Assessment of Intelligence

The first successful attempt to formally measure intelligence was made in 1905 by Alfred Binet and Theodore Simon. When they revised their scale in 1908, they introduced the concept of Mental Age (MA). Mental age is a measure of a person’s intellectual development compared to others of the same age group. For example, a mental age of 5 means that a child’s performance on an intelligence test equals the average performance of a group of 5-year-olds.

Chronological Age (CA) is simply the biological age of a person from birth. A bright child has a mental age higher than their chronological age; a dull child has a mental age below their chronological age. Binet and Simon defined retardation as being two mental age years below one’s chronological age.

In 1912, the German psychologist William Stern devised the concept of the Intelligence Quotient (IQ). IQ is calculated by dividing mental age by chronological age and multiplying the result by 100. The number 100 is used as a multiplier to avoid decimals. When MA equals CA, the IQ is exactly 100. If MA is greater than CA, the IQ is above 100; if MA is less than CA, the IQ is below 100. For instance, a 10-year-old child with a mental age of 12 would have an IQ of 120 (12/10 × 100), while 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, regardless of age. IQ scores are distributed in a way that most people fall in the middle range, with only a few having very high or very low scores. This frequency distribution approximates a bell-shaped curve called the normal curve, which is symmetrical around the central value (the mean).

People with IQ scores in the range of 90–110 are considered to 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. The following table shows the classification of people based on IQ scores: