Individual Differences in Intelligence

Why Are Some People More Intelligent Than Others?

This question has driven decades of research. The answer lies in a complex interplay between what we inherit (nature) and what we experience (nurture). You have already encountered these influences in your Class XI studies.

Intelligence: Interplay of Nature and Nurture

The evidence for hereditary influences on intelligence comes mainly from studies on twins and adopted children.

Twin Studies

Identical twins reared together show an intelligence correlation of almost 0.90. Even when separated early in childhood, identical twins still show considerable similarity in their intellectual, personality, and behavioural characteristics — their intelligence correlates at 0.72. Fraternal twins reared together correlate at about 0.60, while ordinary brothers and sisters reared together correlate at about 0.50. Siblings reared apart, however, correlate at only about 0.25.

Adoption Studies

Children's intelligence is more similar to their biological parents than to their adoptive parents. However, as children grow older, their intelligence level tends to move closer to that of their adoptive parents. Children from disadvantaged homes adopted into families with higher socio-economic status show a large increase in their intelligence scores.

Environmental Influences

Environmental deprivation lowers intelligence. Rich nutrition, good family background, and quality schooling all increase intelligence.

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Assessment of Intelligence

In 1905, Alfred Binet and Theodore Simon made the first successful attempt to formally measure intelligence. In 1908, when the scale was revised, they introduced the concept of Mental Age (MA) — a measure of a person's intellectual development relative to people of their age group. A mental age of 5 means that a child's performance on an intelligence test equals the average performance level of a group of 5-year-olds.

Chronological Age (CA) is the biological age from birth. A bright child's MA is more than their CA; for a dull child, MA is below the CA. Binet and Simon defined retardation as being two mental age years below the chronological age.

In 1912, William Stern, a German psychologist, devised the concept of Intelligence Quotient (IQ). IQ refers to mental age divided by chronological age, multiplied by 100.

The number 100 is used as a multiplier to avoid the decimal point. When MA equals CA, the IQ equals 100. If MA is more than CA, IQ is more than 100. IQ becomes less than 100 when MA is less than CA.

For example, a 10-year-old child with a mental age of 12 would have an IQ of 120 (12/10 × 100), whereas the same child with an MA of 7 would have an IQ of 70 (7/10 × 100).

The average IQ in the population is 100, irrespective of age. IQ scores are distributed in the population such that most people's scores fall in the middle range. Only a few people have either very high or very low scores. The frequency distribution for IQ scores approximates a bell-shaped curve called the normal curve. This type of distribution is symmetrical around the central value, called the mean.

People with IQ scores in the range of 90–110 have normal intelligence. Those with IQ below 70 are suspected to have intellectual disability, while persons with IQ above 130 are considered to have exceptional talents.

Classification of People on the Basis of IQ

All persons do not have the same intellectual capacity; some are exceptionally bright and some are below average. One practical use of intelligence tests is to identify persons at the extremes of intellectual functioning. About 2 per cent of the population have IQ above 130, and a similar percentage have IQ below 70. The persons in the first group are called intellectually gifted; those in the second group are termed intellectually disabled. These two groups deviate considerably from the normal population in respect of their cognitive, emotional, and motivational characteristics.

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Variations of Intelligence

Intellectual Deficiency

On the one hand are the gifted and creative persons. On the other hand, there are children who face enormous difficulty in learning even very simple skills. Those children who show intellectual deficiency are termed as 'intellectually disabled'. As a group, there is wide variation among the intellectually disabled.

The American Association on Mental Deficiency (AAMD) views intellectual disability as "significantly sub-average general intellectual functioning existing concurrently with deficits in adaptive behaviour and manifested during the developmental period."

This definition points to three basic features:

1. Significantly sub-average intellectual functioning — persons having IQs below 70 are judged to have sub-average intelligence.
2. Deficits in adaptive behaviour — adaptive behaviour refers to a person's capacity to be independent and deal effectively with one's environment.
3. Deficits observed during the developmental period — that is, between 0 and 18 years of age.

Individuals categorised as having intellectual disability show significant variation in their abilities, ranging from those who can be taught to work and function with special attention, to those who cannot be trained and require institutional care throughout their lives.

