What is IQ? | Mensa International (2024)

Introduction to IQ

IQ is a type of standard score that indicates how far above, or how far below, his/her peer group an individual stands in mental ability. The peer group score is an IQ of 100; this is obtained by applying the same test to huge numbers of people from all socio-economic strata of society, and taking the average.

The term 'IQ'was coined in 1912 by the psychologist William Sternin relation to theGermanterm Intelligenzquotient. At that time, IQ was represented as a ratio of mental age to chronological age x 100. So, if an individual of 10 years of age hada mental age of 10, their IQ wouldbe 100. However, if their mental age was greater than their chronological age(e.g., 12 rather than 10), their IQ wouldbe 120. Similarly, if their mental age waslower than their chronological age, their IQ wouldbe lower than 100.

When current IQ testswere developed, the average score of the norming sample was defined as IQ 100; and standard deviation(a statistical concept that describes average dispersion)up or downwasdefined as, for example, 16 or 24 IQ points greater or less than 100. Mensa admitsindividuals who score in the top 2% of the population, and theyacceptmany different tests, as long as they have been standardised and normed, and approvedby professional psychologists’ associations.Two of the most well-known IQ tests are 'Stanford-Binet' and 'Cattell' (explained in more detail below). In practice, qualifying for Mensa in thetop 2%means scoring132 or more in the Stanford-Binet test, or 148 or more in the Cattell equivalent.

MeasuringIntelligence -Noteworthy Contributors

Sir Francis Galton

Sir Francis Galton was the first scientist who attempted to devise a modern test of intelligence in 1884. In his open laboratory, people could have the acuity of their vision and hearing measured, as well as their reaction times to different stimuli.

James McKeen Cattell

The world’s first mental test, created by James McKeen Cattell in 1890, consisted of similar tasks, almost all of them measuring the speed and accuracy of perception. It soon turned out, however, that such tasks cannot predict academic achievement;therefore, they are probably imperfect measures of anything we would call intelligence.

Alfred Binet

The first modern-day IQ test was created by Alfred Binet in 1905. Unlike Galton, he was not inspired by scientific inquiry. Rather, he had very practical implications in mind: to be able to identify children who cannot keep up with their peers in the educational system that had recently been made compulsory for all.

Binet’s test consisted of knowledge questions as well as ones requiring simple reasoning. Besides test items, Binet also needed an external criterion of validity, which he found in age. Indeed, even though there is substantial variation in the pace of development, older children are by and large more cognitively advanced than younger ones. Binet, therefore, identified the mean age at which children, on average, were capable of solving each item, and categorized items accordingly. This way he could estimate a children’s position relative to their peers: if a child, for instance, was capable of solving items that were, on average, only solved by children who were two years older, then this child would betwo years ahead in mental development.

William Stern

Subsequently, a more accurate approach was proposed by William Stern, who suggested that instead of subtracting real age from the age estimated from test performance, the latter (termed 'mental age') should be divided by the former. Hence the famous 'intelligence quotient'or 'IQ'was born and defined as (mental age) / (chronological age). It indeed turned out that such a calculation was more in line with other estimates of mental performance. For instance, an 8-year-old performing on the level of a 6-year-old would arriveat the same estimate under Binet’s system as a 6-year-old performing on the level of a 4-year-old. Yet, in Stern’s system, the 6-year-old would geta lower score as 4/6 < 6/8.Experience shows that when they both turn 10, the now 8-year-old is more likely to outperform the now 6-year-old in cognitive tasks;hence Stern’s method proved to be more valid.

Lewis Terman

It was in the US where IQ testing became a real success story after Lewis Terman revised Binet’s test, creatinga much more appropriate norm than the original, and he published it as the Stanford-Binet test (Terman was a psychologist at Stanford University). He was also keen to multiply the result by100, so the final equation for IQ is (mental age) / (chronological age) X 100. Indeed, an IQ of 130 sounds much coolerthan anIQ of 1.3.

This method, however, only works well in children. If a child’s parents were told that their 6-year-old already hadthe mental capabilities of an average 9-year-old and, therefore, his or her IQ was 150, they wouldbe over the moon. But if the child’s grandfather was told that even though he was only 60, his cognitive abilities were on a par with theaverage 90-year-old, he might not take it as a compliment. Obviously, the quotient only works as long as Binet’s original criterion is functional;i.e., as long as older age in general means better abilities. In other words, the method is inappropriate when mental development does not take place any more.

DavidWechsler

DavidWechsler solved the problem of calculating adult IQ by simply comparing performance to the distribution of test scores, which is a normal distribution. In his system the IQ of those whose score equalledthe mean of the age group was100. This way the IQ of the average adult would be100, just like the IQ of the average child in the original system. He used the statistical properties of the normal distribution to assign IQ scores based on the extent of the contemporaries one outscored. For instance, someone whose score was one standard deviation above the mean, and who thus outperformed86% of his or her contemporaries, would havean IQ of 115,and so on.

IQ Tests Today

So, why is it called 'IQ', a quotient, if nothing gets divided? The simplereason is that the concept of IQ had become toopopular for the termto be discarded. Even so, it is interesting to note that in adults it is not really a quotient at all:it is an indication of how well one performs on mental tests, compared to others. Besides extending the concept of IQ, another major step in the development of IQtesting was the creation of group tests; before this, people had beenindividually testedby qualified psychologists. The first grouptest was created for the US army, but they soon spread to schools, workplaces and beyond, becomingone of psychology’s greatest popular successes, and remain so to this day.