital care.
• Changing norms: in December 1973 the American Psychiatric Association's Board of Trustees voted to remove homosexuality from DSM-II, showing that a 'disorder' can reflect social values rather than illness.

The component guide links this history to social control (9.3.9): defining someone as abnormal gives others power over them.

Statistical infrequency

Behaviour is abnormal if it is statistically rare, usually more than two standard deviations from the mean of a normal distribution.

The classic example is IQ (mean 100, SD 15). An IQ below 70 is two SDs below the mean. In a normal distribution about 2.28% of people score below 70, and the same proportion score above 130.

Evaluation

• Objective and easy to measure.
• But some rare traits are desirable (a very high IQ), and some common problems (such as depression, which affects hundreds of millions of people) are not rare.
• The cut-off is arbitrary, and IQ tests reflect Western ideas of intelligence (component guide).

Failure to function adequately

A person is abnormal if they cannot cope with everyday life: working, looking after themselves, keeping relationships.

Rosenhan and Seligman (1989) listed features of personal dysfunction. The more that are present, the more likely the behaviour is seen as abnormal:

Evaluating failure to function

• Strength: it recognises the person's own experience (distress) and links to how clinicians judge severity.
• Weakness: judgements such as 'irrational' or 'observer discomfort' are subjective and depend on culture.
• Weakness: some people are distressed but function well, and some people function but break social norms by choice (for example, risky sports).
• Social control: labelling people who break norms as dysfunctional can be used to control minorities.

Worked example

A student says: 'Anyone with an IQ of 140 is abnormal.' Use the statistical definition to evaluate this.

1. Statistically, 140 is more than two SDs above the mean of 100 (SD 15), so it is rare.
2. So by statistical infrequency alone it counts as abnormal.
3. But a high IQ is desirable and does not need treatment, so rarity on its own cannot define abnormality.
4. Clinicians therefore also consider distress and failure to function.

Answer: Rare is not the same as disordered; a full judgement uses more than one definition.

Check yourself

1. IQ scores are normally distributed with mean 100 and SD 15. What percentage of people score below 70? Give your answer to 2 decimal places.
   Show answer

   2.28 %. 70 is (70 - 100) / 15 = -2 SDs. The area below z = -2 is 2.28%.

2. What percentage of a normal distribution lies within one standard deviation of the mean? Give your answer to 1 decimal place.
   Show answer

   68.3 %. The area between z = -1 and z = +1 is 68.3%.

3. Which is the main weakness of the statistical infrequency definition?
   • It cannot be measured
   • It depends entirely on the patient's feelings
   • Some rare behaviours are desirable and some common ones are distressing
   • It was invented by Rosenhan
   Show answer

   Some rare behaviours are desirable and some common ones are distressing. Rarity does not equal disorder: high IQ is rare but desirable; depression is common but distressing.

4. Which is one of Rosenhan and Seligman's (1989) features of personal dysfunction?
   • Statistical rarity
   • Observer discomfort
   • High intelligence
   • Physical illness
   Show answer

   Observer discomfort. Observer discomfort is one of the seven features.

5. What does the removal of homosexuality from DSM-II in 1973 show about defining abnormality?
   • Diagnosis is always perfectly objective
   • Statistical rarity is the best definition
   • Drug treatments replaced asylums
   • Definitions reflect the social values of the time
   Show answer

   Definitions reflect the social values of the time. The change came from shifting values and evidence, showing definitions are socially shaped.

6. Explain one weakness of the failure to function adequately definition.
   Show answer

   Judging behaviour as irrational or causing observer discomfort is subjective and depends on culture, so the same behaviour may be labelled abnormal in one society but not another.. Subjectivity and cultural relativity are the key weaknesses.

7. Using the same IQ distribution, what percentage of people score above 130? Give your answer to 2 decimal places.
   Show answer

   2.28 %. 130 is +2 SDs; the area above z = 2 is 2.28%, the same as below 70 by symmetry.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification (issue January 2026)
• Pearson IAL Psychology component guide: WPS04 (Issue 2, November 2024)
• Hanlin: Rosenhan and Seligman's features of personal dysfunction
• Drescher (2015), Out of DSM: depathologizing homosexuality, Behavioral Sciences 5(4)
• WHO fact sheet: Mental disorders (Global Burden of Disease figures for 2023)

Last updated 8 November 2026. Spotted a mistake? Tell us.