Explain abnormality. Trace a brief history of how our understanding of psychological disorders has evolved to

Question

Q. Explain abnormality. Trace a brief history of how our understanding of psychological disorders has evolved to its current status.

(OR)

Explain the different psychological models used to explain abnormal behaviour.

CBSECBSE Class XII Board 2019Subjective· 6mImportance★★★★★

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Part (a)Concept understanding — Abnormality Definition

Abnormality Definition — First Principles

Imagine you are looking at a set of data points — say, the heights of all students in your class. Most heights cluster around some average value. A few students are taller, a few shorter, but almost everyone falls within a certain range. Now suppose one student is 8 feet tall. That student's height is abnormal — it lies far outside the pattern of the rest of the group.

That is the core intuition behind abnormality in statistics: an observation that deviates so much from the rest of the data that it stands out as unusual.

The Precise Definition

In formal terms, an abnormality (often called an outlier) is a data point that lies at an abnormal distance from other values in a random sample from a population. There is no single universal cutoff — it depends on the context — but the most common working definition in Indian exam syllabi (especially for statistics and probability) is:

\text{An observation } x \text{ is abnormal if } |x - \mu| > 3\sigma

This is the 3-sigma rule. The reasoning: for a normal (bell-shaped) distribution, about 99.7% of all data lies within three standard deviations of the mean. Anything beyond that has less than a 0.3% chance of occurring naturally — so we call it abnormal.

Why 3-Sigma? The Intuition

The number 3 is not arbitrary. In a normal distribution:

Within 1σ of μ: ~68% of data
Within 2σ of μ: ~95% of data
Within 3σ of μ: ~99.7% of data

A point beyond 3σ is so rare that it is more likely to be a mistake, a special cause, or a genuinely exceptional case — hence "abnormal."

Watch out

The 3-sigma rule assumes the data follows a normal distribution. If the distribution is skewed or has heavy tails, this rule can misclassify normal points as abnormal. Always check the shape of your data first.

A Concrete Example

Suppose the mean height of Indian adult males is 165 cm with a standard deviation of 7 cm. A person who is 190 cm tall:

$z = \frac{190 - 165}{7} = \frac{25}{7} \approx 3.57$ …

Part (b)Concept understanding — Abnormality Definition

Abnormality Definition — First Principles

That is the core intuition behind abnormality in statistics: an observation that deviates so much from the rest of the data that it stands out as unusual.

The Precise Definition

\text{An observation } x \text{ is abnormal if } |x - \mu| > 3\sigma

Why 3-Sigma? The Intuition

The number 3 is not arbitrary. In a normal distribution:

Within 1σ of μ: ~68% of data
Within 2σ of μ: ~95% of data
Within 3σ of μ: ~99.7% of data

A point beyond 3σ is so rare that it is more likely to be a mistake, a special cause, or a genuinely exceptional case — hence "abnormal."

Watch out

A Concrete Example

Suppose the mean height of Indian adult males is 165 cm with a standard deviation of 7 cm. A person who is 190 cm tall:

$z = \frac{190 - 165}{7} = \frac{25}{7} \approx 3.57$ …

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Part (a): Abnormality = maladaptive behaviour (deviance, distress, dysfunction, danger); understanding evolved from supernatural to biological to modern psychological/socio-cultural views.

Part (b): Psychological models of abnormal behaviour = psychodynamic, behavioural, cognitive and humanistic-existential.

Part (a)

Abnormality is behaviour that is considered atypical and maladaptive. A useful way to define it is through the 'Four D's': (i) Deviance — thoughts, emotions or behaviour that deviate markedly from a society's norms and standards; (ii) Distress — behaviour that is unpleasant and upsetting to the person concerned; (iii) Dysfunction — behaviour that interferes with a person's ability to carry out daily activities and adaptive functioning; and (iv) Danger — behaviour that poses a risk of harm to oneself or to others. Abnormality is also described as deviation from social norms and as maladaptive behaviour. …

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Q.Explain abnormality. Trace a brief history of how our understanding of psychological disorders has evolved to its current status.

Part (a)

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