Is the IQ distribution strictly normal?

Medical Psychology and Sociology: Cognition

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Image: "Berlin intelligence structure model according to Adolf Otto Jäger." By Grampians. License: CC BY-SA 4.0


The term cognition is derived from the Latin word cognoscere from, which means something like "recognize" or "experience". It is therefore not surprising that cognition is defined as the entire process that contributes to information processing in a human individual. This includes processes such as the perception of things, memory and remembering or learning something new, attentiveness, thinking and solving problems, language itself and motivation.

Cognition is a characteristic of an intelligent being. These mental processes include what the individual thinks about himself and his environment, both consciously and subconsciously.

Intelligence quotient

Intelligence itself is a hypothetical or latent construct and can be defined in a variety of ways. It can be described, for example, as the ability to find one's way in new situations and under the most varied of influences on the basis of insight and thinking. Operationally, it can be defined in such a way that intelligence is what the intelligence test measures.

There are several components of intelligence: verbal, linguistic intelligence, logical or computational intelligence and the ability to solve problems.

Different tests can be used to determine the intelligence quotient, or IQ for short. For example, in the WAIS IV IQ test, the overall IQ is determined using 15 sub-tests. Four index values ​​are determined here, namely speech comprehension, working memory, logical thinking and processing speed. The IQ value is then always determined from these results in relation to the respective age group and gender.

It is assumed that the IQ follows a normal distribution with a mean value of X ‘= 100. The standard deviation (SX) is 15. The formula with X‘ = mean value, X = value achieved in the test and SX = standard deviation applies:

IQ = 100 + 15 x X-X ‘/ SX

Since the IQ is subject to a normal distribution, it can be said that 68% of all measured IQ values ​​are plus or minus one standard deviation from the mean. That is an IQ between 85 and 115.94% of the measured values ​​lie in an interval of 2 standard deviations plus or minus the mean value, i.e. between 70 and 130. Only 3% of the measured values ​​are below 70 and only 3% of the measured IQ values are over 130.

In the past, the so-called intelligence age was determined. There were specific tests that had to be passed for each age group. A nine-year-old had to be able to solve the tasks of the nine-year-olds and all tasks of the age group below him. If he did not succeed in this, he had a lower intelligence age than age. However, if he was also able to fulfill the tasks up to, for example, those of the 12-year-olds, then he had a higher intelligence age than age. The IQ was then calculated as follows:

IQ = intelligence age / age x 100


There are various theories of intelligence. Three of them will be discussed in more detail below.

Thurstone's intelligence theory is a multiple factor theory. In this model, intelligence is dependent on 7 independent primary factors that are all equally important to one another. These factors are the speed of perception, memory, spatial imagination, logical thinking, arithmetic, language comprehension and the flow of words.

The intelligence structure test (IST) is based on this intelligence theory, which tests the 7 primary factors and determines an IQ for each individual factor. The IST is a group test.

A second intelligence theory was postulated by Spearman. This so-called two-factor theory or g-factor theory divides a person's intelligence into two subsections. The general factor, or G-factor, describes the general mental performance that is involved in all performance of intelligence.
In addition, there are many special individual factors, or S-factors, which are required for special task areas and intelligence services. These S-factors are independent of each other and of the G-factor.

The intelligence test associated with this theory is the Hamburg-Wechsler intelligence test (HAWIE). In addition to the usual verbal part, this test also includes an action part. Here, no distinction is made between the different factors, but an overall IQ is determined directly. The HAWIE is an individual test, which means that only one person can be tested here individually.

In a third intelligence theory by Cattel, a distinction is made between fluid and crystalline intelligence. Fluid intelligence is determined by genes and biology. It is independent of living conditions, culture and upbringing and decreases again from the age of 21. Tasks and problems are dealt with here without resorting to specially acquired knowledge.
Crystalline intelligence, on the other hand, relies on experience-based knowledge and learned skills. Thus, in contrast to fluid intelligence, it is dependent on socio-cultural influences. Besides, it doesn't get lost in old age. On the contrary: it usually tends to increase over the course of life.

Cattel's intelligence theory is not associated with a specific intelligence test.

Popular questions

The correct answers can be found below the references.

1. Which statement about cognition is incorrect?

  1. It includes information processing.
  2. Learning something new, but also remembering is part of it.
  3. Comes from the Latin word cognoscere.
  4. Is a measurable and quantifiable process.

2. A person scores 125 in an intelligence test. The test has a mean of 105 and a standard deviation of 10. What is the IQ score for this person?

  1. 115
  2. 120
  3. 125
  4. 130
  5. 135

3. Which statement is correct in relation to the various intelligence theories?

  1. Spearman divides intelligence into an S-factor and many different G-factors.
  2. Cattel's intelligence theory includes fluid intelligence, which is largely determined by upbringing and cultural background.
  3. In Thurstone's theory with the 5 primary factors, one of them is understanding of language.
  4. The Hamburg Wechsler intelligence test is assigned to Spearman's intelligence theory.
  5. The IST, which is a group test, belongs to the Cattel theory.
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