Levels of Intellectual Disability

• Mild: IQs 55 to approximately 70
• Moderate: IQs 35–40 to approximately 50–55
• Severe: IQs 20–25 to approximately 35–40
• Profound: IQs below 20–25

People with mild disability typically develop more slowly than their peers but can function quite independently, hold jobs, and have families. As the level of disability increases, the difficulties become strongly marked. People with moderate disability lag behind their peers in language and motor skills. They can be trained in self-care skills, and simple social and communication skills, but need moderate supervision in everyday tasks. Individuals with profound and severe disability are incapable of managing life and need constant care for their entire lives.

Intellectual Giftedness

Intellectually gifted individuals show higher performance because of their outstanding potentialities. The study of gifted individuals began in 1925, when Lewis Terman followed the lives of about 1500 children with IQs of 130 and above to examine how intelligence was related to occupational success and life adjustment.

Although the terms 'talent' and 'giftedness' are often used interchangeably, they mean different things. Giftedness is exceptional general ability shown in superior performance in a wide variety of areas. Talent is a narrower term and refers to remarkable ability in a specific field (e.g., spiritual, social, aesthetic, etc.). The highly talented are sometimes called 'prodigies'.

Psychologists suggest that giftedness from the teachers' point of view depends on a combination of high ability, high creativity, and high commitment.

Gifted children show early signs of intellectual superiority. Even during infancy and early childhood, they show larger attention span, good recognition memory, preference for novelty, sensitivity to environmental changes, and early appearance of language skills. To equate giftedness with brilliant academic performance is not correct. Athletes who show superior psychomotor ability are also gifted. Each gifted student possesses different strengths, personalities, and characteristics.

Some important characteristics of gifted children:

• Advanced logical thinking, questioning and problem solving behaviour
• High speed in processing information
• Superior generalisation and discrimination ability
• Advanced level of original and creative thinking
• High level of intrinsic motivation and self-esteem
• Independent and non-conformist thinking
• Preference for solitary academic activities for long periods

Performance on intelligence tests is not the only measure for identifying the gifted. Many other sources of information, such as teachers' judgment, school achievement record, parents' interviews, peer and self-ratings, etc. can be used in combination with intellectual assessment. To reach their full potential, gifted children require special attention and different educational programmes beyond those provided to normal children in regular classrooms. These may include life enrichment programmes that can sharpen children's skills in productive thinking, planning, decision-making, and communication.

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Types of Intelligence Tests

Intelligence tests are of several types. On the basis of their administration procedure, they can be categorised as individual or group tests. They can also be classified as either verbal or performance tests on the basis of the nature of items used. Depending upon the extent to which an intelligence test favours one culture over another, it can be judged as either culture-fair or culture-biased.

Individual or Group Tests

An individual intelligence test can be administered to one person at a time. A group intelligence test can be administered to several persons simultaneously.

Individual tests require the test administrator to establish a rapport with the subject and be sensitive to their feelings, moods, and expressions during the testing session. Group tests, however, do not allow an opportunity to be familiar with the subjects' feelings. Individual tests allow people to answer orally or in a written form or manipulate objects as per the tester's instructions. Group tests generally seek written answers usually in a multiple-choice format.

Verbal, Non-Verbal, or Performance Tests

An intelligence test may be fully verbal, fully non-verbal, or fully performance-based, or it may consist of a mixture of items from each category.

Verbal tests require subjects to give verbal responses either orally or in a written form. Therefore, verbal tests can be administered only to literate people.

Non-verbal tests use pictures or illustrations as test items. Raven's Progressive Matrices (RPM) Test is an example of a non-verbal test. In this test, the subject examines an incomplete pattern and chooses a figure from the alternatives that will complete the pattern.

Performance tests require subjects to manipulate objects and other materials to perform a task. Written language is not necessary for answering the items. For example, Kohs' Block Design Test contains a number of wooden blocks. The subject is asked to arrange the blocks within a time period to produce a given design. A major advantage of performance tests is that they can be easily administered to persons from different cultures.

Culture-Fair or Culture-Biased Tests

Many intelligence tests show a bias to the culture in which they are developed. Tests developed in America and Europe represent an urban and middle class cultural ethos. Hence, educated middle class white subjects generally perform well on those tests. The items do not respect the cultural perspectives of Asia and Africa. The norms for these tests are also drawn from western cultural groups.

It is nearly impossible to design a test that can be applied equally meaningfully in all cultures. Psychologists have tried to develop tests that are culture-fair or culturally appropriate — ones that do not discriminate against individuals belonging to different cultures. In such tests, items are constructed in a manner that they assess experiences common to all cultures or have questions in which language usage is not required. Non-verbal and performance tests help reduce the cultural bias usually associated with verbal tests.