Psychologists use several ways to define abnormality. Here, the focus is on two influential definitions that judge mental health by everyday functioning and by how rare a characteristic or behavior is.

Failure to Function Adequately

Psychologists sometimes define abnormality by asking whether a person can cope with the ordinary demands of daily life. The emphasis is on practical functioning rather than simply on whether behavior seems unusual.

This definition is concerned with whether someone can manage areas such as work, relationships, self-care, and independent living. If psychological difficulties stop a person from carrying out these roles, this may suggest abnormality.

Rosenhan and Seligman identified signs that may show a person is failing to function adequately. These include:

• Personal distress: the individual may feel upset, anxious, or overwhelmed.

• Maladaptiveness: behavior may interfere with wellbeing, safety, or normal life goals.

• Unpredictability: behavior may become difficult for the person or others to understand.

• Irrationality: actions may appear unreasonable or disconnected from the situation.

A key strength of this definition is that it is practical. It focuses on the real impact of psychological problems rather than only on labels. This makes it useful for identifying when someone may need professional support. A person who cannot leave the house, maintain hygiene, or hold a conversation may clearly be struggling, even if the reason is not yet known.

Another strength is that it takes account of personal suffering. Some definitions of abnormality focus on outside judgments, but failure to function adequately looks at whether the individual is experiencing serious difficulty in daily life. This can make the approach more humane and clinically useful.

However, there are important limitations. First, judgments about “adequate” functioning can be subjective. Different people may disagree about how much difficulty counts as abnormal. For example, some individuals choose lifestyles that do not match common expectations, but this does not automatically mean they are mentally ill.

Second, functioning depends on context. Someone may temporarily struggle after bereavement, illness, or major stress. In such cases, poor functioning may reflect a normal response to difficult circumstances rather than abnormality. This means the definition can sometimes pathologize understandable human experiences.

A further weakness is that not all people with a psychological disorder show obvious impairment. Some continue to work, study, and appear outwardly successful while still experiencing serious symptoms. If psychologists rely only on visible functioning, they may miss individuals who need help.

Statistical Infrequency

Another definition focuses on how common or rare a behavior or characteristic is within a population.

This approach assumes that many human traits are distributed around an average.

A labeled normal distribution (bell curve) showing how scores spread around the mean and how rarity increases toward the extremes (“tails”). The standard-deviation markers and percentage bands make it easy to see why only a small proportion of people fall very far from average, which is the logic behind statistical infrequency as a definition of abnormality. Source

Most people fall near the middle, while relatively few are found at the extreme ends. If a behavior or score is very unusual, it may be classified as abnormal.

A common example is IQ. Very low IQ scores are statistically rare and have been used as one indicator in identifying intellectual disability. The key idea is that rarity can provide an objective basis for deciding whether something falls outside the normal range.

One strength of statistical infrequency is that it appears more objective and measurable than many other definitions. Psychologists can compare an individual’s score or behavior with population data rather than relying entirely on opinion.

Two normal curves plotted on the same scale with the same mean but different standard deviations, illustrating how larger $\sigma$ produces a wider, flatter distribution and smaller $\sigma$ produces a narrower, taller one. This helps explain why statistical infrequency depends not just on “distance from the mean,” but also on the population spread used for comparison. Source

This can make the definition useful in assessment and research.

It can also help identify people who may need support or intervention. If a person’s behavior or psychological score is far from the average, this may signal a need for closer evaluation.

Despite this, statistical infrequency has clear weaknesses. The main problem is that rare does not always mean undesirable. Some characteristics are unusual but positive, such as exceptionally high intelligence. A behavior or trait cannot be judged abnormal simply because it is uncommon.

The reverse problem also applies: not all abnormal behavior is rare. Some experiences associated with psychological difficulty are common enough in the population that they would not count as statistically infrequent. This means rarity alone is not a sufficient definition of abnormality.

Another issue is that the cutoff point for “infrequent” can be arbitrary. Psychologists must decide how rare something must be before it is classed as abnormal, and different cutoffs may lead to different judgments. The decision is therefore not as fully objective as it may first appear.

Comparing the Two Definitions

These two definitions ask different questions:

• Failure to function adequately asks whether the person can manage daily life.

• Statistical infrequency asks whether the behavior or characteristic is unusually rare.

Sometimes the two overlap. A person may show a statistically rare pattern and also struggle to function. In other cases, they separate. Someone may have a rare characteristic without any distress or impairment. Equally, a person may be functioning very poorly even though their difficulties are not especially rare in the population.

This is why both definitions have value but also clear limits. Failure to function adequately is useful because it focuses on lived experience and impairment. Statistical infrequency is useful because it offers a more numerical and standardized way of identifying unusual cases. On their own, though, neither definition gives a complete answer to what abnormality is.