What is an intelligence quotient (IQ)?

Introduction

Alfred Binet
and Théodore Simon in France designed the first formal test of intelligence, called the Binet-Simon scale, in 1905. Theirs was an age-based test in that items passed by a majority of children at a particular age were assigned to that age level. For instance, if the majority of nine-year-olds passed a particular item, that item was assigned to the nine-year age level.

William Stern first coined the term “mental quotient” in 1912. Mental quotient was derived by dividing mental age, as assessed by performance on a test such as the Binet-Simon, by chronological age, which yielded a ratio. Children with a ratio of greater than 1 were ahead of their age in mental development, whereas children with a ratio of less than 1 were behind their age in mental development.

In 1916, Lewis Terman published the Stanford Revision and Extension of the Binet-Simon scale, subsequently referred to as the Stanford-Binet. With the publication of the Stanford-Binet, Terman changed the term “mental quotient” to “intelligence quotient,” and he changed Stern’s ratio to a whole number by multiplying the ratio by 100. For example, if a person performed at the seventy-eight-month level on the Stanford-Binet (mental age) and was seventy-two months old (chronological age), his or her intelligence quotient was estimated to be 108.

Change in IQ Score Computation

Shortly after the publication of the Stanford-Binet, fair criticism began to emerge regarding the age-based format of the tests. The alternative suggested was a points-based test for assessing IQ. Tests that use a points-based format assign points based on correctness, quality, and sometimes swiftness of responding. These points are then converted to standard scores, which are then converted into IQ scores. David Wechsler, in the 1930s, was the first to design an intelligence test based on the points format. Current versions of his scales include the Wechsler Preschool and Primary Scales of Intelligence-III (WPPSI-III), the Wechsler Intelligence Scale for Children-III (WISC-III), and the Wechsler Adult Intelligence Scale-III (WAIS-III); these are the most widely used tests of intelligence. Each of the Wechsler scales actually yields three difference intelligence quotients: Verbal IQ, Performance IQ, and a Full Scale IQ that is a combination of the Verbal and Performance IQs.

Interpretation of IQ Scores

IQ scores typically have a mean of 100 and a standard deviation of 15. This means that average intelligence is considered to be any score between 90 and 109. IQs from 110 to 119 are considered to be high average, while scores that range from 80 to 89 are considered to be low average. Scores that fall in the 120 to 129 range are labeled superior, while scores of 130 and above are regarded as very superior. In some instances, individuals who score above 130 are labeled as intellectually gifted. Scores in the 70 to 79 range are considered borderline scores. Those who score below 70 are usually further assessed to determine if a diagnosis of intellectual disability is appropriate.

Factors That Influence IQ

The environmental factors that seem to impact IQ the most include family income, parental education level, parental occupation, and the home atmosphere, which includes the degree to which the parents press for achievement and language development and the provisions they make for their child’s general learning. Although the exact degree to which genetics influences IQ is unclear, estimates are that about 30 to 50 percent of IQ is accounted for by a person’s genotype.

Uses of IQ Scores

One of the primary uses of IQ scores is to assist with the diagnosis and classification of children for special education services. Outside of education, IQ scores have been used to screen applicants for jobs, to help determine the most appropriate job placement within an organization, to assist with vocational counseling, and as part of a complete psychological assessment battery. Relatedly, IQ scores obtained between the ages of three and eighteen have been found to be a significant predictor of educational and occupational success as an adult.

Changes in Conceptualization of Intelligence

Early conceptualizations of intelligence were that one primary underlying cognitive ability permeated all other cognitive skills. Later, researchers have proposed the existence of a number of essential cognitive abilities that should be reflected in the intelligence quotient and have designed tests to measure these different cognitive abilities. For instance, Jagannath Das suggested that the intelligence quotient should reflect a person’s ability to process information simultaneously and sequentially, while Raymond B. Cattell proposed that the two factors that make up intelligence are fluid intelligence and crystallized intelligence. These are two of several possible examples of how the conceptualization of intelligence and the abilities that comprise intelligence have changed.

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