Intelligence quotient, or IQ, is a measure of intelligence, based on numerical scores from standardized tests. The concept of an intelligence scale was pioneered by Alfred Binet, a French psychologist, to evaluate the level of mental ability in children. The term "IQ" refers to scores from several intelligence scales, and is a general indicator of intelligence in adults and children alike. A person's IQ score may differ, depending on the measurement system, and it may also change throughout one's lifetime.

## Binet's IQ Scale

Based on the scale developed by Binet, a child's IQ is calculated by taking into account mental age and chronological age. The exact formula Binet used was "(mental age divided by chronological age) times 100." The chronological age is objective and absolute--it is the biological age of a person--whereas the mental age is based on the mental ability of the person. Using the formula, a 12-year-old child with the mental age of a 15-year-old will have an IQ of 125, calculated as (15 divided by 12) times 100.

## WAIS & WISC

The Wechsler Adult Intelligence Score (WAIS) and Wechsler Intelligence Score for Children (WISC) were developed in the 1930s by American psychologist David Wechsler. The WAIS and WISC scales were different from earlier measures because the scores were based on a standardized normal distribution, rather than a quotient based on age. In Wechsler's system, a mean value of 100 is assigned as the center value on a bell curve (for the standardized normal distribution), with a standard deviation of 15 points. Individual scores are compared to other scores by projecting them on the bell curve.

## Typical IQ Questions

IQ tests include questions on verbal knowledge, spatial visualization, perceptual speed and short-term memory. The commonest IQ measurement tool is the WAIS, in which the tested individual must complete a number of problems in a predefined time frame in a supervised environment. The WAIS test includes 14 parts, half of which are verbal sections containing problems on information, vocabulary, similarity, comprehension, arithmetic, digit span and letter-number sequencing. The other seven parts, emphasizing performance, contain problems on picture completion, picture arrangement, block design, digit symbol coding, object assembly, matrix reasoning and symbol search. Both the WAIS and WISC tests generate scores on individual parts, as well as a total score, indicating overall performance.

## Limitations

An IQ score is not designed to measure capabilities and skills that cannot be directly attributed to mental performance. For example, in a test involving 10 people, the individual with the lowest IQ score may well be the most adept at manual dexterity. Since modern IQ tests such as the WAIS and WISC are based on a normal distribution and bell curve, it follows that the scores are only relevant when they fall under a defined range. The scope of the tests is thus limited, and may not be valid or be able to accurately evaluate very high or very low intelligence.

## Controversies

There is considerable debate over the factors affecting IQ. Some people believe that intelligence may be influenced by environmental factors such as childhood nutrition, breastfeeding in infants and early exposure to toxins, whereas another school of thought maintains that IQ is strictly influenced by upbringing, social status and parents' IQs. Another aspect of IQ that is debated is its relevance. Some scholars argue that intelligence is multidimensional, and thus cannot be fairly and accurately evaluated as a single measure.